WO1992014849A1 - Process and apparatus for de-oiling mill scale waste materials - Google Patents

Abstract

A process for treating oily mill scale waste materials comprises treating the mill scales with an inorganic treatment solution which includes a pH-modifying alkali material such as sodium hydroxide and a dispersant such as sodium silicate. Preferably, the mill is classified into a fines fraction and a coarse fraction prior to treatment, the fines component being treated successively by contact with the treatment solution followed by solids-liquid separation. After removal of oil by skimming, flocculation, and extraction, the inorganic treatment solution is recycled. An apparatus for recovering de-oiled mill scale, which comprises: (a) means for segregating oily mill scale waste by size (203); (b) means for contacting each segregated mill scale component with a treating solution (221 and 207); (c) means for separating the mill scale from the effluent, which includes said oil removed from the substantially de-oiled mill scale (227); (d) means for removing oil from said effluent to provide a substantially de-oiled treating solution (273); (e) means for recovering a de-oiled mill-scale (215 and 259); (f) means for recycling the treating solution (309).

Description

POCESS AND APPARATUS FOR DE-OILING MILL SCALE WASTE MATERIALS

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TECHNICAL FIELD

This invention relates generally to resource recovery. Though this process can have applications to other wastes, it has particular application to 10 the de-oiling of mill scale waste materials.

BACKGROUND ART

Mill scale wastes are metal oxides formed in hot rolling operations in the aluminum and steel industry. The principal application of this technology

15 is in the steel industry where the mill scale waste includes oxides of iron, FeO, Fe₂0₃, and Fe₃0₄. During steel processing operations, layers of mill scale are continually formed on the surface of steel slabs, sheets, and bars when exposed to air at elevated temperatures. The mill scale layers are typically removed by cooling and cleaning the steel with a high pressure water spray, then collected

20 along with the wastewater in a collection system. This wastewater often contains a high level of dispersed oily and hydrophobic materials (hereinafter "oil") and oil-contaminated mill scale materials. The oil may come from a number of sources, such as hydraulic fluid leaking from steel-making equipment, oils from milling operations, and inadvertent dumping of oils and

25 lubricants into the collection system. The dispersed oil may to some extent be removed from the wastewater by skimming the surface of the water from a settling tank; however, a significant amount of the oil remains absorbed on the mill scale.

° 30 Disposing of the contaminated mill scale as industrial waste is

" inappropriate because of environmental problems. Further, the mill scale has value as a recyclable material. For example, the mill scale may be used as resource materials in sintering operations. But the sintering of mill scale having high levels of oil contamination is generally avoided because of unacceptable hydrocarbon emissions and equipment fouling. Thus, a need exists for removing sufficient amounts of the oil from the mill scale to make it useful in sintering and other steel manufacturing operations.

While various processes have been used in attempting to remove the oil from the mill scale, these processes are often inadequate. For example, many systems do not adequately address the problem of different levels of oil contamination on differently-sized mill scale materials. Consequently, the coarser mill scale particles are either treated together with the finer mill scale particles, resulting in inefficient treatment, or else the coarser mill scales are not treated at all.

Other systems such as kiln systems or "hot" processes vaporize oil from the surfaces of the mill scale. However, hydrocarbon emissions from such systems often exceed allowable air quality levels. Furthermore, condensed oils often tend to coat the equipment, thus creating a fire hazard.

Some systems employ organic treating agents such as soaps or surfactants. However, surfactants must be removed before the treating solution can be recycled; otherwise, continuous monitoring of the system to ensure the appropriate surfactant level would be required. But separation of the organic materials from the treating solution requires expensive equipment and constant replenishing of surfactant, often adding tremendous costs to the overall operation.

A continuing need therefore exists for an improved mill scale de-oiling and recovery system which would avoid such shortcomings.

DISCLOSURE OF INVENTION The invention relates to resource recovery and specifically to the recovery of mill scale waste materials. More particularly, the invention relates to a method and apparatus for treating and recovering mill scale materials comprising contacting the mill scale with a recyclable inorganic treating dispersion, then recovering the de-oiled mill scale. The inorganic treating dispersion is an aqueous solution or suspension comprising an inorganic treating agent. Throughout the text, solution will be referred to mean a dispersion. The inorganic treating solution includes a pH modifier selected from the group consisting of potassium hydroxide, sodium hydroxide, calcium oxide, calcium hydroxide, sodium carbonate, and potassium carbonate and a dispersant selected from the group consisting of sodium silicate, potassium silicate, sodium phosphate and potassium phosphate.

In a preferred embodiment, the invention comprises a closed loop wastewater treatment system. The treating solution, mixed with an oily wastewater/mill scale mixture, is continually recycled or recirculated through the system without having to add additional water. Preferably, the method includes a recycled or recirculated inorganic treatment solution capable of removing significant levels of oil from the mill scale particles, to the level such that the mill scale can be reused. A particular advantage of the invention is that while the treating agent concentration in the treating solution may be high, e.g., about 10 percent by weight (for heavy oil contamination), the treating solution may also be effective at low treating agent concentrations, e.g., down to about 0.05 percent by weight (for light oil contamination). Selection of the treating agent concentrations for the different oily mill scales will depend on the degree of oil contamination and the type of oily material present. Generally, high viscosity oils will require a high concentration of the treating agent in the process. Light oils are defined as those with relatively low viscosity and high vapor pressure, such as hydrocarbons having carbon chain lengths of 15 or less. Heavier oil contaminants are hydrocarbons with carbon chain lengths greater than 15. Due to the considerable diversity of oil contaminants present in the mill scale waste, the oil concentration needs to be evaluated through bench scale testing as illustrated in the Examples to properly determine the required treating agent concentration. A preferred embodiment of the invention comprises separate classification, treatment, recycling and recirculation steps. In this embodiment, the mill scale materials are classified according to size into an oversized coarse particle component and an undersized fine particle component. The treating step comprises contacting each component separately with the treating solution, which preferably consists of inorganic treating agents and more preferably comprises effective amounts of an inorganic pH-modifier and inorganic dispersant, ranging from about 0.05 percent by weight (for light oil contamination) to about 10 percent by weight (for heavy oil contamination). The treating solution may be heated from ambient temperature to boiling, with higher temperatures within the range of 160°F to 200°F being preferred. The temperature for the treating solution is dependent upon the level of oil removal required and the type and oxidation level of the oil. Operating at a higher temperature will improve the overall rate of oil removal from the mill scale waste. The coarse particles are treated after classification, or as required, which may need a one stage reactor, depending on the oil content and the type of oil present. The fine oily mill scale particles may be stirred vigorously in a treatment vessel containing the treating solution, followed by separation of the de-oiled fine mill scale particles from the treating solution. In an alternate embodiment, the same procedure is followed except the de-oiled fine mill scale particles are rinsed after stirring but before the solid-liquid separation. Preferably, to obtain the desired degree of substantial oil removal, it is essential to repeat this fine particle treatment and solids-liquid separation several times. Treatment vessels, where the oily mill scale is contacted with the treating solution, and their corresponding solid-liquid separators are connected in a cascading series as shown in Figures 1 and 2. Substantial oil removal in the steel industry depends upon the makeup of the sinter plant feed and the type of air pollution control facility used. Typically, for dry systems, the oil limit is about 0.1 percent by weight and for wet systems, the oil limit is about 1.0 percent by weight. Nevertheless, higher oil-containing mill scale waste can be blended appropriately with cleaner mill scale waste as feed to the sinter plant, so long as the final feed meets the operational specifications of the sinter plant.

Separation of both fine and coarse de-oiled mill scale particles from the treating solution may be accomplished with a solids-liquid separation step including filtering or hydrocycloning. In the hydrocycloning procedure, the underflow consists of the treated fine mill particles, while the effluent, or specifically, the overflow, consists of the treating solution, containing oil, to be recycled or recirculated. In the filtering procedure which may contain a vacuum horizontal belt filter, the overflow consists of the mill particles, while the effluent, or specifically, the filtrate consists of the treating solution, containing oil, to be recycled or recirculated. During the recycling stages, an organic flocculant such as Clearfloc 751 or Jayfloc 911 should be added to the treating solution after the treatment stages to remove trace amounts of oil and solids. But in a preferred and alternate embodiment, substantially no organic ingredients are present during the mill scale treatment stages.

After the separation step, the remaining wastewater effluent contains treatment agents as well as microscopic oil droplets and ultra-fine mill scale. An important aspect of the process is the recycle or recirculation step of the treating solution. Recycle of the treating solution comprises removing sufficient oil from the wastewater effluent to provide a treating solution in condition for reuse, then recycling the treating solution back to contact the oily mill scale waste. In addition to recycling, one embodiment of the invention recirculates a portion of the treating solution to contact the oily mill scale before recycling. Recirculating the treating solution comprises contacting it with "upstream" fine oil mill scale which has a higher content of oil than the fine mill scale just contacted by the treating solution. Recirculating the treating solution allows a more efficient use of the treating solution in that the recirculated treating solution contacts oily mill scale in different treatment vessels before the treating solution is recycled. The recirculating feature is possible in both embodiments, but is illustrated in Fig. 2. In a preferred and alternate embodiment, the recycle step further comprises contacting the effluent with a flocculant in an amount sufficient to remove oil and ultra-fine solids from the effluent. The flocculant is a polymer preferably comprising a cationic quarternized amine having a molecular weight in the range of 50,000 to 40,000,000 and charge density in the range of 5 to 100%, preferably quarternized for high pH (>11.0) applications. The success of the flocculant used is evaluated on the basis of total suspended solids and the oil content of the resultant clarified solution. After fiocculation, the aggregated or flocculated material ("floe") is separated from the clarified effluent. In an alternate embodiment, the effluent is filtered through a deep bed sand filter to remove residual flocculant from the clarified effluent. The success of the flocculant used is evaluated on the basis of total suspended solids and the oil content of the resultant clarified solution. The concentration of any leftover flocculant in the effluent may then be reduced, preferably using an oxidizing agent, to produce a substantially inorganic treating solution for recycling. The oxidizing agent comprises hydrogen peroxide, ozone, permanganate, hypobromous acid, chlorine dioxide, hypochlorous acid, hypoiodous acid, or a member selected from the group consisting of chlorine, bromine and iodine.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic flowsheet of an alternate embodiment of the present invention;

Figure 2 is a schematic flowsheet of a preferred embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

Although directed to resource recovery in general, in a particular aspect this invention relates to a method for recovering oil-contaminated mill scale solids comprising treating the oily mill scale with an inorganic treating solution to remove oil from the surfaces of the mill scale. Preferably, after the mill scale is contacted with the treating solution in accordance with the invention, oil is removed from the resulting wastewater effluent to provide a reusable treating solution which may then be recycled back to contact additional oily mill scale.

One embodiment of the process is described below with reference to

Figure 1. This embodiment comprises classifying the mill scale into fine and coarse particle components; removing the various types of oil and lubricants from the fine and coarse mill scale materials in the treatment stages; separating the de-oiled mill scale from the liquid; and recovering the de-oiled mill scale for use in sintering operations. Preferably, an improved recycling stage is provided which includes removing oil from the wastewater effluent (hereinafter "effluent") to provide a substantially de-oiled effluent stream comprising the inorganic treating solution and recycling the treatment solution back for additional mill scale treatment. Preferably, the recycling stage includes a flocculation step in which any remaining microscopic oil in the effluent may be contacted with a small amount of an organic flocculant to remove the oil. The flocculant in the effluent is degraded from the effluent to obtain a substantially inorganic treating solution which may be recycled back to the treatment stage(s) and contacted with the oily mill scale.

In a preferred embodiment, the treating solution in an aqueous inorganic solution comprising an alkali treating agent which modifies the pH of the mill scale particle surfaces and a dispersant to modify the mill scale surface and facilitate oil removal. In a preferred embodiment, it is essential that the treating solution be inorganic, based on the discovery that treating solutions containing organic materials such as surfactants or organic detergents are not easily recycled. As discussed in the background section, such a treating solution may require extensive separation or monitoring prior to recycle. Advantageously, the treating solution of a specific embodiment of the present invention may contain a low concentration of treating agents, e.g., below 1 or 2 percent and in some cases as low as 0.1 percent by weight, and still be effective. Thus, only periodic replenishment of the treating agents is necessary. In a preferred embodiment, the treating agent comprises a separate pH- modifier such as sodium hydroxide together with a separate dispersant such as a sodium silicate.

In accordance with a specific embodiment of the invention, the treating solution comprises sufficient pH-modifier to alter the pH of the mill scale surface to a desired level. Generally, the pH-modifier is an alkali or base which raises the pH of the treating solution or slurry. The pH of the fine mill scale surfaces may be approximated by measuring the pH of the slurry (often referred to as "bulk pH"), while the pH of the coarse mill scale surfaces may be approximated by measuring the pH of the treating solution rinsing stream in the treatment rinser. The pH-modifier preferably comprises potassium hydroxide or sodium hydroxide. Other pH-modifiers which may also be used include sodium chloride, sodium or potassium bicarbonate, and magnesium hydroxide. Monovalent pH-modifiers such as sodium hydroxide are preferred because, although divalent pH-modifiers may be used, they often tend to agglomerate the mill scale fine particles.

The pH-modifier should be added in an amount sufficient to obtain a pH from 7 to 9 for light oil contaminants and 9 to 12.5 for heavier oil contaminants. Thus, lower concentrations of pH-modifiers are generally required for light oil contamination than for heavy oil contamination. Light oils are defined as those with relatively low viscosity and high vapor pressure, such as hydrocarbons having carbon chain lengths of 15 or less. Heavier oil contaminants are hydrocarbons with carbon chain lengths greater than 15. Because of the different desired pH ranges for different types of oil contamination, in a specific embodiment of the invention the mill scale particles are classified based on the type of oil contamination (e.g., light oil versus heavy oil contamination) prior to size classification. In such an embodiment, mill scale contaminated with light oils are directed to one collection device (not shown) while mill scale contaminated with heavier oils are directed to another collection device (not shown). Each component may then be size-classified and/or treated separately as discussed below.

The dispersant of this invention broadly includes any surface active agent which promotes separation of the mill scale particles, and may include polymeric electrolytes or polyphosphates. The function of the dispersant is to modify the surface of the mill scale materials to facilitate removal of oil and lubricants during the treatment stages. In a preferred aspect, the dispersant of the present invention is inorganic and preferably comprises a silicate or phosphate of sodium or potassium. More preferably, the dispersant comprises sodium silicate. In a preferred aspect, the treating solution comprises sodium hydroxide in addition to the sodium silicate dispersant. However, because certain dispersants, such as sodium silicate, may also modify pH, a treating solution consisting of water and sodium silicate may under certain circumstances constitute a treating solution which both modifies pH and the surfaces of the mill scale to facilitate oil removal.

FIGURE 1

As illustrated in Figure 1, one embodiment of the invention, the mill scale materials are fed to a classifying stage which comprises classifying or segregating the mill scale feed materials into a "coarse" component and a "fines" component. In a specific embodiment, the mill scale feed materials are passed over a size separator, e.g., a classifying screen 11, which mechanically classifies the mill scale particles into overflow "coarse" particles and underflow "fine" particles. A stream of recycled treating solution may be directed downward onto the classifying screen 11 , along line 13. Alternatively, hot water (not shown) may be directed downward onto the classifying screen 11. Thus the fine particles are pushed through the line 17 which exits the classifying screen 11 and enters the treatment vessel 33. The overflow coarse particles are then gravity fed through line 15 to the treatment rinser 19.

The coarse particles should preferably be 8-mesh or greater with the fine particles being smaller than 8-mesh. More broadly, however, the division between "coarse" and "fine" particles should vary from case to case depending on the particle size distribution of a particular oily mill scale feed, the type and extent of oil contamination, and the desired level of oil removal. It has been discovered that finer mill scale particles have a proportionately higher amount of oil contamination relative to their particle weight than do the coarser mill scale particles. A typical oil and grease distribution is shown in Table 1. Therefore, in accordance with this invention, it is generally desirable to treat the fine particles with greater amounts of treating agents for longer contact periods than the coarse particles.

Following classification, each component is separately contacted with the treating solution in a variety of treating steps. Preferably, the fines component is stirred vigorously in a vessel containing the treating solution, followed by rinsing and separation of the de-oiled fine mill scale particles from the treating solution. Preferably, the coarse component is treated by spraying the component with treating solution, then separating the de-oiled coarse mill scale from the treating solution. In either case, after sufficient contact with the treating solution the de-oiled mill scale should be separated from the treating solution with a solids-liquid separation step including filtering or hydrocycloning. After separation, the resulting liquid or effluent comprises water, oil, the inorganic treating agent (e.g., sodium hydroxide and/or sodium silicate), and ultra-fine mill scale particulates.

I. COARSE MILL SCALE

The oily coarse mill scale materials are preferably rinsed with a treating solution at treatment rinser 19. In a preferred embodiment, the treatment rinser 19 includes means for contacting the mill scale with a treating solution stream, e.g., a spraying device 23. The treating solution stream is sent from the reclamation vessel 125 along line 27 and then connecting to line 25. Additionally, treatment rinser 19 contains a solids-liquid separator 21, including a vacuum horizontal belt filter or hydrocyclone, to separate the solids from the liquid after contacting the mill scale with the treating solution. The spraying device, belt filter, and hydrocyclone are themselves conventional devices, are recognized by persons skilled in the art, and will not be described herein in detail.

The treating solution and water may be heated from ambient temperature to boiling, with higher temperatures within the range of 160°F to 200°F being preferred. The temperature for the treating solution is dependent upon the level of oil removal required and the type and oxidation level of the oil. Operating at a higher temperature will improve the overall rate of oil removal from the mill scale waste. The temperature can be monitored by placing a sensor above the reclamation vessel 125. Following rinsing and separation, the de-oiled coarse mill scale may be recovered as resource material pumped through line 29. Preferably, the recovered coarse mill scale is then dried and directed to a sintering plant, wherein it may be heated at temperatures sufficiently high to cause sintering. Although the coarse mill scale may be sintered separately, it may also be desirable to mix the de-oiled coarse mill scale with de-oiled fine mill scale, prior to sintering. The filtrate from the treatment rinser 19, comprising treating agent, water and oil, is directed to settling tank 81 along line 31.

II. FINE MILL SCALE

To treat the fines component, the fines mill scale (preferably less than

8-mesh) are placed in a treatment vessel 33 which includes a slurry of treating solution and preferably the recycled treating solution directed from the classifying screen 11. In operation, the fines mill scale feed materials are preferably directed along line 17, as underflow from classifying screen 11 to treatment vessel 33 along with water or recirculating treating solution to form the slurry. Preferably, the treatment vessel 33 includes a stirrer 35 and heating means, e.g., a heating coil or steam (indicated by arrows), to increase the temperature of the slurry. The .temperatures of the slurry may range from ambient to boiling, with higher temperatures within the range of 160°F to 200°F being preferred. The temperature for the treating solution is dependent upon the level of oil removal required and the type and oxidation level of the oil. The temperature can be controlled by placing appropriate sensors over the treatment vessels 33, 51, and 67. Additionally, the temperature in the reclamation vessel 125 is monitored with a temperature sensor. Recycled treating solution from reclamation vessel 125 may be directed on line 27 to treatment vessel 33. Preferably, a constant stream of treating solution is recycled to treatment vessel 33 from reclamation vessel 125 with treating solution also directed optionally on lines 41, 63, and 71 to other treatment rinsers 45, 57, and 73 respectively. Preferably, chemicals such as sodium hydroxide and sodium silicate are added to the reclamation vessel 125 either manually or through dispensers 127 and 129, to make the inorganic treating solution comprise a pH-modifier and dispersant in the desired amounts. The pH is monitored by testing the recycled solution within reclamation vessel 125. Dispensers 127 and 129 may include any means for introducing the chemicals separately into the recycled treating solution. The chemicals are preferably dispensed in a controlled fashion, for example, from chemical metering pipes through pipes or tubes. Treatment chemicals may also be added to the solution at virtually any point in the system; to one of the treatment vessels 33, 51 or 67, or the treatment rinsers 45, 57 or 73.

In one embodiment, the slurry in treatment vessel 33 is stirred vigorously using stirrer 35 for a time sufficient to remove oily material from the surfaces of the mill scale. The stirring time can be varied depending on the type and amount of oil contamination, and should be determined experimentally. Following stirring, the fines are removed from treatment vessel 33 and directed on line 37 to fines treatment rinser 45, which may be designed in a manner similar to that of coarse mill scale treatment rinser 19.

Preferably, treatment rinser 45 comprises a spraying device 43 for dispensing a stream of treating solution from line 41 onto the fine mill scale and a solids-liquid separator 29, e.g., a vacuum belt filter or hydrocyclone, for separating the liquid from the mill scale and preferably for removing as much treating solution and oil from the mill scale as possible. In an embodiment which comprises a vacuum belt filter, the fines may be rinsed with the treating solution prior to drying under a vacuum. Preferably, after separation in the vacuum belt filter or hydrocyclone, treatment of the mill scale fines is repeated several times using treatment vessels 51 and 67, each followed by rinsing and separation in treatment rinsers 57 and 73, to produce the desired removal of oil contamination, each yielding a de-oiled fine particle stream and an oily filtrate stream.

In the cases of both the fines component and the coarse component, prior to mill scale recovery, the rinsing stage may be followed by conventional means of solids-liquid separation, including treatment with a dewatering screen, a vacuum or gravity drained belt, or a centrifuge. A preferred solid-liquid separator is a hydrocyclone. In a preferred embodiment, the recovered de- oiled mill scale (both coarse and fines component) has less than 0.1 percent by weight oil contamination. In a preferred embodiment of the invention, it is considered essential for reducing oil contamination to below about 0.1 percent to include several stages as shown in Figure 1. Each stage includes the steps of treating the mill scale by contacting it with treatment solution, then performing solids-liquid separation to remove the treatment solution with oil, leaving a less oily mill scale.

The overflow from treatment rinser 45 is sent through line 47 to the treatment vessel 51, while the filtrate from treatment rinser 45 travels through line 49, which connects to line 31 and ends in the settling tank 81. In addition, the treating solution from reclamation vessel 125 is sent into treatment vessel 51 through line 53.

After stirring, all the contents of treatment vessel 51 are sent, via line 55, to treatment rinser 57. There, the mill scale are sprayed with the treating solution from reclamation vessel 125 which is sent along line 63. The overflow from treatment rinser 57 is pumped along line 61 to treatment vessel 67. The contents of treatment vessel 67 are contacted with the treating solution from reclamation vessel 125 along line 65. The filtrate from treatment rinser 57 is sent through line 59 connecting with line 31 and ending in the settling tank 81.

From treatment vessel 67 all the contents are sent through line 69 to the treatment rinser 73. There, the mill scale are contacted with the treating solution sent from reclamation vessel 125 on line 65 which connects with line 71. The mill scale are additionally contacted with the slurry sent from lines 87 and 99. The filtrate from treatment rinser 73 is sent along line 77 to connect to line 31, ending in the settling tank 81. The de-oiled fine mill scale which are sent through line 75 are recovered and sent on to the sintering plant similar to the coarse mill scale.

III. RECYCLE SYSTEM

Another aspect of the invention involves the recycling or recirculation stages. The importance of a recycling system stems from both environmental and economic factors. A mill scale treatment system which produces wastewater effluent creates a need for additional treatment or disposal facilities to handle contaminated liquids. Furthermore, as discussed in the background section, disposing of the effluent creates a need to replenish the supply of treatment chemicals. Accordingly, this invention preferably includes a recycling stage for treating the liquids from the treatment stages.

Referring to the embodiment shown in Figure 1, the recycling stages comprise directing the filtrate from the coarse particle treatment rinser 19 through line 31 and from one or all of the fine particle treatment rinsers 45, 57 and 73 via lines 49, 59, and 77 respectively, to settling tank 81. In a one embodiment, the system is a "closed-loop" system, wherein all the filtrate from the treating stages is directed to the settling tank 81 and continuously recycled back to the treatment stages.

In settling tank 81, the effluent, composed mainly of treating solution and oil, as well as any other material not separated and recovered with the mill scale, is permitted to settle into layers, e.g., a layer comprising treating solution and a surface layer of oil. A small remnant of solid particles, including ultra fine mill scale not retained with the solids in the solids-liquid separator, e.g., either by the filter or with the cyclone overflow, may settle at the bottom of settling tank 81 and be periodically removed. These solids may be recovered for disposal. Preferably, they are either recycled or sent for further treatment depending on the oil/grease content. The oil/grease content is determined on a dry weight basis, and the following ranges are for example only and do not dictate the requirements of the invention. A very high oil/grease content would be greater than 2 percent by weight. A high oil /grease content would be 0.4 - 0.5 percent by weight. A marginal oil/ grease content would be 0.15 percent by weight. A clean oil/grease content would be <0.1 percent by weight.

If the oil/grease content is low enough, valves 79 and 89 will be closed and the particles will travel along line 85, connect with line 87 and get sent to treatment rinser 73 to be recovered as coarse mill scale.

If the oil/ grease content is marginal, it may be desirable to close valves 79 and 86 and to open valve 89 to direct these solids through line 91 into treatment vessel 93 wherein an oxidizing agent such as ozone or peroxide may be added. The oxidizing agent degrades absorbed oil from the surfaces of the oily ultra fine particles.

If the oil/grease content is high, valves 89 and 86 will be closed. The material will go through valve 79 via line 83 and will be returned to treatment vessel 33 for further treatment. If the oil/grease content is very high, it may be desirable to close valves 79 and 86 and to open valve 89 to direct these solids through line 91 into treatment vessel 93 for storage until it is removed for disposal or outside processing beyond the scope of this invention.

The solids in treatment vessel 93 may be separated from any remaining oily phase and treating solution by allowing the solids to settle at bottom. The oily phase may be removed, e.g., by skimming, from treatment vessel 93 through line 95 and collected or disposed of. The remaining materials with a high oil/ grease content may be directed back to treatment vessel 33 through line 97 (which connects to line 137) for further treatment. If the materials have a low oil/ grease content they will be sent to treatment rinser 73 through line 99 to be blended with de-oiled mill scale.

After settling, the effluent from settling tank 81, which comprises oil and treating solution, is preferably directed over weir 101 to skimming vessel 103, where any oily phase may be skimmed from the surface and directed through line 105 which connects with line 109 for collection or disposal. This oil may be combined with extracted oil from extractor vessel 135 which is also sent through line 109.

To extract the oil in extractor vessel 135, the floes from settling tank 115 are preferably treated with an oxidizing agent (e.g., peroxide or ozone) or a solvent (e.g. diesel oil or alkyl amine) as shown by arrow 143. The treated floes may be directed to a gravity separator (not shown) to remove the oil. The remaining solids-liquid phase, if it has an acceptably low level of oil and grease, may be directed through line 87 to one of the treating rinsers, e.g., treatment rinser 73. If the level of oil and grease is unacceptably high, valve 136 may be opened and the solid-liquid phase may be directed through line 137 back to the slurry in treatment vessel 33.

After removing the oil from the effluent by skimming, the remaining effluent in sl mming vessel 103 (which may still contain trace amounts of oil and fine particles) may then be directed through line 107 to a means for removing additional oil which preferably comprises a flocculation vessel 111, which preferably includes a stirrer 35. In flocculation vessel 111, the effluent is preferably contacted with a polymer flocculant as shown by arrow 139. The polymer flocculant preferably comprises a high molecular weight cationic quaternized polymer having a molecular weight in the range of 50,000 to 40,000,000. In flocculation vessel 111 the flocculant is preferably mixed with the effluent in a sufficient amount and for a sufficient time.

An especially preferred flocculant is a quaternized amine cationic polymer having a charge density in the range of 5 to 100%. Of the various polymers tested, the two found to be the most effective were Clearfloc 751 sold by Clearwater, Inc., Pittsburgh, Pa. and Jayfloc 911 Cationic Flocculant sold by Callaway Chemical Company, Columbus, Georgia.

Jayfloc 911 Cationic Flocculant is an extremely high molecular weight, high charge density cationic emulsion polymer. Clearfloc 751 is a high molecular weight, very high charge density cationic polyacrylamide. The product is supplied as a low viscosity water-in-oil emulsion and it generally performs in the pH range of 2.5 to 12.5.

Polymer addition is monitored using a streaming current detector, preferably a detector by Milton Roy. This on-line streaming current measurement device measures the charge density on the particles. If the charge density is high, more polymer is added. Similarly, if the charge density is low, less polymer is added. The Milton Roy coagulant control center is an on-line electrokinetic charge analyzer with measuring, recording and control functions. It is the only on-line instrument which directly measures the result of coagulation addition by instantaneously measuring the electric current is generated. One monitor would be placed on line 107, before the polymer is added (at arrow 139). A second monitor would be placed on line 123, after the hydrogen peroxide is added, to ensure the polymer has been destroyed. The second monitor would detect the charge, thus if the particles are more negatively charged then the polymer has been destroyed.

From flocculation vessel 111, the effluent is directed through line 113, to a secondary settling tank 115. At this stage, the flocculated effluent is separated, with the clarified effluent in the upper layer being recovered through line 117, and the aggregated solid and oil fraction being directed through line 133 to extractor vessel 135.

To remove the residual flocculant from the clarified effluent and to produce a recyclable, substantially inorganic treating agent, the effluent should be filtered through a deep bed sand filter 119. From the sand filter 119, the filtered effluent, traveling along line 120, is treated in a final treatment vessel 121 by mixing it with an oxidizing agent, e.g., peroxide or ozone, as shown by arrow 141. If insubstantial amounts of flocculant remain in the clarified effluent, the effluent in stream 117 may be directed immediately to treatment vessel 121, bypassing sand filter 119. The recycle stream 123 preferably comprises aqueous inorganic treating solution in condition for recirculating back to reclamation vessel 125, where either water or a pH modifier and/or dispersant may be added as necessary to the recycled solution (through lines 127, 129,and 131, respectively) to obtain the desired composition of treating solution for use in any one of the previously described treating steps. The pH is monitored by testing the recycled solution from within the reclamation vessel 125.

FIGURE 2

As illustrated in Figure 2, a second embodiment of the invention, the process begins with a classifying stage where the mill scale feed materials are separated into a "coarse" component and a "fine" component. In a specific embodiment, the mill scale feed materials are passed over a size separator, e.g., a classifying screen 203, desirably of 8-mesh, which mechanically classifies the mill scale particles into overflow "coarse" particles and underflow "fine" particles.

A stream of treating solution travelling along line 201, is directed downward onto the screen 203 to push the "fine" particles through the screen into treatment vessel 221, while the "coarse" particles are sent to treatment rinser 207. Alternatively, water (not shown) may be directed downward onto screen 203.1n the illustrated embodiment, coarse particles are 8-mesh or greater, and fine particles are smaller than 8-mesh. The mesh size used to separate "coarse" and "fine" particles may vary. The factors affecting this variation are the particle size distribution of a particular oily mill scale feed, the type and extent of oil contamination, and the desired level of oil removal.

I. COARSE MILL SCALE

The mill scale classified as "coarse" by screen 203, is separately contacted with the treating solution in a variety of treating steps. The coarse component is treated by spraying it with a treating solution from the plant's recycled treating solution, and then separating the de-oiled coarse mill scale from the treating solution, with a solids-liquid separation step. The liquid or "filtrate" resulting from the separation comprises water, oil, the inorganic treating agent (e.g., sodium hydroxide and/or sodium silicate), and ultra-fine mill scale particulates.

More specifically, the oily coarse mill scale materials are gravity fed to the treatment rinser 207 along line 205. In a preferred embodiment, the treatment rinser 207 includes means for contacting the mill scale with a stream of treating solution, e.g., a spraying device 209. The treating solution and water may be heated from ambient temperature to boiling, with higher temperatures within the range of 160°F to 200°F being preferred. The temperature for the treating solution is dependent upon the level of oil removal required and the type and oxidation level of the oil. Operating at a higher temperature will improve the overall rate of oil removal from the mill scale waste. The temperature may be controlled by placing sensors above reclamation vessel 309.

The treatment rinser 207 includes a solid-liquid separator, comprising a vacuum horizontal belt filter 213. This filter separates the solids from the liquid as the mill scale is contacted with the plant's recycled treating solution from line 211.

Following contact with the plant's recycled treating solution, the de-oiled coarse mill scale is sent through line 215 to be recovered as resource material. Preferably, the recovered coarse mill scale is then dried and directed to a sintering plant. There it may be heated at temperatures sufficiently high to cause sintering. Although the coarse mill scale may be sintered separately, it may also be desirable to mix the de-oiled fine mill scale with the de-oiled coarse mill scale, prior to sintering.

The liquid or "filtrate" from the treatment rinser 207 is separated from the mill scale by the vacuum horizontal belt filter and directed through line 217 to the second sump 269. The filtrate is comprised of the treating agent, water, oil, and ultra-fine mill scale particulates.

II. FINE MILL SCALE

Those particles smaller than 8-mesh are classified as "fine" by screen 203. Generally, the "fine" mill scale particles have a proportionately higher amount of oil contamination relative to their particle weight than do the coarser mill scale particles. (A typical oil and grease distribution is shown in Table 1.) Therefore it is generally desirable to treat the fine particles with greater amounts of treating agents and for longer contact periods, than the coarse particles.

In operation, the "fine" mill scale feed materials are preferably directed as underflow from classifying screen 203 along with the treating solution, through line 219 to a first treatment vessel 221, in which the fine mill scale particles are stirred vigorously along with the treating solution. Next, the de- oiled "fine" mill scale particles are separated from the treating solution.

More specifically, to treat the "fine" component emerging as underflow from the screen 203, the underflow is channeled through line 219 to a first treatment vessel 221, which includes a slurry of treating solution. In the present embodiment, the treating solution in the first treatment vessel 221 is provided by three sources, namely solution recirculated from the treating vessel 235 through line 229, material recirculated from line 275, and material recirculated from the overflow of the second hydrocyclone 227 through line 239. Preferably a constant stream of treating solution is recirculated to the first treatment vessel 221 by these means.

The treatment vessel 221 includes a stirrer 223 and a heating means, e.g., a heating coil or steam (indicated by arrows), to increase the temperature of the slurry. The temperature of the slurry may range from ambient to boiling. Again, higher temperatures within the range of 160°F to 200°F are preferred, because operating at higher temperatures will improve the overall rate of oil removal. The temperature can be controlled by placing appropariate sensors above the treatment vessels 221, 235, and 243. Additionally, the temperature in reclamation vessel 309 may be monitored in a similar fashion.

The top layer of the slurry in the first treatment vessel 221 contains more oil than in the second and third treatment vessels 235 and 243. The first treatment vessel 221 is stirred vigorously using stirrer 223 for a time sufficient to remove oil and lubricants from the surfaces of the mill scale. The residence time for stirred treatment depends on the type and amount of oil contamination, and is preferably determined empirically.

Following the stirring in treatment vessel 221, the slurry from the first treatment vessel 221 is directed through line 225 to a solid-liquid separator, such as a first hydrocyclone 227. In the present embodiment of the invention, it is considered essential to include several stages of treatment so as to reduce oil contamination of the recovered mill scale to below 0.1 percent by weight. Each stage includes treating the mill scale by contacting it with treatment solution, then perfoπning solid-liquid separation to remove the treatment solution with oil and to leave a less oily mill scale. The less oily mill scale is treated further to produce the desired level of reduced oil contamination.

The first hydrocyclone 227 separates the liquid from the mill scale and preferably removes as much treating solution and oil from the mill scale as possible. The first hydrocyclone 227 has the highest oil output of the three hydrocyclones. The overflow of liquid from hydrocyclone 227 is sent along line 231 to the second sump 269.

The underflow from the first hydrocyclone 227, comprising the partially treated mill scale fines, is directed through line 233 to the second treatment vessel 235. The contents of the second treatment vessel 235 are vigorously stirred by agitator 223, and additionally, the recirculated liquid from the first sump 263 is added via line 236. This addition of recirculated liquid also helps to maintain a liquid balance within treatment vessel 235. The flow of liquid depends on the demand for water in treatment vessel 235. The water is needed to adjust the solid content level. The flow of liquid into treatment vessel 235, from line 236, can be controlled by valve 267 located between lines 265 and 236.

The overflow of liquid from treatment vessel 235 is sent back to the first -24-

The ultra-fine oily mill scale, not retained with the solids in the solid-liquid separator, may settle at the bottom of settling tank 273 and be periodically removed. These solids are recovered for recycle through line 275 back to the fines processing first treatment vessel 221. 5

After settling, the effluent from settling tank 273, which now comprises oil and treating solution, is preferably directed over weir 279 where any oily phase may be skimmed from the surface and directed through line 283 for collection or disposal. 10

After the oil is removed from the effluent by skimming, the remaining effluent may then be directed through line 285 to a means for removing additional oil and generating a recycled solution.

15 III. RECYCLE AND RECIRCULATION SYSTEM

Another aspect of the invention involves the recycling and recirculation stages. The importance of a recycle and recirculation system stems from both environmental and economic factors. A mill scale treatment system which

20 produces wastewater effluent creates a need for additional treatment or disposal facilities to handle contaminated liquids. Furthermore, as discussed in the background section, disposing of the effluent creates a need to replenish the supply of treatment chemicals. Accordingly, this invention preferably includes a recirculation stage for treating the liquids from the treatment stages.

25

Recycled treating solution from reclamation vessel 309 may be directed to treatment rinser 207 along line 211. Preferably, a constant stream of treating solution from reclamation vessel 309 is recycled to treatment vessel 243 and treatment rinser 255 along lines 245 and 257. Preferably, chemicals such as

30 sodium hydroxide and sodium silicate are added to the reclamation vessel 309, either manually or through dispensers 311 and 313 to make the inorganic treating solution comprise a pH-modifier and dispersant in the desired -23- treatment vessel 221 along line 229, while the mill scale are sent on to the second hydrocyclone 227 along line 237. After solid-liquid separation in the second hydrocyclone 227, the overflow liquid (water and oil) is sent back to the first treatment vessel 221 along line 239 and the underflow mill scale are sent for further treatment by line 241 into the third treatment vessel 243.

The plant's recycled treating solution is introduced, via line 245, into the third treatment vessel 243. After stirring using stirrer 223 for a time sufficient to remove oil and lubricants from the surfaces of the mill scale, the mill scale are sent to the third hydrocyclone 249, by line 247. The overflow liquid from the top of the third hydrocyclone 249 travels along line 251 to the first sump 263. The liquid in sump 263 has the lowest concentration of oil and grease throughout the system. The underflow mill scale from hydrocyclone 249, now devoid of oil and grease, are sent through line 253 to a vacuum horizontal belt filter 255 and are sprayed again with the plant's recycled treating solution sent from line 257. The solids which are de-oiled mill scale, are sent along line 259 to be recovered, together with the coarse mill scale, as resource material to be sintered.

The liquid drawn from the vacuum horizontal belt filter 255 is sent to the first sump 263. When valve 267 is open, the liquid flows through line 236 back to the second treatment vessel 235. This is done to maintain a balance within the second treatment vessel. The liquid going into treatment vessel 235 from line 236 should equal the liquid exiting line 237. If valve 267 is closed, the liquid travels to sump 269 through line 265, where it is combined with the filtrate from the coarse processing (line 217) and the overflow (line 231) of hydrocyclone 227. This combined effluent is then sent to settling tank 273 through line 271.

In settling tank 273, the effluent is composed of the treating solution, oil and any other material not separated and recovered with the mill scale. The effluent settles into layers of oil, treating solution and ultra-fine oily mill scale. amounts. The pH is monitored by testing the system's recycle solution within reclamation vessel 309. Dispensers 311 and 313 may include any means for introducing the chemicals separately into the recycled treating solution. The chemicals are preferably dispensed in a controlled fashion, for example, from chemical metering pipes through pipes or tubes.

Referring to the embodiment shown in Figure 2, the recirculation stages comprise directing the effluent from both the fine particle treatment rinser 255 (along line 261) and the overflow of the third hydrocyclone 249 (along line 251), into the first sump 263. If valve 267 is closed, the contents of the first sump 263 are sent through line 265 to the second sump 269. Additionally, the overflow from the first hydrocyclone 227, (travelling along line 231) and the filtrate of the coarse treatment rinser 207 (travelling along line 217) are sent to the second sump 269. From the second sump 269, the effluent is directed to the settling tank 273 through line 271.

In settling tank 273, the effluent, composed mainly of treating solution and oil, as well as any other material not separated and recovered with the mill scale, is permitted to settle into layers, e.g., a layer comprising treating solution and a surface layer of oil.

A small remnant of solid particles, including ultra fine mill scale not retained with the solids in the solids-liquid separator, e.g., the hydrocyclone overflow, may settle at the bottom of settling tank 273 and be periodically removed. These solids may be recycled through line 275 back to the fines processing treatment vessel 221.

After settling, the effluent from settling tank 273, which comprises oil and treating solution, is preferably directed over weir 279 to skimming vessel 281, where any oily phase may be skimmed from the surface and directed through line 283 for collecion or disposal. After removing the oil from the effluent by skimming, the remaim^'ng effluent in skimming vessel 281 (which may still contain trace amounts of oil and fine particles) may then be directed through line 285 to means for removing additional oil which preferably comprises a flocculation vessel 289, which preferably includes a stirrer 223. In flocculation vessel 289, the effluent is preferably contacted with a polymer flocculant 287, preferably comprising a high molecular weight cationic quaternized polymer having a molecular weight in the range of 50,000 to 40,000,000.

An especially preferred flocculant is a quaternized amine cationic polymer having a charge density in the range of 5 to 100%. Of the various polymers tested, the two found to be the most effective were Clearfloc 751 sold by Clearwater, Inc., Pittsburgh, Pennsylvania, and Jayfloc 911 Cationic Flocculant sold by Callaway Chemical Company, Columbus, Georgia.

Jayfloc 911 Cationic Flocculant is an extremely high molecular weight, high charge density cationic emulsion polymer. Clearfloc 751 is a high molecular weight, very high charge density cationic polyacrylamide. The product is supplied as a low viscosity water-in-oil emulsion, and it generally performs in the pH range of 2.5 to 12.5.

Polymer addition is monitored using a streaming current detector, for example a detector by Milton Roy of St. Petersburg, Florida. This on-line streaming current measurement device measures the charge density on the particles. If the charge density is high, more polymer is added. Similarly, if the charge density is low, less polymer is added. The Milton Roy coagulant control center is an on-line electrokinetic charge analyzer with measuring, recording and control functions. It is an on-line instrument which directly measures the result of coagulation addition by instantaneously measuring the electric current is generated.

Preferably, one such monitor would be placed on line 285, before the polymer is added (at arrow 287). A second monitor would be placed on line 307, after the hydrogen peroxide is added, to ensure the polymer has been destroyed. The second monitor would detect the charge. Thus if the particles are more negatively charged, the polymer has been destroyed.

In flocculation vessel 289 the flocculant is preferably mixed with the effluent in a sufficient amount and for a sufficient time using stirrer 223. From flocculation vessel 289, the effluent is directed to the second treatment vessel 293 through line 291. After additional stirring with stirrer 223 to ensure complete mixing, the effluent is sent along line 295 to the clarifier 297. At this stage, the flocculated effluent is separated, with the clarified effluent in the upper layer being directed through line 301, and the aggregated solid and oil fraction being recovered through line 299. The filtered effluent is treated in a final treatment vessel 303 by mixing it with an oxidizing agent, e.g., hydrogen peroxide 305 using stirrer 223. The resulting recycle stream 307 preferably comprises aqueous inorganic treating solution in condition for recirculating back to reclamation vessel 309, where a pH modifier and/or dispersant, such as sodium hydroxide or sodium silicate,may be added as necessary to the recycled solution (through lines 311 and 313 respectively) to obtain the desired composition of treating solution for use in any one of the previously described treating steps.

The invention will be more fully understood from the following examples. The following examples are in accordance with various aspects of the invention for illustrative purposes only, and are not to be considered as limiting in any way the scope of the invention.

EXAMPLES

Example 1

This example relates to size classification of typical mill scale waste materials. Contaminated mill scales from an actual steel mill were screened into various size fractions using a series of classification screens. The particle size distribution obtained is summarized as follows:

Table 1 Particle Size and Oil Distribution Analysis of Mill Scale

0.04

1/4 in. to 8-mesh 86.90 17.9 76.5 0.10

0.086

Fines Fraction

8- to 28-mesh 160.80 33.1

0.188

28- to 48-mesh 79.52 16.4

0.254

48- to 100-mesh 84.17 17.3

0.352

100-mesh 47.16 9.7

0.40

After screening the particles into their various size fractions, the particles were separated into a "coarse" particle fraction (8-mesh or larger) and a "fines" particle fraction (smaller than 8-mesh). Treatment of each fraction is discussed in the following examples.

Example 2 This example relates to treatment of the coarse particle fraction. One hundred grams of the coarse mill scale fraction were transferred to a Buchner funnel fitted with a polyester filter medium. The mill scale particles were spread out on the filter medium. A measured volume of an aqueous solution containing treatment chemicals (A) and (B) was poured into the funnel, mixed with the mill scale using a spatula, and filtered under vacuum. In these examples, treatment chemical (A) is sodium hydroxide and treatment chemical (B) is sodium silicate. The mill scale particles thus treated were removed for a standard oil and grease analysis. The results are summarized in Table 2. It may be desirable to treat the coarse fraction several times to obtain less than 0.1% oil content.

Table 2 Chemical Rinsing of Coarse Fraction

Test No. Chemical Dose (lb/Ton) Oil & Grease (Wt%) __} (B_ Feed Product

1 1.5 1.5 0.09 0.05 2 5 5 0.89 0.32

Example3 This example relates to treatment of the fines particle fraction from

Example 1. Seven tests were conducted, the results of which are summarized in Table 3. Referring to Test No. 1, in the first stage of the treatment step, 100 grams of the fine (smaller than 8 mesh) mill scale fraction were transferred to a beaker to which measured amounts of chemicals (A) and (B) were added. The concentration of chemicals (A) and (B) was 5 lbs./ton, each on a dry weight basis. The particles in the beaker were slurried to a predetermined solid concentration of 70 percent by weight using preheated tap water. The temperature of the slurry thus prepared was maintained at about 200 °F by controlling the temperature of the hot plate on which the beaker sat. The temperature was continuously monitored with a thermocouple while the slurry was being mixed with an electric stirrer.

After mixing the slurry for a predetermined period at constant stirrer speed and temperature, the slurry was poured into the Buchner funnel with the same type of filter medium as that used in the treatment of the coarse particles in Example 2. After gravity drainage of liquid was complete, a measured volume of preheated treating solution containing predetermined amounts of chemicals (A) and (B) was added to the Buchner funnel. The liquid was drained under vacuum.

The solid fraction retained on the filter medium in the Buchner funnel was subjected to further treatment in the funnel as described above, for a total of three stages. The filtrate was retained for further treatment. Six additional tests were run in the same manner, varying the slurry solid content, oil/grease content, temperature, and a number of stages. Test results are summarized in Table 3. It can be seen that the higher chemical concentrations and solution temperature of Test No. 5 in comparison with those of Test Nos. 4 and 6 increased the amount of oil removal.

Example 4

This example relates to treatment of the wastewater filtrate. The filtrate collected from the processing and rinsing stages in Example 3 was allowed to settle under gravity for 30 minutes in a first beaker. Supernatant from this beaker was transferred into another beaker and maintained as a slurry using an electric stirrer. To this second beaker a high molecular weight cationic polymer was introduced at a controlled rate. Two polymers found to be most effective were Clearfloc 751, a quarternized copolymer sold by Clearwater, Inc., Pittsburgh, Pennsylvania and Jayfloc 911, a quarternized methacryloxyethyl trimethyl ammonium methyl sulfate sold by Calloway Chemical Co., a division of Exxon Co., Columbus Georgia. Polymer addition and mixing were continued until a complete clarity of water was achieved. The clarified effluent was decanted and was passed through a sand bed filter. Hydrogen peroxide was added to the effluent for further clarification while the flocculated sludge was set aside for further solid-liquid separation.

Example 5

This example relates to the recirculation of the clarified effluent as treating solution. Prior to reusing the clarified effluent from Example 4 as a recycled treating solution, the concentrations of chemicals (A) and (B) were determined. Any loss in their concentrations was compensated for with the addition of their desired respective amounts. The treating solution was then brought to the desired temperature level, after which it was used in the process as a processing and rinsing solution. The effectiveness of the recycled effluent by its repeated use was tested by going through the processing sequence as discussed above in Examples 2-4. Table 4 shows the effectiveness of a typical treated and recycled effluent on the oil and grease content of oily mill scale particles.

Table 4

Effect of Recycled Treated Effluent on De-oiling of Mill Scales

No. Recy. Chem. Dose Slurry O & G Content CWt%) Temp. No. lb/Ton Solid Cont Feed Prod. (^βF) Stages (A) (B) (Wt%)

Fresh 5 5 70 1.17 0.27

1 5 5 70 1.17 0.27

2 5 5 70 1.17 0.29 3 5 5 70 1.17 0.18

It will be apparent from the foregoing that particular embodiments of the mvention have been described and that modifications may be made therein without departing from the spirit of the invention. Accordingly the scope of the invention should be determined not only by the illustrated embodiments but by the appended claims and their legal equivalents.

Claims

CLAIMSHaving thus described the invention, what it is desired to claim and thereby protect by Letters Patent is:

1. A method of recovering mill scale from oily mill scale waste, which comprises the steps of:

(a) contacting oily mill scale waste with a recycled treating solution consisting essentially of an aqueous dispersion of an inorganic treating agent in an amount sufficient to remove absorbed oil from the mill scale surface;

(b) separating the treated, substantially de-oiled mill scale from an effluent which includes said treating solution and oil removed from said substantially de-oiled mill scale;

(c) removing oil from said effluent to provide a substantially de-oiled inorganic treating solution; and

(d) recovering the de-oiled mill scale; and

(e) recycling said substantially de-oiled inorganic treating solution back to step (a).

2. The method of claim 1, wherein the treating solution is continually recycled without having to add additional water, whereby a closed loop is provided for circulation of the treating solution.

3. The method of claim 1, wherein the inorganic treating agent consists essentially of a pH-modifier and a dispersant.

4. The method of claim 3, wherein said treating agent includes a pH- modifier selected from the group consisting of potassium hydroxide, sodium hydroxide, calcium oxide, calcium hydroxide, sodium carbonate, and potassium carbonate, and a dispersant selected from the group consisting of sodium silicate, potassium silicate, sodium phosphate and potassium phosphate.

5. The method of claim 1, further comprising a first step of classifying the oily mill scale waste into an over-sized coarse fraction of oily coarse mill scale waste and an under-sized fines fraction of oily fine mill scale waste and then performing steps (a), (b) and (d) on the oily coarse mill scale waste.

6. The method of claim 1, further comprising a first step of classifying the oily mill scale waste into an over-sized coarse fraction of oily coarse mill scale waste and an under-sized fines fraction of oily fine mill scale waste and then: performing step (a) on the oily fine mill scale waste in a treatment vessel; transferring the contents of the treatment vessel to a solid-liquid separator; performing step (b) therein; and performing step (d) on the treated, substantially de-oiled fine mill scale.

7. The method of claim 6, wherein the solid-liquid separation is a hydrocyclone, and step (d) is performed on a vacuum horizontal belt filter.

8. A method of solid-liquid separation, wherein the oily fine mill scale waste is contacted with recycled treating solution in multiple treatment vessels and separated from the effluent utilizing multiple means of separation comprising the steps: (a) feeding the oily fine mill scale waste and treating solution from a treatment vessel to a means for separation;

(b) feeding the oily fine mill scale waste from the means of separation of (a) into a subsequent treatment vessel with recycled treating solution; and

(c) feeding the oily fine mill scale waste and treating solution from the treatment vessel of (b) into a subsequent means for separation.

9. The method of claim 8, wherein the means of separation comprises a hydrocyclone comprising an overflow of treating solution and oil and an underflow of substantially de-oiled fine mill scale waste.

10. The method of claim 9 wherein the overflow of treating solution and oil is recirculated to contact oily fine mill scale waste that has a higher content of oil than the oily fine mill scale waste just contacted by the overflow.

11. The method of claim 8, wherein the means of separation comprises a vacuum horizontal belt filter comprising an overflow of substantially de- oiled fine mill scale waste and a filtrate of treating solution and oil.

12. The method of claim 1, additionally comprising a first step of classifying the mill scale into a fraction comprising mill scale contaminated with light oil contaminants and a fraction mill scale contaminated with heavy oil contaminants.

13. A method of treating mill scale waste having undesired oil on its surface, wherein the method comprises: (a) contacting oily mill scale waste with a treating solution which consists essentially of an aqueous dispersion of a treating agent, wherein the treating agent is a substantially inorganic alkali pH- modifier and dispersant, in an amount sufficient to remove oil from the surfaces of the mill scale waste and to provide substantially de-oiled mill scale;

(b) separating the treated, substantially de-oiled mill scale from an effluent which includes said treating solution and oil removed from said substantially de-oiled mill scale;

(c) recovering the substantially de-oiled mill scale; and

(d) separating oil from said effluent to provide a substantially de- oiled effluent.

14. The method of claim 13, wherein the pH of a slurry produced by the contacting step (a), is about 7 to about 12.5, and wherein the treating agent modifies the surfaces of the mill scale waste to facilitate the removal of the oil therefrom.

15. The method of claim 13, wherein said treating agent includes a pH- modifier selected from the group consisting of potassium hydroxide, sodium hydroxide, calcium oxide, calcium hydroxide, sodium carbonate, and potassium carbonate, and a dispersant selected from the group consisting of sodium silicate, potassium silicate, sodium phosphate and potassium phosphate.

16. The method of claim 13, wherein said contacting step is carried out at a temperature of from about 40°F to boiling.

17. The method of claim 16, wherein the temperature is monitored between the range of 160°F and 200°F at contacting step (a).

18. The method of claim 13, wherein the pH is monitored at separating step (d).

19. The method of claim 13, said contacting step (a) comprises the substeps of:

(i) mixing said inorganic treating solution and oily fine mill scale waste with treating solution and maintaining said oily fine mill scale in suspension by agitation; and

(ii) mixing the inorganic treating solution and oily fine mill scale waste within an externally heated treatment vessel.

20. The method of of claim 13, wherein the means for step (b) comprises a vacuum horizontal belt filter, wherein the substantially de-oiled coarse mill scale is the overflow, ana the effluent, comprising the treating solution and removed oil is the filtrate.

21. The method of claim 13, further comprising a first step of classifying the oily mill scale waste into oily coarse mill scale waste and of oily fine mill scale waste, wherein the means of step (b) for oily fine mill scale waste comprises a hydrocyclone, wherein the substantially de-oiled fine mill scale is the underflow, and the effluent, comprising the treating solution and removed oil is the overflow.

22. The method of claim 13, wherein the separating step (d) comprises the substeps of:

(i) adding a polymer flocculant to the effluent to aggregate oil and soϋds, and

(ii) removing the aggregated oil and solid floe

to provide a substantially de-oiled inorganic treating solution.

23. The method of claim 13, wherein the separating step (d) further comprises the substeps of:

(i) directing the effluent into a settling tank for a time sufficient to provide an oily surface phase, a middle effluent phase and a bottom solids phase;

(ϋ) skimming the oily surface phase from the liquid in the settling tank and recovering the skimmed oil phase to provide a substantially de-oiled effluent comprising essentially the treating solution; and

(iii) treating the substantially de-oiled effluent with a flocculant to provide a clarified liquid floe.

24. The method of claim 23, wherein the flocculant consists of a cationic polymer.

25. The method of claim 24, wherein the cationic polymer has a molecular weight of about 50,000 to about 40,000,000 daltons.

26. The method of claim 24, wherein the cationic polymer is a quarternized compound having a charge density in the range of between about 5 and 100 percent.

27. The method of claim 23, wherein the charge density on particles within the effluent is measured in step (i) to determine the amount of flocculant needed.

28. The method of claim 27, wherein the charge density on the particles within the effluent is monitored by a streaming current detector.

29. The method of claim 23, wherein the separation step (d) of claim 13 further comprises the substeps of:

(iv) feeding the effluent into a settling tank, and

(v) allowing the floe to settle.

30. The method of claim 29 wherein the effluent is passed through a paniculate filter.

31. The method of claim 23, further comprises treating the clarified liquid with oxidizing agent.

32. The method of claim 31, wherein said oxidizing agent comprises hydrogen peroxide, ozone, permanganate, hypobromous acid, chlorine dioxide, hypochlorous acid, hypoiodous acid, or a member selected from the group consisting of chlorine, bromine, or iodine.

33. A method of recovering mill scale from oily mill scale waste, said method comprising:

(a) classifying the oily mill scale waste into an over-sized coarse fraction of oily coarse mill scale waste and an under-sized fines fraction of oily fine mill scale waste;

(b) treating the coarse particle component by rinsing the oily coarse mill scale waste with an aqueous treating solution comprising a pH-modifier and dispersant in an amount sufficient to remove oil from the surfaces of the oily coarse mill scale waste and to provide substantially de-oiled coarse mill scale;

(c) recovering the substantially de-oiled coarse mill scale;

(d) treating the oily fine mill scale waste by mixing the oily fine mill scale waste in a treatment vessel with a heated slurry which consists essentially of an aqueous treating agent consisting essentially of a pH-modifier and dispersant, removing oil from the surfaces of the oily fine mill scale waste, and providing substantially de-oiled fine mill scale; and

(e) removing the substantially de-oiled fine mill scale from the treatment vessel and contacting the substantially de-oiled fine mill scale with a treating solution comprising a pH-modifier and dispersant in an amount sufficient to remove oil from the surfaces of the substantially de-oiled fine mill scale and thereafter recovering the treated fine mill scale.

34. The method recited in claim 33, wherein said pH-modifier and dispersant are present in an amount sufficient to increase the pH of the slurry to from about 7 to about 12.5.

35. The method recited in claim 33, comprising separating the substantially de-oiled fine mill scale from the liquid treating solution, then treating the separated substantially de-oiled fine mill scale with treating solution, said treating and separating being done a sufficient number of times to reduce the oil content of the fine mill scale to about 0.1 percent by weight.

36. The method recited in claim 33, wherein the recycled treating solution is substantially inorganic.

37. An apparatus for recovering de-oiled mill scale, which comprises:

(a) means for segregating oily mill scale waste by size;

(b) means for contacting each segregated mill scale component with a treating solution;

(c) means for separating the mill scale from the effluent, which includes said oil removed from the substantially de-oiled mill scale;

(d) means for removing oil from said effluent to provide a substantially de-oiled treating solution;

(e) means for recovering a de-oiled mill-scale; and

(f) means for recycling the treating solution.

38. The apparatus of claim 37, wherein the means (a) comprises a classifying screen.

39. The apparatus of claim 37, wherein the means (b) comprises a spraying device for dispensing a stream of treating solution.

40. The apparatus of claim 37, wherein the means (b) comprises a treatment vessel.

41. The apparatus of claim 40, further comprises a means for heating the treatment solution to a temperature between about 40°F and boiling.

42. The apparatus of claim 40, further comprising means for agitating the contents.

43. The apparatus of claim 37, wherein means (c) comprises a series of solid-liquid separators.

44. The apparatus of claim 43, wherein the series of solid-liquid separators comprises a plurality of hydrocyclones followed by a vacuum horizontal belt filter.

45. The apparatus of claim 43, wherein the solid-liquid separators are vacuum horizontal belt filters.

46. The apparatus of claim 37, wherein the contacting means (b) comprises a plurality of treatment vessels for contacting the oily fine mill scale and wherein the separating means (c) comprises a plurality of solid-liquid separators for separating the oily fine mill scale.

47. The apparatus of claim 37, in which each treatment vessel is connected to a corresponding solid-liquid separator in a cascading series.

48. The apparatus of claim 37, wherein means (d) comprises:

i. a settling tank having a weir over which the effluent passes;

ii. an extractor vessel having a means for adding an oxidizing agent or solvent;

iii. a skimming vessel having a means for removing oil from the effluent; iv. a flocculation vessel and means for adding flocculant to the effluent;

v. a secondary settling tank;

vi. a particulate filter, and;

vii. means for adding water, pH-modifier or dispersant to the treating solution which is to be recycled in the system.

49. The apparatus of claim 37, wherein means (d) comprises:

i. a settling tank having a weir over which the effluent passes;

ii. a flocculation vessel and means for adding flocculant to the effluent;

ϋi. a treatment vessel;

iv. a clarifier having means for recovering aggregated solid and oil;

v. an extractor vessel having means for adding an oxidizing agent or solvent;

iv. means for adding water, pH-modifier or dispersant to the treating solution which is to be recycled in the system.

50. The apparatus of claim 37, wherein the treating solution is recycled in a closed loop.