WO2006103713A2

WO2006103713A2 - Use of chromiferous tanning waste for obtaining a chromium-oxide based product, mainly directed to the ceramic industry

Info

Publication number: WO2006103713A2
Application number: PCT/IT2006/000204
Authority: WO
Inventors: Leonardo Regoli; Luca Lemmi
Original assignee: Leonardo Regoli; Luca Lemmi
Priority date: 2005-04-01
Filing date: 2006-03-30
Publication date: 2006-10-05
Also published as: ITPI20050035A1; WO2006103713A3

Abstract

The invention concerns the use of tanning bath waste to obtain a chromium- oxide based product for use in other industrial fields and in particular in the ceramics field, refractory materials field, and for use in pigments for glass and colorants for metallic foil. For this purpose, it is possible to use a process wherein the precipitation of a solid is induced using a reactive precipitant, the process further comprising the steps of solid/liquid separation, washing, a successive solid/liquid separation, and a drying action. If necessary, the solid can also be heated to separate residual organic impurities.

Description

TITLE

USE OF CHROMIFEROUS TAKNING WASTE FOR OBTAINING A CHROMIUM-OXIDE BASED PRODUCT, MAINLY DIRECTED TO THE

CERAMIC INDUSTRY DESCRIPTION

Field of the invention

The present invention relates to a new system for obtaining a chromium-oxide based product for use in various industrial sectors, especially but not exclusively the ceramics industry.

Background of the invention

Chromium-oxide (Cr₂O₃) based products are widely used for the synthesis of dyes and pigments in the ceramics industry, but also and more in general, in the glass industry, in the industry of colorant substances for metal foil and in the refractory materials industry, as well known by any expert in these sectors .

The only market source of chromium usable in the chemical industry is chromite, a mineral with the chemical formula FeCr₂O₄. Various methods for producing chromium oxide are known, the most common being those that pass through the synthesis of sodium chromate and dichromate

(in particular by means of sulphuric acid, carbon dioxide, electrolysis) . Then sodium chromate and dichromate, as a result of the reaction with ammonium sulphate, produce ammonium dichromate, that is finally used to produce chromium oxide via a decomposition in the presence of sodium sulphate. Direct synthesis of chromate with elementary sulphur is also possible, by processing in a surge tank, known as Slonim processing. All these methods are relatively lengthy, expensive and pose problems concerning the obtainment of raw material supplies, not to mention environmental aspects . Summary of the invention

According to the present invention, a process has now been developed for obtaining a chromium-oxide based solid product with optimal characteristics for the synthesis of ceramic dyes and pigments, at extremely reduced prices, and above all with extremely advantageous results from the standpoint of ecology and impact on the environment . Essentially, as defined in the first of the appended claims, in the process according to the present invention a chromium-oxide based product is obtained from chromiferous tanning bath wastes. Other features of the invention are described in the dependent claims . The advantages resulting from the present invention consist basically in the fact that it is possible to obtain a chromium-oxide based product in solid state, in a simple and economic way, recovering it from tanning waste, without any kind of negative effects, and on the contrary, with the advantage that the above mentioned waste recovery reduces the problems of the disposal thereof in the environment. In fact, from an environmental viewpoint, thanks to the process at issue the waste water to be eliminated will have a chromium content which is lower than the minimum limit normally imposed by the Authorities for discharge in sewer systems.

This is a very important achievement, since the problem of recovering industrial waste material is becoming more and more felt, above all in those areas where, because of concentrated industrial complexes, environmental contamination must be reduced to a minimum. More specifically, given the negative environmental impact resulting from the discharge of tanning wastes containing chromium into the water system, companies that produce chrome-tanned leather are forced to dedicate considerable time and effort in an attempt to eliminate these substances safely, or to recover and reuse them.

On the other hand, compared with a possible recycling in a tanning plant, where the recovered product has a lower quality than that of virgin chrome, mainly because of the high content of organic residue and the difficulty in resolubilizing it in water without using large amounts of acid, the invention provides for the possibility of supplying users of chromium-oxide for ceramic dyes and pigments, with a product having outstanding characteristics. In fact, the content of organic matter present in the product obtained does not affect the good result of the inorganic syntheses of ceramic dyes and pigments, because these syntheses occur at temperatures far higher than 600⁰C, temperatures at which organic matter degrades to volatile compounds (mainly water and carbon oxides) ; in other words, to substances that, in practice, remain inert in the synthesis. In many cases the syntheses of inorganic pigments are performed in reducing environments to the establishment of which the organic compounds already present in the product may also contribute. In cases where the organic content is not necessary, it is possible to remove almost all of it by means of a simple heat treatment consisting in keeping the product at a temperature under 500⁰C for a few seconds. Moreover, as a result of originating from the precipitation and separation of a powdery solid, the product obtained with the said method has a granulometry sufficiently fine to permit the product to attain a high reactivity level in the characteristic solid-solid reactions that occur during ceramic pigment and dyes syntheses. When the granulometric distribution of the solid is not deemed sufficiently homogeneous for the specific purposes, it will still possible to add a final dry grinding step after drying or possible heat treatment.

Another essential aspect is represented by the fact that the metal salt content that may interfere with pigment preparation can be diminished by simple washing with water based solutions of binding and complexant agents used under appropriate reaction conditions. Therefore, it is possible to choose the most suitable purification method according to the final use. The considerations above are applicable also to possible uses of the product in the glass and refractory material industries, and in colorants for metal foils.

The process uses chemical reactives not considered toxic or harmful to the environment, and no hexavalent chromium is produced at any stage. From an economical viewpoint this permits the supply of a low cost raw material for the ceramics industry, or for other industrial fields, such as those previously mentioned, with the full elimination of a waste product from the tanning productive cycle.

Furthermore, the whole process, with the exception of the final heating step (that is in any case optional) , is performed in cold conditions, thus providing considerable energy saving during the processing stage. In any case, even the possible heating treatment, given the temperatures involved and above all the short time the products are exposed to said temperatures, cannot have a significant influence on the economical aspects of the process. The chemical products used in the process are easily and cheaply available on the market . The industrial plant for the preparation of the final product is composed of simple, easily accessible modules.

Brief description of the appended drawing The characteristics and advantages of the method and plant for obtaining a chromium-oxide based product according to the present invention will be apparent from the following description of embodiments thereof, provided as an example but by no means limiting, with reference to the appended drawing in which:

- figure 1 shows a layout scheme of a plant for carrying out a method according to the invention. Description of a preferred embodiment

As previously mentioned, according to the invention, the chromium contained in the wastes derived from chromiferous tanning baths used in the tanning industry is recovered and made suitable as a raw material to be used mainly for the synthesis of chromium-oxide based ceramic pigments and dyes. This product can also be used as a raw material in glass industry, the industry of colorants for metal foils and in the refractory materials industry.

Because of its intrinsic nature, the industrial waste (staring point of the process) has a composition that is not constant. Consequently, the specific operating conditions for the recovery process will be subject to slight variations, due to the nature of the waste processed, or the type of tanning process in question. Then, the reaction parameters (pH and concentrations) will be indicated in the form of ranges, and not with a single exact value, in order to take into consideration the nature of the treated waste.

Obviously, assuming that the invention is used in a whole industrial district with the treatment of all the waste, the final reaction conditions in the industrial application of the invention will reasonably not be very distant from a given average value. Said average value will be subject to slight changes over a period of time, due to the different leather articles treated in the tanning process that originates the waste, or due to changes in the productive processes applied in the reference district. Normally, in tanning wastes rather a complex system can be found, composed mainly of trivalent chromium hydroxides and sulphates, coordinated to organic ^■ molecules (proteins from leathers, sugars and agents used in tanning processes as masking agents, and water) . The average chromium content is about 40 g/1 in equivalent Cr₂O₃.

In brief, the waste from chromium tanning or pre- tanning processing in the tanning industry is treated initially with a precipitant reactive, more precisely a basifying solution for bringing the pH to a value between 6.8 and 7.8, under agitation. Once the best possible pH level has been reached, the system is left to rest for the short time necessary for the precipitated solid to separate. The liquid fraction, separated from the solid component, presents chemical characteristics that are not particularly critical in order to be discharged into the sewer system. After washing with an aqueous solution, and the resulting further solid/liquid separation, the solid is dried and in this way is made available for use as a raw material, e.g. and typically for the synthesis of dyes or pigments in the ceramics industry. The nature of the basifying agent can vary according to the reaction conditions and/or the economy of the process. In any case the choice should be directed to minimizing the metallic impurities present in the final product. The concentration of the basifying agent can be appropriately chosen according to the volumes of the liquid fraction that is required: the greater the concentration, the lesser the liquid volume obtained although with increasing difficulty in the handling of more concentrated base solutions.

The reaching of any pH value within the indicated range, or even slightly outside the range, will not preclude the obtainment of the final product: however, the accuracy in obtaining a pH value as close as possible to the intermediate value of the indicated range will permit the recovery of practically all the chromium contained in the tanning wastes, although this requires greater carefulness in performing the process.

The separation of the solid from the liquid can be performed using any method suitable for the process : decanting, filtering, centrifuge, filter pressing, etc. The more effective is the technique, the greater is the amount of recovered product, and therefore the lower is chromium content in the liquid fraction to be discharged. The washing with suitable solutions of chelating or complexant agents used in the appropriate reaction conditions (for example ambi- or multident complexant reactives, EDTA solutions buffered at suitable pH) will remove most of the metal salts present (such as sodium, calcium, potassium, iron, aluminum, manganese, magnesium and zinc sulphates or hydroxides, etc.) or any organic matter. The choice of the conditions under which washing should be performed depends on the subsequent process that the recovered chromium must undergo, by establishing an economic balance between the advantages of the purification and the cost of the process. On the other hand, even with the use of simple water there will be a reduction, though a very slight one, in the salts inside the solid produced.

Further final heating treatment may be advisable; however, it must be taken into consideration that the presence of organic matter has a very slight influence on the reactions involved in ceramic treatments. In any case, the operating temperatures will depend on the waste under processing, and will be less than 500⁰C, and applied only for a short time.

The same kind of considerations should be applied to possible dry grinding. This treatment step should be envisaged only when the granulometric distribution requires a determined homogeneity, or when the average size of the particles needs to be especially fine for particular applications.

Entering into further detail, the following is a treatment formula exemplifying the process, with the relevant resulting data. Using an aqueous solution of sodium hydroxide at 30% as a basifying agent, in the amount of 23 ml for each liter of tanning bath waste (in order to obtain the complete liquid/solid separation) an amount of about 35 grams of dry solid product is obtained for each liter of tanning bath waste. The concentration of the residual chromium in the waste is approximately 0.2 mg/liter.

Following the washing of the solid product with an aqueous solution of EDTA 0,1M (buffered at pH 8.7; 50 ml of solution per gram of dry solid) , an aqueous washing solution is obtained with a chromium concentration equal to 1.1 mg/liter, and an amount of dry solid product is obtained equal to 57% in relation to the starting amount subjected to the washing process.

The chemical characteristics of the product obtained are as follows:

Cr : 33% Na : 0.9% Al : 0.5% Fe : 0.2% Ca : 0.2%

Organic (weight loss at 600⁰C) : 48%. The composition of the product obtained expressed in oxides is as follows: Cr₂O₃ : 53.3% ZnO : 0.01% Al₂O₃ : 1.0% CaO : 0.19% Na₂O : 0.40% Fe₂O₃ : 0.20% Organic : 45%.

With the final heat treatment it is possible to change the product characteristics as follows: by heating gradually up to a temperature of 200⁰C, about 7% of the total mass is lost through degradation of organic matter; by heating gradually up to a temperature of 360⁰C a further 18% of the total mass is lost through degradation of organic matter; by heating gradually up to a temperature of 490⁰C a further 10% of the total mass is lost through degradation of organic matter; by heating gradually up to a temperature of 600⁰C further degradation occurs until all the organic matter present is eliminated.

Again as an example, the following data are obtained after a washing treatment using water only (50 ml of water per gram of dry solid) . The aqueous washing solution will have a chromium concentration of 0.9 mg/liter, producing an amount of dry solid product equal to 60% of the initial amount subjected to washing.

The chemical characteristics of the product obtained are as follows:

Cr : 29.3% Na : 0.4% Al : 0.5% Fe : 0.2% Ca : 2%

Organic (weight loss at 600⁰C) : 44% The composition of the product obtained expressed in oxides is as follows:

Cr₂O₃ 50.4%

ZnO 0.02%

Al₂O₃ 1.10% CaO 1.56% Na₂O 0.65% Fe₂O₃ 0.27%

Organic 46%

Sodium hydroxide represents the cheapest basifying agent and also the easiest to use in these specific conditions. However, the reactive precipitants can also generally include other alkaline hydroxides or earth alkalines and magnesium oxide, adapting the reaction conditions to the reactive used.

With reference to figure 1, a plant for carrying out the invention can be made up according to a very simple layout, that is also quite versatile depending on the amount of waste to be processed and on the type of product that is to be obtained. Essentially, there is required a storage tank 2 for the waste 1 coming from the tanning plant, that can be discharged into a basin 3 equipped with a feeding device 4 for the addition of the reactive precipitant solution. The basin 3 is provided with an agitating system (not shown) and an internal pH control system 5 for assisting and controlling the formation of the solid chromium hydroxide. The basin 3 can be emptied using a pump if the product (pumpable solid) is passed through a filter, filter press or centrifuge 8. Alternatively, a liquid pump can be sufficient to extract the surplus liquid in cases where the solid washing action is performed in the same precipitation basin 3.

The subsequent washing step can be carried out in a second basin 6 according to installation choices, (depending on the amounts to be processed and the efficiency required in the installation) . This washing step can also be carried out in the first basin 3, thereby, in practice, the whole group included within the square in dotted lines, indicated at "A", can be replaced by a single basin with appropriate feeding and control means . The second basin 6, also equipped with a mechanical agitating device, is fed with washing liquid stored in an adjacent tank 7. To obtain the washed product with elimination of the contaminating salts, the second basin 6 must be equipped with a pump for the successive transfer of the pumpable solid to the aforesaid filter, filter press or centrifuge 8. The solid obtained on exit from the filtration unit 8 already possesses the characteristics necessary to be employed as a raw material for the applications envisaged above. If necessary, in order to optimize the transport costs of the product, a drying or desiccation unit 9 can be positioned directly at the exit of the plant to completely eliminate the water contained in the solid.

The drying unit 9 can be equipped with a forced ventilation system or an oven able to attain temperatures up to 100⁰C. As stated previously, in order to eliminate almost all the organic impurities present in the solid, a further module can be added to heat the material to at least 600° C (this system can be realized simultaneously and in the same device envisaged for the drying action, by adopting a more powerful heating system) . The further addition of a unit for dry-grinding the product (not shown) will provide a material with a more homogeneous granulometry . The filtration can be performed by a single unit 8 for both the solid/liquid separation steps (the first after precipitation, and the second after the washing action) , or two separate filter sub-units can be installed.

A number of variants and/or modifications can be brought to the method and plant for obtaining a chromium- oxide based product according to the present invention without departing from the protective scope of the invention itself.

Claims

1. A method for obtaining a chromium-oxide based product to be used as a raw material in the synthesis of ceramic pigments or dyes, in the glass industry, in the one of metal foil colorants and in the refractory materials industry, characterized in that it comprises a treatment of a chromiferous bath waste from the tanning industry.

2. The method according to claim 1, comprising the following steps : - treatment of the said tanning bath waste with a reactive precipitant for precipitating the chrome; separation of the liquid from the solid thus obtained; washing of the solid with an aqueous solution; - separation of the solid from the liquid; and drying of the solid.

3. The method according to claim 2, further comprising the step of separating the organic impurities through the heating of the solid.

4. The method according to claim 2 or 3 , wherein said reactive precipitant is a solution of NaOH at 30% added under agitation until a pH between 6.8 and 7.8 is obtained.

5. The method according to claim 4, wherein said precipitation occurs with 2-3% of volume of NaOH solution at 30% until a pH of about 7 is obtained.

6. The method according to claim 2, wherein said reactive precipitant comprises a reactive chosen from alkaline hydroxides or earth alkalines and magnesium oxide, the reaction conditions being adapted depending on the reactive precipitant employed.

7. The method according to any one of the claims from 2 to 6, wherein said solid-liquid separation step is carried out through filtration, centrifugation or filter pressing.

8. The method according to any of the claims from 2 to 7 , wherein said solid washing step is carried out by using an aqueous solution of a complexant agent.

9. The method according to claim 8, wherein said solid washing step is carried out by using an aqueous solution of EDTA 0. IM until a pH of about 8.7 is obtained.

10. The method according to claim 8, wherein said complexant agent comprises ambi- or multident complexant reactive agents.

11. The method according to any of the claims from 2 to 10, wherein said solid drying step is carried out through simple ventilation of the solid or via a modest heating action.

12. The method according to any of the claims from 2 to 10, wherein said solid drying step is carried out through heating to a temperature lower than 500⁰C.

13. The method according to claim 12, wherein said heating involves at least an initial step below a temperature of about 200⁰C, with an approximate organic matter loss of 7%, a possible second step reaching a temperature up to 360⁰C, with a further organic loss of 18%, and a possible third step up to a temperature of about 490⁰C, with a further organic loss of 10%.

14. The method according to any of the claims from 3 to 13 , wherein said separation of the organic impurities occurs by heating the solid up to a temperature of 600⁰C.

15. The method according to any of the claims from 2 to 14, further comprising the step of dry grinding to obtain a homogeneous granulometric distribution of the solid.

16. A product obtained using the method according to any of the previous claims.

17. A plant for carrying out the method according to any of the claims from 1 to 15, characterized in that it comprises at least, in sequence, the following: - a waste storage tank; at least one basin for the precipitation and the washing of the precipitated solid; reagent feeder units towards said at least one precipitation and washing basin; - a filtering unit.

18. The plant according to claim 17, comprising separate basins respectively for precipitation and washing, arranged in sequence .

19. The plant according to claim 17 or 18, further comprising a heat treatment unit to heat the solid, arranged downstream of said filtering unit .

20. The plant according to any of the claims from 17 to

19, comprising a grinding unit for obtaining a homogeneous granulometry of the solid, arranged downstream of said filtering unit or said heat treatment unit .

21. The plant according to any of the claims from 17 to

20, wherein said solid precipitation basin is equipped with a mechanical agitating device and pH continuous control device .

22. The plant according to any of the claims from 17 to

21, wherein said solid washing basin is equipped with a mechanical agitation device.

23. The plant according to any of the claims from 19 to

22, wherein said heat treatment unit comprises an oven with a heating capacity up to at least 600⁰C.