WO2016209093A1

WO2016209093A1 - Method for chromium separation from tanning waste

Info

Publication number: WO2016209093A1
Application number: PCT/PL2015/000099
Authority: WO
Inventors: Piotr Grzybowski
Original assignee: 23 Rs Coras Sp ZOO
Current assignee: 23 Rs Coras Sp ZOO
Priority date: 2015-06-23
Filing date: 2015-06-29
Publication date: 2016-12-29
Anticipated expiration: 2017-12-23
Also published as: PL412821A1

Abstract

The present invention provides a method of chromium separation from sewage, including tanning sewage, as a result of a double exchange reaction yielding insoluble salts of chromium and organic acids which are very easy to filtrate and enable chromium separation from waste using this method. Collected precipitates of chromium salts of some organic acids are then regenerated.

Description

Method for chromium separation from tanning waste

The subject of the invention relates to a method for Cr³⁺ separation from sewage, in particular tanning sewage.

The chromium Cr³⁺ salts are commonly used in leather tanning. The tanning process involves cross-linking of collagen proteins comprising the main ingredients of animal skin using chromium ions, leading to the formation of extremely strong bonds. The leather becomes resistant to moisture as well as to action of bacteria and fungi. Salts of metals other than chromium are also used, but leather tanned using the chromium techniques are of very high quality and, nowadays, it is the dominating tanning technology. While soaking leather in tanning worts, as well as during leather cutting or thinning, sewage and wastes containing chromium are generated. Chromium is a heavy metal causing intoxication if consumed in its ionic form, and even deaths in higher amounts. Chromium ions penetrating into the environment become gradually oxidised to their Cr⁶⁺ form, in which they additionally show carcinogenic properties. Chrome elimination from tanning waste and sewage is thus very important both from the point of view of environment and from the point of view of health. In addition, chromium is expensive and its recovery from waste has an additional, economic aspect. Returning chromium separated from wastes back to the production cycle seems to be the most proper approach. Theoretically, separation of chromium seems to be very simple, since when warm, chromium in its Cr³⁺ form forms its completely insoluble hydroxide Cr(OH)₃ that can be separated from the waste stream. Unfortunately, such hydroxide obtained from aqueous solutions forms strongly hydrated flocks forming compressible precipitates. These flocks lead to filter clogging and block fluid passage through filters. Thus use of typical cake filtration techniques for separations seems to be nearly impossible. PL171 170 discloses a method of chromium(III) regeneration from used tanning solutions including subjecting chromium(III) compounds contained in used tanning solution to adsorption on a waste protein bed, preferably on a bed containing cuttings of chromium leathers, activated by treatment with alkaline substances, preferably lime, sodium carbonate of magnesium oxide, with the activation of protein bed and chromium compounds adsorption is performed at least twice, and chromium(III) is recovered as salts by treating the bed with acid, preferably with sulphuric acid.

Polish patent application P.299475 discloses a binding technology of post- neutralisation, high chromium content galvanic wastes, wherein ingredients of such wastes, hazardous to the environment, are permanently bound within ceramic products. In result such waste can be utilised in construction ceramics. The utilisation method includes decreasing of chromium content achieved by appropriate mixing of high chromium content precipitates with wastes having different characteristics and by permanent chromium binding within a ceramic product by performing the sintering in reducing atmosphere and in that the chromium, calcium and alkali contents are lowered thanks to the use of appropriate raw materials, mixing of post-neutralisation wastes having various compositions in order to achieve proper balance between the metals, and finally, by optional addition of lead-containing compounds leading to chromate binding in the form of extremely insoluble compounds, followed by waste biding within a ceramic product.

Polish patent application P.307908 discloses a method wherein radioactive chromate waste is reduced with sugar, followed by neutralisation of sulphuric acid using sodium lye. Thus obtained solution, continuously mixed, is introduced into cement and the entire material hardens in a metal container.

US4340571 Al discloses a method of chromium recovery after tanning procedure. Chromium is recovered from the ash obtained after combustion of industrial wastes, in which chromium is substantially present in its hexavalent form only. The disclosed method includes stages of ash mixing with sulphuric acid and water in order to obtain a suspension of ash in the acid; addition of a reducing agent to the suspension in order to transform the hexavalent chromium into trivalent chromium; filtration of the suspension and of the mixture of reducing agent in order to obtain the extraction solution of trivalent chromium and residues; washing the residues with water in order to remove residues of the extraction solution and to obtain an aqueous solution containing the recovered chromium.

US4483829 discloses a method of chromium recovery from tanning products, in which waste products from leather tanning using chromium are washed and then ground down in water, followed by hydrolysis at pH of at least 10.5, at the boiling point or at a temperature close to the boiling point. The obtained suspension is filtered and a polypeptyde solution is obtained together with a cake containing solids, substantially containing chromium hydroxide. The solution is neutralised, water is evaporated, the solution is cooled and filtrated in order to obtain substantially chromium- free protein material which can be used as feed for animals or as a plant fertiliser; solids are washed, filtrated and acidified obtaining chromium sulphate which may be re-used at the tanning factory.

The disclosure in US4569830 provides a method of trivalent chromium salts recovery from water obtained during leather tanning using so called "chromium" system, including stages in which all trivalent chromium salts in the form of chromium hydroxide are flocculated by treating them with an aqueous solution containing 15 to 30% (weight) of a salt obtained from a weak acid and a strong base, with pH of 6.9 to 7.2 at a temperature of 22°C to 26°C, followed by separation of thus obtained chromium hydrate from the aqueous dispersion by concentrating the dispersion in a device designed for purification and flocculation, followed by concentrate filtration until a precipitate containing ca. 20 %wt. of dry mass is obtained. Then, the precipitate is dissolved in sulphuric acid, at pH of 1.8 to 2 and at temperature over 50°C, preferably at 70°C. The invention also presents an installation enabling implementation of said method.

Surprisingly, it was found that some insoluble salts of organic acids and trivalent chromium form precipitates that are crystalline and easily filtrated. Thus, separation of chromium from tanning waste became possible and very easily achievable in practice.

The subject of the invention is a method of Cr³⁺ isolation from sewage, including tanning sewage, by precipitating thereof as an insoluble salt with selected organic acids. Preferably, organic acids insoluble in water are used, more preferably organic acids are used, of which sodium, potassium and ammonium salts are soluble in water, said acids are selected from the group including terephthalic acid, salicylic acid, benzoic acid, stearic acid, palmitic acid, sebacic acid, azelaic acid and mono- or polycarboxylic aliphatic and/or aromatic acids.

A preferred embodiment of the method includes filtration, sedimentation and/or centrifugation of insoluble, crystalline chromium salts of organic acids.

Preferably, chromium from insoluble, crystalline chromium salts of organic acids is recovered by treating chromium salts with mineral acid solutions, preferably with sulphuric acid.

The process includes pH adjustment of wastes, aimed at maintaining a pH value of 7 or lower, followed by introduction of soluble salts of appropriate organic acids to such wastes. A double exchange reaction yields insoluble salts of chromium and organic acids which are very easy to filtrate and enable chromium separation from waste using this method. Collected precipitates of chromium salts of some organic acids are then regenerated. The regeneration includes treatment with aqueous solution of a mineral acid, e.g. sulphuric acid. The reaction enables recovery of the organic acid in the form insoluble in water, and of a soluble chromium salt, e.g. chromium sulphate. The chromium salt in aqueous solution can be easily isolated from the precipitate of the organic acid, and its solution after concentration may be used for separation of pure chromium salt.

Example 1

480 kg of„wet blue" tanning waste was placed in a 4 m tank reactor equipped with a mixer, a heating mantle, and 2 m of water containing 40 kg NaOH was added. The mixture was heated to a temperature close to the boiling point and maintained under these conditions, mildly stirring, for 4 hours. During this time, hydrolysis of collagen proteins comprising the main constituent of leather occurred and chromium released from their structures formed chromium hydroxide, according to the following equation:

Cr³⁺ +3 OH^" = Cr(OH)₃|

This precipitate floated in the post-reaction mixture comprising an aqueous solution of amino acids and oligomers from collagen decomposition with the mixer running and precipitated on the bottom of the reactor when the mixer was switched off. Then, 250 kg of 20% H₂S0₄ solution was added to the reactor, resulting in dissolution of the chromium hydroxide precipitate. Thus obtained solution was filtered through a cloth filter, removing all insoluble contaminations present in the processed waste, such as mineral particles as e.g. sand. 850 kg 20% of aqueous solution of disodium terephtalate was added to the clear, post-reaction solution. An exchange reaction yielded an insoluble salt of terephthalic acid and chromium(III). Crystals of this salt were easily filtrated in a filter-press, and the filtration cake obtained was washed with hot water, removing residues of protein hydrolysate solution. The precipitate of the salt of terephthalic acid and chromium was then washed using 430 dm^J of 20% aqueous solution of sulphuric acid which washed chromium from its salt with terephthalic acid, leaving a precipitate of insoluble terephthalic acid on the filter. Then the precipitate was washed with a small amount of water in order to remove as much chromium(III) sulphate solution as possible, combining the washings with the previously collected solution. Regeneration of the solution of sodium terephthalate included washing the terephthalic acid cake with 785 kg of warm water containing 65 kg NaOH. The obtained solution of chromium(III) sulphate was concentrated and 218 kg of crystalline chromium sulphate Cr₂(S0₄)3 · 12 H₂0 was obtained. Example 2

"Wet blue" tanning waste was used in a similar amount, and was subjected to hydrolysis as described in Example 1. 150 kg of terephthalic acid was added to the reactor containing the post-reaction mixture comprising aqueous solution of collagen protein hydrolysis products and suspension of chromium(III) hydroxide. The mixture was heated up to nearly 100°C and mixed for period of 3 hours. Then, the suspension was pumped to the filter-press and filtered. The filtrate comprised an aqueous solution of protein hydrolysate. The precipitate contained the chromium salt of terephthalic acid and the unreacted excess of terephthalic acid. The filtration cake contained all contaminations present in the starting„wet-blue" waste. The filtration cake was washed with hot water to remove remaining of the hydrolysate solutions. 430 dm³ of 20% aqueous solution of sulphuric acid was passed through the filtration cake, wherein said sulphuric acid washed chromium from its salt with terephthalic acid, leaving a precipitate of insoluble terephthalic acid on the filter. Following washing chromium(III) out of the precipitate as chromium(III) sulphate, the precipitate was washed with hot water, and the washings were added to the previously obtained salt solution. The recovered precipitate of terephthalic acid was ready to be used in the next batch. The obtained solution of chromium sulphate was concentrated and 223 kg of crystalline chromium sulphate Cr₂(S0₄)3 · 12 H₂0 was obtained. Insoluble contaminations present in the processed "wet blue" waste remain in the terephthalic acid precipitate and they should be removed along with the evaluation of circulating contamination rate of the terephthalic acid. This acid can be conveniently purified by dissolution in a NaOH solution, filtration of insoluble contamination and repeated precipitation of the terephthalic acid by acidification of its sodium salt solution.

Claims

1. A method for isolation of chromium Cr from sewage, in particular from tanning sewage, by precipitating chromium as an insoluble salt with selected organic acids.

2. The method according to claim 1, wherein organic acid insoluble in water are used.

3. The method according to claim 1 or 2, wherein organic acids are used, of which sodium, potassium and ammonium salts are soluble in water.

4. The method according to any of the claims 1-3, wherein terephthalic acid, salicylic acid, benzoic acid, stearic acid, palmitic acid, sebacic acid, azelaic acid and similar mono- or polycarboxylic aliphatic and/or aromatic acids are used.

5. The method according to any of the claims 1-4, wherein said method comprises the steps of filtrating, sedimentation and/or centrifugation of insoluble, crystalline chromium salts of organic acids.

6. The method according to any of the claims 1-5, wherein said method comprises the step of recovering chromium from insoluble, crystalline chromium salts with organic acids by treating chromium salts with mineral acid solutions, preferably with sulphuric acid.