IE49370B1 - Recovery of cyanides from solutions - Google Patents

Abstract

Soluble cyanides contained in rinse solutions are transformed by chemical reaction exemplified by (M(CN)u)s+ n MC1 = n MCN into insoluble precipitates which are recovered.

Description

RECOVERY OP CYANIDES FROM SOLUTIONS This invention concerns a simple process for recovering for re-use cyanides from solutions, particularly rinsing solutions, from industrial methods for the electrodeposition of metals.

The electroplating industry seeks a more economical and practical process for purifying rinsing solutions.

The work developed in this sector of electrochemistry is rather complicated and requires specialized personnel trained for the use of the equipment employed; it is io expensive due to high costs of energy and specialized labour. The large amount of chemical products lost in the electroplating industry is common knowledge, creating, as a result of treatment for destroying poisons, high heavy metal content slurries, which are lost.

Everyone in the trade knows that the treatment of cyanide bearing rinsing solutions is attained mainly through the poison elimination process by destruction of the cyanide. Another known process, the recycling process, is based upon the evaporation of the solvent in the rinsing solutions, by using energy to concentrate the fluid which will be re-used in the respective dips. Other processes are.also known which use ion exchangers, involving high - 3 capital investment for equipment and the loss of chemical products through their destruction.

The present invention provides a process for the recovery of a cyanide from a solution from a cyanldric electrodeposition of a metal, the solution containing a soluble complex cyanide of an alkali metal and a non-alkali metal of the class consisting of gold, silver, zinc, copper and cadmium, which process comprises adding to the solution a soluble simple salt of the same non10 -alkali metal of said class such that it reacts with the complex cyanide and any free cyanide ions in the solution to form and precipitate an insoluble cysmide of the non-alkali metal, the pH and any acids added and any salts added chosen so that substantially all the free and complex cyanide in the solution is precipitated and in the case of the non-alkali metal being copper so that the precipitate is substantially pure insoluble cyanide of copper, and separating the precipitate for re-use.

The present process recovers chemical products in the cyanide bearing solutions, transforming the soluble cyanide into insoluble cyanide, thus allowing its removal by filtration or deposition. The insoluble cyanide may, therefore, be easily re-used industrially.

The process allows the total precipitation of the cyanides in the insoluble form, together with the cyanide metals and their recovery for industrial re-use.

In the present process, the pH and any acids and salts (i.e. the essential added salt and any other salts) are so chosen that substantially all the free and complex cyanide precipitates. It is essential when the non-alkali metal is copper and preferred when the non-alkali metal is other than copper, that the pH and any acids and any salts are so chosen that the precipitate is substantially pure insoluble cyanide of the non-alkali metal.

In a particular embodiment, an acid salt is added to adjust the pH and to adjust the valency of the non-alkali metal of the simple salt. 48370 The present solutions are preferably rinsing solutions from a cyanidric electrodeposition of a metal, and the invention is described particularly with reference thereto.

According to the amount of cyanide in the rinsing solutions, adequate quantities of metallic salts are added; these correspond to the metal associated with the cyanide in the rinsing solution and in the electroplating dip. As examples, zinc sulphate and/or chloride for rinsing solutions of the cyanidric sine dip, or else copper sulphate and/or chloride for rinsing solutions of the cyanidric copper dip, associated with acid or alkaline salts which control the pH of the rinsing solution and the valency of the metallic ions, according to the type of metal, thus obtaining a full transformation of all the free cyanide and cyanides associated in complex salts to a simple metallic cyanide, insoluble in water with an adequate pH in the rinsing solutions, without releasing cyanidric gases.

All the cyanide in the water can thus be transformed into an insoluble salt, and may be removed from the rinsing solution by deposition or filtration.

By using for each type of metal in the electrodeposition of metals process separate rinsing tanks without danger of Intermixing metals, the metallic cyanides precipitated by means of the present process may be re-used directly in the respective dips after rinsing with water the insoluble precipitate which has been filtered or deposited, until the rinsing water attains a density of 1.000 or 0’ Be, 48370 thus releasing the precipitated, cyanide iron the soluble components. The same process may also he used for the precipitation and/or recovery of cyanide from any cyanidric liquid already used in the electroplating of zinc, cadmium, copper, silver, gold and their alloys.

The present process is simple. There is no need for specialized technical personnel, nor special equipment; neither does it consume chemical products for later destruction. It reclaims efficiently valuable chemical products from the solution, the value of the metallic salt used for the precipitation and recovery being two to six times lower than that of the reclaimed material.

In most countries, the copper, the zinc, the cadmium, the silver, the gold and cyanides of the solutions treated by the present process are imported materials, thus justifying avoidance of the loss of these products in the rinsing solutions in the electroplating industry.

The electroplating industry is up against problems concerning compliance with the standards of public bodies, which are responsible for the preservation and protection of the ecology. They demand an efficient and complete treatment of industrial effluent waste products.

The high toxicity of cyanides is well known and, therefore, the necessary and strict controls by public bodies over the electroplating industry are well justified.

The present process allows the treatment of waste waters containing cyanides with profit to the users through the recovery of the material via visual control of the treatment, thus eliminating the necessity for high costing equipment and avoiding any impairment of the ecology.

The following four chemical equations illustrate the development of the reactions of the insoluble copper cyanide with the dissolved potassium cyanide to form the complex salt of soluble copper cyanide, demonstrating further how the complex soluble copper cyanide, by the addition of copper chloride, is converted into the insoluble copper cyanide at the rinsing solution treatment stage: - 1) CuCN + KCN = K(Cu(CN)2) - 2) K(Cu(CN)2) + KCN = K2(Cu(CN)3) - 3) K2(Cu(CN)3)£ K2(Cu(CN)2)~ (Cu(CN)3)2* (Cu(CN)4)3~ - 4) (Cu(CN)2>3>4) ’ ’ + nCuC1 B ncuCN insoluble. 48370 In formula four, there would be no difficulty in exchanging the copper chloride for copper sulphate.

By using the insoluble sine cyanide with the sodium cyanide in an assembly of sine dips, a soluble complex zinc cyanide salt is obtained end this may be precipitated with zinc sulphate by the present rinsing solution treatment according to the equation: Zn(CN)42 + ZnSo* > 2Zn(CN)g insoluble The precipitation equation may be generalized considering as -M-the symbol of the present ηοη-alkali metal, thu3 permitting the objective of recovering the cyanides and the respective metals is the rinsing solutions: (K(CB) 2,3,4^ 1,2,3~ + nMCl . nMCN M(CB)2 + MSO. . nM(C»)o 4 2 The present process may thus be seen as the addition of metallic salts along with salts and/or acids to correct the pH of the water and associated with ealte, gases and/or acids to correct the valency of metallic ions, where necessary.

To demonstrate the technical and economic value of the invention, it will now he illustrated by the following Examples: Example 1 Cyanidric zine dip with the following formulations - 60 g/litre of zinc cyanide at Cr? 130.00/kg - CrM 7-60 50 g/litre of sodium cyanide at Or# 100.00/kg « Crif 5«00 g/litre of caustic soda at Or# 18.00/kg Crg 1.26 One litre of zinc dip .................. Crg 13.06 When one litre of the zinc dip is flushed to the rinsing tank, the following materials are needed to precipitate the soluble cyanide and part of the sodium hydroxide (caustic soda): - 200 g of zinc sulphate at CrS 14.00/kg . Cr£ 2.80 The value of the recovered and precipitated material afterwards is: “ 120 g of zinc cyanide at Cr? 130,00/kg Cr£ I5.6O g of zinc hydroxide at Cr# 40.00/kg · Crg 0.64 Value of recovered material Crg 16.2ft This means that the value of the recovered material is 5.8 times the value of the material used for recovery.

The water after this treatment contains no cyanides and no Cri 180.00/kg » Cri 10.80 Cri 100.00/kg - Cri 8.10 heavy metals; there remains only sodium sulphate and traces of sodium hydroxide, which are easily neutralized. Example 2 Cyanidrlc copper dip, with the following formulation: - 60 g/lltre of copper cyanide at 81 g/lltre of sodium cyanide at g/lltre of sodium hydroxide at Cri 18.00/kg « Crj 0.27 One litre of copper dip .....................= Cri 19,17 Therefore, a litre of copper dip when flushed to the rinsing tank needs the following amount of materials to precipitate the soluble cyanide: - 161 g of copper chloride at i Cri 111.60/kg χ Cri 17.96 The value of the recovered and precipitated material will then be the following: - 210 g of copper cyanide at CrjS 180.00/kg χ Cri 37.80 The value of the reclaimed material is therefore 2.1 times the value of the metallic salt used for the recovery.

The water after the treatment has no cyanides and no heavy metals, and there remains only sodium chloride, which is no problem for the water treatment.

These two Examples give a good demonstration of the technical and economic value of the present process.

The traditional processes for the treatment of cyanide bearing waters have the following values concerning the usage of chemical products for the destruction of the unreclalmable cyanides: Comparative Example 1 Γογ one litre of tine bath dip: - 360 g of chloride · Cr? 2.88 410 g of sodium carbonate - Cr? 7.38 5 Total ........ Cr? 10.26 Comparative Example 2 For one.litre - 430 g of chloride - Cr? 3.44 490 g of sodium carbonate > Cr? 8.82 10 Total ........ Cr? 12.26 These costs do not Include the cost of filtering and separating the slurry, the capital investment for equipment and the running expenses for storage of the residues in adequate locations.

In this comparison, the physical values are constant internationally; the values in money terms may differ in relation to the cost of the chemical products in each country. The monetary values must be considered for comparisons, and the approximate equivalent in Brazil of one United States Dollar is thirty cruzeiros: (1.00 - Cr? 30.00).

Claims (10)

1. A process for the .recovery of a cyanide from a solution from a cyanidric electrodeposition of a metal, the solution containing a soluble complex cyanide of an alkali metal and a non-alkali metal of the class 5 consisting of gold, silver, zinc, copper and cadmium, which process comprises adding to the solution a soluble simple salt of the same non-alkali metal of said class such that it reacts with the complex cyanide and any free cyanide ions in the solution to form and precipitate 10 an Insoluble cyanide of,the non-alkali metal, the pH and any acids'added and any’salts added being chosen so that substantially all the free and complex cyanide in the solution is precipitated and in the case of the non-alkali metal being copper so that the precipitate is substantially 15 pure insoluble cyanide of copper, and separating the precipitate for re-use.

2. A process according to claim 1 wherein an acid salt is added to adjust the pH and to adjust the valency of the non-alkali metal of the simple salt, 20

3. A process for the recovery of a cyanide from a solution from a cyanidric electrodeposition of a metal, the solution containing a soluble complex cyanide of an alkali metal and a non-alkali metal of the class consisting of gold, silver, zinc and cadmium, which 25 process comprises adding to the solution a soluble simple salt of the same non-alkali metal of said class such that it reacts with the complex cyanide and any free cyanide ions In the solution to form and precipitate an Insoluble cyanide of the non-alkali metal, the pH and any acids added and any salts added being chosen so that substantially all the free and complex cyanide in the solution is precipitated, and separating the precipitate for re-use.

4. A process according to claim 3 wherein the non-alkali metal is gold.

5. A process according to any one of the preceding 10 claims wherein the simple salt is a sulphate or chloride.

6. A process according to claim 3 wherein the non-alkali metal is zinc and the simple salt is zinc sulphate or zinc chloride.

7. A process according to claim 1 or 2 wherein the non15 -alkali metal is copper and the simple salt is copper sulphate or copper chloride.

8. A process according to any one of the preceding claims wherein the solution is a rinsing solution from a cyanidrle electrodeposition of a metal. 20

9. A process according to claim 8 wherein the separated cyanide is re-used for the cyanidric electrodeposition of a metal.

10. A process according to claim 1 performed substantially as described herein. 25 11· A process according to claim 1 performed substantially as described herein In either of the Examples.