GB2036083A - Recovering Metal from Metal Complex Containing Liquors - Google Patents

Abstract

The process comprises the steps of:- (a) adjusting the pH of the liquor e.g. metal refinery effluent as necessary to a pH of 6.0+/-0.5; (b) removing by filtration any precipitate or other solid matter from the so-treated liquor of step (a); (c) heating the filtrate from step (b), or, in the absence of step (b), the treated liquor of step (a) to not less than 70 DEG C; (d) electrolysing the heated filtrate or liquor of step (c) at a substantially constant temperature of at least 70 DEG C so as to destroy the stable complexes and thereby to precipitate at least a proportion of the said metal in the form of hydrated oxides or hydroxides, and (e) filtering-off the precipitate from step (d). Complexes present in effluents which,are otherwise difficult to break down and cause problems in purification and control are said to be successfully destroyed by this method. e

Description

SPECIFICATION The Electrochemical Destruction of Stable Complexes This invention relates to the treatment of aqueous liquors such as effluents. More particularly it relates to the destruction of compounds or complexes which are normally very stable and which pose problems in the purification and control of effluents.

Certain very stable and water soluble compounds or complexes of metals are sometimes encountered which resist all normal chemical and electrochemical attempts to remove them from solution. These compounds or complexes, which, throughout the remainder of this specification will be referred to for convenience as "complexes", usually occur only in small proportions relative to the total content of the complexed metal. They are normally only detected as the differences in the results between analyses based on a whole sample, and analyses involving concentration steps other than evaporation. The stable complexes escape such concentration steps and are therefore lost and the said differences are usually explained as experimental error.

in the case of, for example, the platinum group metals, their high intrinsic value makes recovery worthwhile. In other cases, recovery may be required to prevent infringement of discharge limits. It is, therefore, an object of this invention to provide a process for destroying these stable complexes and thereby permitting recovery of small traces of metals which are not usually recoverable.

In the course of work to recover the values from the effluent from a platinum refinery, it was found that all the possible methods which gave excellent results on synthetic solutions failed to do so when actual effluent was treated. After treatment, the apparently barren effluent was found by spectroscopic examination to contain up to 100 mgl-' of various precious metals in solution. A method for recovering these traces of the platinum group metals has been described in U.S. Patent No. 4,127,458 but experience has shown that the method is no longer adequate.

Changes in refinery technique have resulted in changes in the type and proportion of the stable complexes in the effluent. This present invention provides a more general treatment which is successful in destroying all the classes of stable complexes so far encountered. The method is best applied to effluent streams which have received no previous recovery treatment.

According to a first aspect of this invention, a process for recovering metal present as one or more stable complexes in an aqueous liquor such as an aqueous effluent comprises the steps of: (a) adjusting the pH of the liquor as necessary to a pH of 6.0+0.5; (b) removing by filtration any precipitate or other solid matter from the so-treated liquor of step (a); (c) heating the filtrate from step (b) or, in the absence of step (b) the treated liquor of step (a), to a temperature not less than 700 C; (d) electrolysing the heated filtrate or liquor of step (c) at a substantially constant temperature of at least 700C so as to precipitate at least a proportion of the said metal as oxides or hydroxides, and (e) filtering-off the precipitate from step (d).

The invention is particularly applicable to the recovery of platinum group metal present as one or more stable complexes in an aqueous effluent from a refinery, the expression "Platinum group metal" both here and throughout the remainder of the specification being taken to mean one or more of the platinum group metals platinum, palladium, rhodium, iridium, osmium and ruthenium.

According to a second aspect of this invention therefore, a process for recovering platinum group metal present as one or more stable complexes in an aqueous effluent comprises the steps of: (a) adjusting the pH of the effluent to a value of 6.0+0.5; (b) removing by filtration any precipitate or other solid matter from the so-treated effluent of step (a); (c) heating the filtrate from step (b), or, in the absence of step (b), the treated effluent of step (a) to a temperature not less than 700C;; (d) electrolysing the heated filtrate or effluent of step (c) at a substantially constant temperature of at least 700C so as to destroy the stable complexes and thereby to precipitate at least a proportion of the platinum group metal (as herein defined) in the form of hydrated oxides or hydroxides and (c) filtering off the precipitate from step (d); In the process of this invention all the aqueous discharges likely to contain platinum group metals are collected and mixed together under controlled conditions. Additions of acid, usually hydrochloric acid, or alkali, usually sodium hydroxide or sodium carbonte, are made to bring the mixture to pH 6.0+0.5. The bulk of the base metals such as iron, copper and nickel and amphoteric metals such as arsenic, antimony, tin, zinc, solenium and tellurium are then removed by filtration as their hydroxides, oxychlorides and so on.The filtrate is variable in composition but usually contains 500-1 500 mgl-' of platinum group metal, up to several hundred milligrams mgl-l of copper, nickel and zinc as ammines and also traces of other metals. Sodium chloride is present usually in the range 5000 to 20,000 mgl-1 together with ammonium salts. Other water soluble salts are also present in small quantity.

The filtrate or effluent is taken at pH 6+0.5, heated to 700C and electrolysed at current densities capable of effecting the evolution of oxygen and hydrogen at the anode and cathode respectively. Electrolysis conditions are usually defined in terms of current density or electrode potentials but the variable nature of the effluent liquors to be treated, and of the subsequent reaction products, make it impossible to make definitive statements of these conditions for the present invention. Experiment has shown, however, that it is necessary for the cathode potential to be sufficient to effect hydrogen evolution at the cathode and for the potential at the anode to be sufficient to effect the evolution of oxygen at the anode, although neither of these processes will necessarily occur continuously under constant current conditions.

Electrolysis is continued until the pH rises to 8.0 at which point the destruction of the stable complexes is complete. The temperature is maintained at at least 700C throughout the electrolysis. Although some metal is deposited at the cathode, the majority is precipitated in the form of highly oxidised hydrated oxides or hydroxides. The products are similar to those produced by the methods given in U.S. Patent No.

3,806,591 (JC.461 ). This invention, therefore, also includes a method of preparation of a hydrated oxide or hydroxide of a platinum group metal.

These products are readily decomposed in dilute acids and are also slowly decomposed in sodium hydroxide. If the electrolysis is continued the pH will rise rapidly and at values above pH 8.0, platinum group metal will be redissolved. The products are, therefore, recovered by filtration at pH 8.0 leaving a filtrate containing 5-1 5 mgl-l of mixed platinum group metals. These residual traces are no longer complexed in unreactive forms and may be recovered easily by any of the known techniques, e.g., high contact area cathode electrolysis, spinning cathode electrolysis, or by chemical means such as sulphiding. Final discharges containing less than 0.5 mgl-' total platinum group metals are easily achieved.

The nature of the stable complexes is not known. It is believed that tetramine platinum II chloride and pentamine rhodium chloride are typical examples, but organic complexes are sometimes also present. Together these complexes make up approximately 10% to 20% of the platinum group metal content of refinery effluent. As they are water soluble and chemically unreactive, their isolation from such small concentrations in such a complex system has not hitherto been attempted.

During the electrolysing step (d) in a process according to the invention, a number of reactions take place in succession and are best understood with reference to the attached Figure 1 which shows, diagrammatically, the changes of pH with time in a process according to the invention. In stage 1 of the process, electrolysis of uncomplexed metal salts gives cathode deposits and liberates acid. The pH falls slowly. This being a chloride system, the liberated hydrochloric acid is next decomposed to yield chlorine and hydrogen. The chlorine reacts with ammonia to form chloramine.At 706C or above, chloramine will react with more chlorine or with itself to decompose to nitrogen and more hydrochloric acid as follows:- 2NH3+2CI2=2NH2CI+2HCI NH2Cl+C12=NHCl2+HCl NH2Cl+NHCl2=N2+3HCl 2NH3+3CI2=N2+6HCI, thus giving the rapid fall in pH in stage 2. Simple metal amine complexes are decomposed by similar reactions. Stage 3 of the electrolysis is simply the destruction of the liberated acid and the discharge of chlorine during which stage the pH remains very low. The cell voltage is also at a minimum.

When the acid is exhausted the reaction changes at stage 4 effectively to the electrolysis of sodium chloride. Chlorine liberation at the anode is replaced by the generation of hypochlorite ions OCI-whilst hydroxyl ions OH+ are produced at the cathode. The pH and the cell voltage begin to rise, and the hypochlorite ions help to decompose the first type of stable compounds previously mentioned. This is demonstrated by the sudden increase in colour often observed at this stage. The reaction do not occur in chemical simulations of this stage or if the current in the cell is too low.

The pH continues to rise rapidly until at about pH 4.5 precipitation begins. The rate of increase of pH slows down as hydroxyl ions are removed from the electrolyte to form the precipitate. Again, if the current density is too low the desired reactions do not take place, and additions of alkali to the electrolyte at this stage do not give the desired destruction of the second type of stable complexes. Electrolysis must therefore be continued. At about pH 6.0 the rate of pH increase slows down even more, and in some cases may show a temporary fall as indicated by dotted lines in the attached Figure 1. It is believed that organic amines are being destroyed at this stage. if the temperature is allowed to fall, chloramines can once again be detected and black tarry deposits build up around the top of the cell.Again the destruction of the complexes only occurs if the current density is high enough. The reactions at this stage continue until the pH increase becomes rapid. The electrolysis is judged complete when pH 8.0 is reached.

The time taken for each of the stages to be completed depends on the quantities of the compounds reacted in each stage that are present in the sample under test. In the variable discharge from the refinery, samples representing a week's arisings were taken for each of twelve weeks, and showed total process times varying from 2 to 10 hours. The variation in processing times for individual stages was from 5 minutes to 6 hours.

However, by electrolysing to pH 8, recovery of the platinum group metal so as to leave less than 0.5 mgl-l in the final discharge was achieved in every case. Experience has shown that it is preferable for each and every stage to be allowed to proceed to completion in order to ensure substantially complete recovery of platinum group metal values.

The invention will now be illustrated by means of the following three examples.

Example 1 After adjustment of the pH value to pH 6.0+0.5 and filtration, a sample of mixed effluents contained:~ Pt 1000 mgl-' Pd 1000 Ir 50 Ru 70 Rh 100 A 750 ml sample was electrolysed at 5.0 amps between Rh/Pt electrodes (anode 80 cm2 cathode 50 cm2) at a temperature above 700C and the pH of the electrolyte returned to pH 8.0 after 10/5 hours. After filtration the electrolyte contained.~ Pt 10.5 magi~' Pd NIL Ir NIL Ru NIL Rh 0.1 and, after sulphiding, the contents were reduced to Pt 0.08 mgl-l Sulphiding the untreated liquor left 28 magi~' total platinum group metals.

Example 2 A different sample of mixed effluents was treated as in Example 1. The contents before electrolysis were.~ Pt 125 mgl-l Pd 250 Ir 6.3 Ru 10 Rh 15 This sample again required 10 hours to return to pH 8.0. The electrolyte after filtration then contained:~ Pt 8.5 mgl-' Pd 1.2 Ir NIL Ru NIL Rh 0.7 After further electrolysis using a high cathode area and high turbulence, the platinum group metal content was reduced to 0.15 mgl-1.

Example 3 Yet another mixture of effluents which contained:~ Pt 550 mgl-' Pd 450 Ir 8.0 Ru 12.0 Rh 20 was electrolysed under the same conditions as in Example 1 and took six hours to reach pH 8.0.

After filtration the filtrate contained:~ Pt 1.2 mgl-l Pd NIL Ir 0.6 Ru NIL Rh 0.1 In this example secondary treatment was not used.

From these examples it can be seen that the metal content of the solution has little bearing on the total time required for the electrolysis. It is also noteworthy that if the electrolysis is not continued until the pH value of the electrolyte reaches 8.0, between 5% and 25% of the original platinum group metal is left in solution.

The current densities given in the foregoing examples are those which were found to be optimum for the particular conditions in the Examples.

If the contents of the effluent liquor to be treated were always consistent, a series of cells in which the conditions for each of the four successive stages in the pH/reaction time diagram of Figure 1 could be used, the effluent or filtrate being transferred from one cell to the next succeeding one the completion of the appropriate stage in the electrolysis. This would then probably lead to a reduction in power consumption.

Claims (9)

Claims

1. A process for recovering metal present as one or more stable complexes in an aqueous liquor comprises the steps of: (a) adjusting the pH of the liquor as necessary to a value of 6.0+0.5; (b) removing by filtration any precipitate or other solid matter from the so-treated liquor of step (a); (c) heating the filtrate from step (b) or, in the absence of step (b) the treated liquor of step (a), to a temperature not less than 700C; (d) electrolysing the heated filtrate or liquor of step (c) at a substantially constant temperature of at least 700C so as to destroy the stable complexes and thereby to precipitate at least a proportion of the said metal in the form of hydrated oxides or hydroxides, and (e) filtering-off the precipitate from step (d).

2. A process according to claim 1 in which the metal is platinum group metal and the liquor is refinery effluent.

3. A process according to claim 1 or 2 wherein the electrolysis in step (d) is continued until the pH of the filtrate, liquor or effluent reaches a value of pH 8.0.

4. A process according to claim 1, 2 or 3 wherein the electrolysis in step (d) is conducted at such values of current density as will effect the liberation of hydrogen and oxygen at the cathode and anode respectively.

5. A process according to claim 4, wherein electrolysis is carried out in a cell including two electrodes one of which presents a relatively large area and the other electrode presents a relatively small area to the electrolyte.

6. A process according to claim 5 wherein the electrode which presents the relatively large area is the anode and the electrode which presents the relatively small area is the cathode.

7. A process according to claim 6 wherein the ratio of areas presented to the electrolyte by the anode and the cathode falls within the range 60-1 and 102-1.

8. A process according to claim 7 wherein during electrolysis a current density within the range 0.15 and 0.25 A cm~2 is maintained at the anode.

9. A process according to any preceding claim for the preparation of a hydrated oxide or hydroxide of a metal.