Historical Articles

December, 1952 issue of Plating

 

Metal Recovery by lon Exchange

C. F. PAULSON
The Permutit Company, New York, N. Y.

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ONE OF THE FIELDS in which ion exchange holds the most promise is the recovery of metals. In a vast number and variety of manufacturing processes, dilute solutions are produced containing metals whose recovery would be most attractive.

We commonly think of metals as cations when in solution, and it was logical for the first attempts to be made using cation exchangers. However, some of the most important and valuable metals are also frequently present as anions. Table I illustrates some of the forms in which industrially important metals are present in solution. This list is by no means exhaustive, but the eases given are typical, and will be expanded upon to show the different methods of attack.

Metal Recovery from Acid Solution
The examples of acid plant wastes containing small quantities of metals are too numerous to mention. However, a large and typical one is zinc in the waste from rayon plants. Zinc sulfate in an acid solution containing large quantities of sulfuric acid and sodium sulfate is used to coagulate the cellulose strands to form rayon, and when this solution is rinsed from the rayon, a large volume of dilute solution is produced. Mindler has reported1 on tests using a sulfonated crosslinked polystyrene resin. Many of the earlier types of cation exchangers had been investigated unsuccessfully for this purpose. They would not remove metals in the presence of hydrogen ions. The capacities at various acid concentrations when treating solutions containing 500 ppm zinc are shown in Table II. It can be seen that the capacity falls as the acid concentration rises. These data show the capacity to a point where 50 ppm of zinc appears in the effluent. At higher acid concentrations than 1 per cent, the slippage of zinc rose and the capacity fell.

Using customary regeneration techniques it would be possible to recover solutions containing several per cent of zinc. However, if a much stronger solution can be recycled into the manufacturing process, special regeneration techniques may be resorted to. The counter current principle in a modified form is used to extract the zinc from the cation exchanger at the most useful concentration. Several regenerant segments are passed through the cation exchanger in order of decreasing zinc concentration and increasing acid concentration. To achieve highest capacity, the final segment must be a strong sulfuric acid solution, and to achieve high regenerant effluent zinc concentration, many segments must be used.

Metal Recovery from Anionic Complex by Cation Exchange
A novel application of cation exchange to metal recovery has been reported¹. In tin plating from alkaline baths, the rinse water contains recoverable amounts of tin as sodium stannate. A sulfonated cross-linked polystyrene resin in the hydrogen cycle was employed to treat such a rinse solution. Hydrogen ions were exchanged for the sodium, causing reduction in pH and precipitation of metastannic acid. The tin-bearing solution was sent through the bed upflow at a rate sufficient to fluidize the bed. This prevented clogging of the voids in the bed by the precipitate. Formation of an insoluble end product drove the reaction to completion, and the very satisfactory capacity of 1400 meq/l was obtained. Sludge filtration in a conical vessel was employed on the effluent solution to concentrate the sludge. The overflow from this unit was water at a pH of 3.0 containing 30 ppm of sodium and no trace of tin. This water could be neutralized and used for further rinsing, being of better quality than most plant waters, and since the entire process was conducted at elevated temperatures, the heat in the rinse water was conserved.

The precipitate drawn from the bottom of the sludge unit could be dissolved in strong hot caustic to yield a solution satisfactory as makeup to a tin plating bath.

Recovery of Cationic Complex by Cation Exchange
In the cuprammonium rayon industry vast quantities of copper-bearing water are produced from rinsing the filaments. Most of this water is quite alkaline and contains Cu probably as the tetramine, NH₄, Na, S0₄, and small quantities of other ions. Recovery of the copper in Germany by cation exchange has been reported² and a similar process is in operation in this country. Since the quantity of copper is quite low, and the amount of suspended material may be appreciable, the flow rate is high and the use of a high capacity synthetic resin is not indicated. A sulfonated coal is used, and at the high pH, its carboxylic acid and phenolic groups come into play giving a capacity in the neighborhood of 750 meq/l of copper. The copper is taken up eventually as the diamine, and since this is held very strongly by the exchanger, the other cations, present in considerable excess compared to the copper, pass through. The copper is then regenerated with 4 per cent sulfuric acid. The acid solution containing copper sulfate may be readily recycled into the process, but if the need arose, concentration along the same lines as described for zinc could be performed.

Recovery of Chromium by Cation Exchange
The processes mentioned above for copper and zinc can be applied very well to the recovery of cationic chromium. However, relatively little of the chromium which is produced by metal treating processes is initially present as the chromic cation. Chromium is generally used as chromic acid in such processes as electroplating, etching, anodizing, copper stripping, brightening after electroplating other metals, etc. In these processes, the acid rapidly becomes contaminated with metallic cations and must be discarded. Costa has described³ a prolonged series of tests on the removal of interfering cations from chromic acid so that the baths may be reused. This process makes use of one of the outstanding characteristics of the sulfonated polystyrene cation exchange resins, their resistance to oxidation. There have been published^{4, 5, 6} a number of papers on treating brass and copper mill wastes by ion exchange but they almost all found that if hexavalent chromium were present, both the cation and anion exchangers were oxidized and lost their effectiveness. However, the polystyrene cation exchangers are resistant to chromic acid up to 15 per cent and have an excellent capacity for metallic cations in any solution of lower concentration.

In practice the method is best applied to anodizing baths, where approximately one pound of aluminum is dissolved for each pound that goes into the oxide coating on the metal, and hard chrome plating baths where the prolonged contact of the strong acid solution with the plated surface causes some of the cationic metal to go into solution. By withdrawing each day a portion of this bath and treating it by cation exchange, the bath may be permanently maintained at the point of maximum coating efficiency. Former practice was to allow the metal content to build up to a point where the coating was adversely affected and then dump a portion of the bath and refill with fresh CrO₃. This created a very serious waste disposal problem and was expensive.

Chromium Recovery by Anion Exchange
An extension of the process described above has been developed recently. After a chromic acid treatment, it is customary to rinse excess acid from the metal. There are many ways of operating rinsing facilities with one or more flowing rinses occasionally preceded by a still rinse. But in most cases a solution must eventually be disposed of which contains from 10 to 100 ppm of chromate anion. A new anion exchanger, Permutit S, is resistant to attack by such solutions and will adsorb the chromate anion. The rinse water may then be recirculated. The Permutit S is then regenerated with sodium hydroxide and the regenerant effluent sent back to the anodizing tank through a cation exchange resin which forms the free acid. In plant scale operations this process has been shown to be profitable. Fig. 1 shows this equipment. The process conserves the chromic acid, rinse water, and heat in the rinse water and eliminates a serious waste disposal problem. It costs less than the purchase of an equivalent quantity of chromic acid. In anodizing operations, it has also been shown to yield a more corrosion resistant coating.

Fig. 1--Chromate Recovery Equipment

Removal of Metals from Acids by Anion Exchange
An interesting type of recovery has been reported by Kraus and Moore⁷. They find that many metals customarily considered to be present in solution as cations are present as anions in acid solution, actually probably as coordination compounds with the acid. Such compounds as Fe(PO₄)₄^-,FeCl₄^-, and AlF₆^--- are typical. Even in acid as strong as 9 N hydrochloric, these anions may readily be held by highly basic anion exchangers. Kraus and Moore have applied their efforts chiefly to separations of hafnium, columbium, tantalum, protoactinium, and zirconium, but have demonstrated the ready manner in which metals which form coordination compounds can be separated from those which do not. The process has been applied successfully in several locations for the removal of traces of iron from muriatic acid.

Precious Metal Recovery by Anion Exchange
Recovery of gold and silver from ore and electroplating wastes can be accomplished by anion exchange. In these solutions the precious metal is generally present in alkaline solution as the cyanide complex.: Conditions here are somewhat different than in previous situations discussed as the very dilute form in which the precious metals are present results in low capacity. Also, the complex precious metal anions are only regenerated or the anion exchanger with difficulty. Efforts are being directed toward development of special anion exchangers for precious metals and effective special regeneration procedures have been developed.

Anion exchange has also been attempted to recover gold from ore. Hussey has reported a series of investigations along this line. Customary extraction processes depend upon the percolation of a cyanide solution through the finely ground ore. However, an increasing number of ore bodies being worked contain the gold mixed with clays and other minerals which form slimes and prevent percolation. An anion exchanger may be mixed with the ore during leaching. The anion exchanger holds the metal cyanide as it is dissolved allowing complete leaching of the ore. The granular exchanger may then be filtered from the slime and regenerated with caustic. Special ion exchange equipment has been developed for this purpose.

Recovery of Electroplating Wastes
To show how some of these techniques may be combined to solve a difficult disposal problem, we will consider a plant doing chromium plating on top of nickel and copper. Fig 2 shows the flow plan of such a plant and how ion exchange might be incorporated. For simplification only one rinse is shown after each step; except chromium plating, but more rinses might be necessary. The material to be plated is cleaned, plated with copper from a cyanide bath, plated with nickel from a sulfate bath, and plated with chromium from a chromic acid bath, with rinses between each of the, steps. Various recovery methods including ion exchange have been proposed for such a system, but they almost all were based upon treatment of the mixed wastes and yielded some, if not all of the metal values in forms which were difficult to recycle to the system. A typical system might involve reduction of chromate, neutralization of the chromic cation and precipitation with lime, and oxidation of the cyanide with chlorine.

Fig. 2--Plating Recovery Flow Sheet

As analyses of individual wastes from such plants are rather hard to locate, and generally vary from time to time to such a degree as to be unreliable, the analyses in Table III have been assumed.

Demineralized water is used for making up all the baths and the copper rinse. The copper rinse, containing the copper as a complex anion, is pumped from the overflow of the rinse tank through a bed of a highly basic anion exchanger such as Permutit A which will hold the cyanide, and copper complex. The effluent of this unit will contain hydroxide equivalent to the anions removed and the run will be halted when cyanide breaks through. Part of this water will be recy-cled and the remainder will go to the nickel rinse bath. The highly basic anion exchanger will be regenerated with caustic soda and the regenerant effluent which will be highly alkaline will be returned to the plating bath. Regeneration rinse waters and any other waters which might contain cyanides can be given a chlorine treatment to insure freedom from toxicity and used to neutralize the acid rinse waters.

Fig--3. Closed System Ion Exchange Installation

Rinse water from the nickel rinse will go to the bed of sulfonated cross-linked polystyrene resin such as Permutit Q where the nickel will be removed. At the low pH, little if any of the sodium will be held. The effluent from this unit can be recycled to another bath where traces of sodium sulfate would not be harmful. Upon regeneration with sulfuric acid, this cation exchanger will yield a solution containing 2 per cent Ni and 2 per cent free H₂SO₄ which can be returned to the plating bath. The excess acidity may have to be lowered by dissolving nickel carbonate in the regenerant effluent before addition to the bath.

The chromate recovery system will be a completely closed system. Fig. 3 shows such an ion exchange system. The concentration of chromate in the still rinse will be allowed to rise to about 10 per cent. At frequent intervals this still rinse will be passed through a cation exchange unit to remove metallic cations, and occasionally part of the treated acid will be returned to the plating tank. It may be necessary to use an evaporator to concentrate this solution although experience in many locations has indicated that a 10 per cent solution may be returned directly. While the dragout from the plating rinse contains some metallic cations, the dragout from the still rinse will be almost pure chromic acid. This running rinse will pass through a highly basic styrene base anion exchanger and then be recycled. When this unit is exhausted, it will be regenerated with caustic soda and the regenerant effluent sent through the cation exchange unit for removal of sodium and then to the still rinse tank. Throughout the cycle of the two units working on chromate bearing liquors, it is possible to operate in such a manner that all the liquor containing chromates will go to either the plating bath, still rinse or running rinse and none will go to waste. The regenerant from the cation exchange unit will be sent to waste as it will contain a mixture of metals which would not be worth separating.

Let us examine the savings that such a system would involve. It would be possible to oxidize the cyanides with 7 parts of chlorine per part of cyanide, reduce the chromates with copperas or sulfur dioxide and mix the three rinses and neutralize to precipitate the hydroxides. These hydroxides could be removed by settling or filtration. A conventional disposal house for these solutions would cost not less than $25,000. The sludge from such a unit would contain approximately 5 per cent solids which could be lagooned. It is possible that after settling, the precipitate could be sold for reworking.

Neutralization would require 122 pounds per day of lime, reduction of chromate would require 216 pounds of copperas, and oxidation of cyanide, 39 pounds of chlorine.

To treat the cyanide waste would require 3 cu. ft. per day of anion exchanger. Actually the flow rate would necessitate the use of a larger unit. A satisfactory unit would be 30 inches in diameter and hold 14 cu. ft. of highly basic anion exchanger. Each day the regenerant needed would be 4 pounds per cu. ft. each of caustic soda and sodium cyanide.

The nickel recovery unit would also have to be sized according to flow rate as approximately 4 cu. ft. of styrene base cation exchanger per day would be needed. This unit also would be 30 inches in diameter and would require 40 pounds per day of 66° Be H₂SO₄.

The chromate recovery anion exchanger would be similar to the copper cyanide unit in size. It would require 5 pounds of caustic soda per day as regenerant. The chromate recovery cation exchanger is difficult to estimate accurately as too many factors are involved but it is assumed that operation would be similar to what was found in a chromic acid anodizing plant where the sizes of anion and cation exchangers are similar. Twenty pounds of sulfuric acid are used as regenerant for this unit. One extra regeneration would be required to take care of anion regenerant so this unit ‘ would operate 2 cycles per seven days.

As on any day only one ion exchange unit would require regeneration and this would require little attention, no cost for labor has been included.

In estimating the value of the recovered materials, the market prices of the chemical in solution is used. Thus the value of the copper may seem rather high. Since chromium plating baths are more concentrated than most others, dragout is more serious. Probably if ion exchange were not used for this recovery, some other means of minimizing this loss would be utilized, so the value of chromate recovered is somewhat high. A cost balance indicates a saving of $3.82 per’ day over no waste treatment or of $28.32 over a conventional system. This saving would pay for the equipment in 300 working days. Also, since most of the water is reclaimed and reused, the water cost will be reduced from $14.40 to $4.80 per day.

Recirculating Rinse Water
In many locations plating tank and piping layouts are such that individual rinse solutions cannot be isolated for recovery of the valuable constituents. Here also ion exchange can be profitably employed in the waste disposal scheme. Rather than conventional waste holding tanks and chemical feeders, it is possible to demineralize the entire waste stream, at an equipment cost comparable to the tanks and feeders, to produce a high quality water for all plant processes. The ion exchangers in the demineralization step may be regenerated when exhausted to yield only a small volume of concentrated waste which may readily be rendered non-toxic by conventional means. Such a process offers major economies in labor, and of course the demineralized water results in freedom from staining and water spots of the finished work, freedom from sludges in the baths and dull, pitted, blistered, or brittle deposits. Frequently the cost of treatment for such recycled water is less than the cost for treating the’ plant raw water to make it suitable for plating operations.

REFERENCES
1. A. B. Mindler, M. E. Gilwood and G. H. Saunders, Ind. Eng. Chem. 43, 1079 (1951).
2. Combined Intelligence Objectives Subcommittee, Chem. & Met 52, 214 (1945).
3. R L. Costa, Ind. Eng. Chem. 42, 308 (1950).
4. H. A. Bliss, Chem. Eng. Progress 44, 887 (1948).
5. W. S. Wise Sewage Works J. 20, 96 (1948).
6. D. E. Bloodgood and F. J. Losson, Jr., Proc. 3rd Ind. Waste Conf. (Purdue U.) 64, 196 (1947).
7. K. A. Kraus and G. E. Moore, J. A. C. S. 71, 3555 (1949) 71, 3263 (1949); 72, 4293 (1950).
8. S. J. Hussey Bureau of Mines, R. I. 4374 (Jan. 1949).

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