Adsorption and elution of metal from ion exchange resin
This invention relates to a method and apparatus for recovering gold in the form of potassium gold cyanide contained in the rinse water used to rinse plated articles in plating and stripping operations. 
     BACKGROUND OF THE INVENTION 
     It is standard practice to rinse articles such as electrical terminals or connectors which have been electroplated to remove the chemicals utilized in plating and clean the articles. The resulting rinse water is then treated to remove any residual precious metals for recovery with the remaining effluent passed through appropriate waste treatment A wide variety of mechanical and chemical processes such as dialysis and membrane filters have been tried and are in fact used to treat rinse water from plating operations. These processes work with varying degrees of success in recovery. Typically, processes require many stages of treatment with each stage removing a percentage of the precious metal or compound thereof but complete recovery, limited by the cost of energy and the chemicals used in recovery which in all events should not exceed the value of the recovered material or the cost of alternative disposition of such effluent. 
     The present invention has as an object the provision of a method and apparatus for recovering precious metals such as gold drag-out from plating and stripping operations in an economic and effective manner. The invention has a further object the provision of a method and apparatus for recovering essentially all of the precious metal found in the rinse water utilized in plating operations. Yet a further object is to provide a simple and compact apparatus for recovering potassium gold cyanide from rinse water in a manner leading to a highly efficient reuse of chemicals and water employed in the process. 
     SUMMARY OF THE INVENTION 
     The present invention operates to recover precious metal from rinse water from plating and stripping operations by pumping rinse water through a weak base, ion exchange resin. Thereafter, the metal cyanide anion is extracted from the resin by treating the resin with a solution of an inorganic base followed by treatment with an inorganic salt solution the resultant eluent thereafter being evaporated to a point of the solubility limit of the metal salt which is then crystallized out of solution by chilling. In practice, the apparatus of the invention pumps a rinse solution such as a gold rinse solution through bed columns containing a macroporus weak-base anion exchange resin in the form of a cross-linked polystyrene until the adsorption of metal-cyanide anion by such resin is exhausted as measured by the presence at the outlet of the column of the metal salt such as gold. Thereafter, in a recovery cycle, potassium hydroxide is first pumped through the column in a concentration of 1 molar releasing gold in the form of potassium gold cyanide to a substantial percentage from the resin. The bed column is then flushed with six-bed volumes of water with the resulting solution of potassium hydroxide, potassium gold cyanide, and water then evaporated until the potassium gold cyanide approaches 90 percent of saturation. The water condensate from evaporation is then available for reuse and the remaining solution is chilled whereupon the potassium gold cyanide crystallizes and can be removed. The potassium hydroxide solution being returned for reuse in the recovery cycle for extracting the metal cynadide anion from the resin in the various bed columns. As a final part of the process, the resin in the given bed column is then treated with potassium thiocyanate pumped through the column followed by a six-bed volume flush of water with the resulting solution of potassium thiocyanate, the remaining potassium gold cyanide and water being subsequently collected and evaporated with the condensate water being made available for reuse and the evaporation maintained until the potassium gold cyanide salt approaches saturation in the solution. That remaining solution is then chilled with the potassium gold cyanide salt crystallizing out and the potassium thiocyanate solution is returned for reuse in regeneration of resin. The apparatus of the invention features a plurality of columns with means to detect the presence of gold discharged therefrom as a control to switch columns from lead to lag and to facilitate regeneration of the resin therein. The second step utilizing potassium thiocyanate following potassium hydroxide has been discovered to remove up to better than 95 percent of the potassium gold cyanide from the resin. 
    
    
     IN THE DRAWING 
     FIG. 1 is a schematic representation of the apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an apparatus 10 is shown to include a microprocessor 12 connected to sequentially and selectively operate a four-way valve 14 having as a supply four separate effluents. These include a source for water, a source for sulfuric acid, and sources for potassium hydroxide and potassium thiocyanate. The outlet from valve 14 connects via a supply pipe 16 to a series of valves 18 each associated with a bed column, three bed columns, B-1, B-2, and B-3 being shown in FIG. 1. The bed columns are, in a typical construction, comprised of large size PVC tubing having dimensions, for example, on the order of 3 inches by 40 inches and each approximately 80 percent filled with a suitable macroporus weak-base anion exchange resin R to be described hereinafter. Each of the bed columns includes an outlet valve 20 connected to an outlet pipe 22 which discharges the effluent into a suitable waste treatment system. The valves 18 further valve an inlet from a pipe 24 in turn valved at 25 and pumped from a reservoir 26 of gold cyanide rinse W by a pump 27. The valves 20 further connect to an outlet pipe 28 which in turn connects to sampling unit shown representatively as D in FIG. 1 including means associated therewith to detect the presence of gold cyanide anion in the effluent discharge through valves 20. Sampling means D can be a sample bottle. A third outlet, pipe 30, connects to each of the bed columns and discharges to one or the other of two reservoirs 32 and 34 via valves 31 and 33; one reservoir containing potassium hydroxide and potassium gold cyanide salt along with water and the other reservoir containing potassium thiocyanate, potassium gold cyanide salt, and water. 
     The various valves 18, 20, 25, 31 and 33 and pump 27 may be provided with a solenoid action under the control of the microprocessor 12 (through cables not shown) in conjunction with the four-way valve 14. Each of reservoirs 32 and 34 include an outlet pipe 38 connected to an evaporator 40 suitably heated by a heater 46 to cause the water constituent to evaporate from the solution, the water vapor being condensed and extracted as by a discharge pipe 42. Further shown in FIG. 1 is a reservoir 44 which is made to receive the concentrated residual solution from reservoir 40 which is then chilled by refrigerant 48 until reaching the point of crystallization of the potassium gold cyanide salt whereby the salt solutions being crystallizes and the crystals are removed with the particular inorganic base or salt drawn off via a discharge pipe 36 to be returned to the appropriate supply source for subsequent in the system reuse. 
     The invention method contemplates a particular operation of apparatus 10 which will now be described. With the bed columns B-1-B-3 filled to an 80 percent level with a suitable resin, for example, Duolite A-368 from Diamond Shamrock Corporation, Cleveland, Ohio, the first step of the cycle initiated by microprocessor 12 is to open valve 14 supplying H 2  SO 4  via outlet pipe 16 to bed column B-1, valves 18 and 20 being controlled to supply H 2  SO 4  to B-1, and outlet pipe 22 to sulfate the resin. Thus we have: 
     
         2 R.sub.Z NR.sub.2 +H.sub.2 SO.sub.4 ⃡(R.sub.Z NR.sub.2).sub.2.H.sub.2 SO.sub.4 
    
     Next, valve 14 is operated to supply H 2  O to bed column B-1 to flush away excess H 2  SO 4 . At least six bed column volumes of H 2  O should be passed through B-1. This procedure is repeated with respect to bed columns B-2 and B-3, the flush water being valved via 22 to waste treatment. 
     Next, microprocessor 12, having closed valve 14, opens valve 25, and operates pump 27 to supply gold rinse solution via pipe 24 to B-1, the valve 18 thereof being opened to supply B-1 and the valve 20 of B-1 being connected to an outlet pipe 21 connecting to the inlet of B-2 and the outlet of valve 20 of B-2 being connected to an outlet pipe 23 supplying B-3. The rinse water will flow through B-1 engaging the resin therein as follows: 
     
         (R.sub.Z NR.sub.2).sub.2.H.sub.2 SO.sub.4 +2KAu(CN).sub.2 →2R.sub.Z NR.sub.2.H Au(CN).sub.2 +K.sub.2 SO.sub.4 
    
     The potassium gold cyanide of the rinse water will, in the process of flowing through B-1, combine with the resin therein, releasing the sulfuric acid constituent. During the pump through of bed column B-1, the output therefrom is monitored via the connecting pipe 28 for analysis by an atomic absorbance unit for the presence of gold ions which presence indicates exhaustion of the resin in B-1. During this time the bed column B-1 serves as a lead column with the columns B-1 and B-3 serving as lag columns. The columns B-2 and B-3 will, of course, be far from exhausted as long as column B-1 is functioning and no gold is detected at D. Upon the detection of gold at D, the valves 18 and 20 of B-1 are operated to close off B-1 to the gold rinse supply and the valves 18 and 20 of bed column B-2 are opened to receive such rinse supply via pipe 24. The bed column B-2 now becomes the lead column with bed column B-3 becoming the lag column. 
     Bed column B-1 is then treated to provide a regeneration by an appropriate manipulation of valves 14 and the valves 18 and 20 of bed column B-1 to admit first potassium hydroxide to the bed column B-1, the discharge being via pipe 30 into the reservoir 34. At this time the reaction is as follows: 
     
         R.sub.Z NR.sub.2.HAu(CN).sub.2 +KOH→R.sub.Z NR.sub.2 +KAu(CN).sub.2 +H.sub.2 O 
    
     The solution of KOH supplied to B-1 is in a concentration of one Molar which is pumped through the bed column followed by six bed columns of water supplied by manipulation of valves 14 and 33 under control of microprocessor 12. The solution including the potassium gold cyanide, potassium hydroxide, and water is collected in reservoir 34. Next, by an appropriate manipulation of valves 14 and 31 the bed column B-1 is pumped with a one Molar concentration of potassium thiocyanate followed by six bed columns of water, the resulting solution flowing into reservoir 32. This results in the following: 
     
         R.sub.Z NR.sub.2.HAu(CN).sub.2 +KSCN⃡R.sub.Z NR.sub.2 +HSCN+KAu(CN).sub.2 
    
     Thereafter, through appropriate manipulation of the valves 14 and 18 and 20 of bed column B-1, a 5 percent solution of sulfuric acid is pumped through B-1 followed by a flushing with six bed columns of water and thereafter B-1 is reinstalled in the process as a lag column until such time as it becomes designated as a lead column upon the exhaustion of the other columns. 
     The solution in reservoir 34 is then separately pumped by means not shown into the evaporator 40 wherein sufficient heat is supplied at 46 to boil off the water portion of the solutions the water being condensed and taken off via pipe 42 for reuse in the system. The remaining solution is monitored by means not shown until the concentration of potassium gold cyanide in solution approaches saturation as for example, 90 percent of saturation, at which time the residual solution is removed to the reservoir 44 via pipe 45. The solution in reservoir 46 is chilled to the point of crystallization by 48 with the potassium gold cyanide salt crystallizing out and the crystals are removed for reuse in the plating operation. The remaining potassium hydroxide solution is then pumped off for storage via pipe 36 for reuse in the apparatus 10 via pipe 36. An identical procedure is then repeated with respect to the potassium thiocyanate, potassium gold cyanide, and water in reservoir 32. 
     It has been discovered that the use of only one of either potassium hydroxide or potassium thiocyanate removes no more than 60 or 70 percent of the potassium gold cyanide from the resin and requires a repeating of the step many times over whereas the use of both the base and acid constituents with respect to regeneration achieves a better than 95 percent removal of the potassium gold cyanide transferred to the resin from the gold rinse water. 
     With respect to the apparatus 10 and the use of the Duolite A-368 resin, a reuse by regeneration may be expected to have a life of about four years. The operating temperature of the fluid system for apparatus 10 should be in the range of room temperature up to 140°. The resin capacity of Duolite A-368 is about 250 tray ounces of gold per cubic foot of resin when operated under ideal conditions, the capacity under more typical operating conditions being from about 100 to 200 tray ounces per cubic foot, depending upon the waste stream. 
     It is contemplated that other resins having a higher operating capacity, higher resistance to oxidation and in essence being macroporus polystyrene in a cross-length matrix may be employed. Duolite A-368 has an operating pH range of 0 to 9 and a maximum operating temperature on the order of 140 degrees Fahrenheit in the free base form. 
     It is believed that the invention method and apparatus may be utilized with other precious or noble metals with respect to extraction from rinse water and the like. It is also contemplated that the method and apparatus of the invention may be utilized with respect to extraction of precious and noble metals found in relation to other processes than electrodeposition. 
     Having now described the invention in terms intended to enable a preferred practice thereof, I now define the invention through the appended claims: