Desorption

There are two alternatives which are radically different for recuperating the gold and the silver from the loaded carbon. The first, sometimes adapted in small operations where the capital is limited for investing in installations, is to sell it directly to the smelter to recuperate these elements through calcination of the product and fusion of the residue. The second alternative, which is more prefferred than the previous one mentioned, consists in disorbing the precious metals and afterwards recuperate them by electrolysis, allowing to use the carbon again.

The first practical procedure of desorption or elusion of gold and silver was developed in 1952 by the U.S. Bureau of Mines, and is frequently referred to as the Zadra procedure. In this, the operation is performed in a column where one places the loaded carbon and you make it pass in an ascending formto a flux of between 1,0 and 1,3 B.V./hour (#), one solution to the 1% NaOH + 0,1% NaC, to 93°C and at an atmospheric pressure. The eluid containing the disorbid gold and the silver is fed in counter-current to three circular electrolythic cells in series, provided of a stainless steel cylindric anode, and a cathode of steel shaving within a basket of perforated plastic where one goes depositing these values in the measure which they are removed from the carbon. The cathode of the first cell is removed periodically from the circuit while the other two advance a place, placing a fresh cathode in the last one. This technique of simultaneous desorption and electrolysis with return of exhaust electrolyth occupies a minimum inventory of solution, reducing the necessities of reactives.

Not withstanding, the desorption of the gold through this procedure proved to be excessively slow in the practice, needing 48 hours or more of operation to download the carbon to economical limits, acceptable in the industry (22, 23). The strong dependence of the temperature found in future studies, lead the same U.S. Bureau of Mines to develop a new method of desorption through pressure which reduces the time of operation to less than 8 hours in making the solution pass by the bed of carbon to a flux of nearly 6 B.V./hour to 150°C and 52 psi; in these conditions the addition of cyanide to the eluyent solution can be ommitted, and the concentration of caustic soda, diminish to up to 0,4%. Thedesorption os equally effective with water by pressure and temperature, after conditioning during thirty minutes the loaded carbon with the solution to the 1% NaOH + 0,1% NaCN used originally by Zadra. The development of this method of desorption through  by pressure was accompanied by a perfectioning in the design of the electrolythic cells.

In further studies it was possible to determine that the gold and the silver can be desorbed in a very fast velocity of les than two hours with deionized water at 110°C and in a pressure of 12 to 18 psi, after a pretreatment of soaking in heat of the loaded carbon with a solution to the 1% NaOH + 3% to 5% NaCN during 15 to 30 minutes; depending of the pelletmetry of the carbon, the deionized water is fed to the column of desorption to a flux between 3 and 5 B.V./hour. A substantial change in the speed and in the efficiency of desorption is obtained also by adding soluble alcohol (methanol or ethanol) to the alkaline solution of cyanide by reason of a 20% in volume, reducing to only six hours of operation the 48 or more required by the method of Zadra. This procedure, which uses the same concept of simultaneous desorption and electrolysis, has the advantage of operating at only 80°C and at atmospheric pressure.