All minor elements react with the electrolyte. For example bismuth present in the anode is converted into bismuth oxychloride and if hydrochloric acid is not present in sufficient quantity, it will precipitate in the anode sludge. The greater part of iridium and other elements of the platinum group metals, except platinum and palladium, remain undissolved and is precipitated with the sludge. Lead present in the anode is dissolved and forms lead chloride until the electrolyte is saturated. When this occurs, it crystallizes with the sludge. If large amount of lead is present in the bullion anodes, both electrodes and the internal surface of the tanks and the surface of the electrolyte become coated with crystals of lead chloride and it advisable to avoid inconvenience of the formation of these coatings of lead chloride crystals by adding to the electrolyte sulphuric acid about equal in volume to the hydrochloric acid present.
It has been noted that lead forms an insoluble sulphate, which passes to the anode sludge. If the anode bullion is rich in silver or lead, it may become coated with a layer of silver chloride and lead chloride or sulphate as interfere with the electrochemical action. Then, the anode surface must be kept clean by an automatic scraper that acts periodically at intervals depending upon the rapidity with which the layer is formed. Part of lead and bismuth, all the platinum and palladium pass into solution as chlorides, but they are not precipitated with the gold and their presence is favorable to the formation of a dense and pure deposit of gold on the cathode.
Platinum is recovered from the electrolyte from time to time when sufficient quantity is dissolved. In this case, gold is precipitated by means of ferrous sulphate and then the platinum as ammonium platinum chloride by adding ammonium chloride to the solution. The palladium may be precipitated as iodide by the addition of potassium iodide. The gold obtained has a fineness of over 99.98% and the main impurity is silver, which is dissolved by the hydrochloric acid present in the electrolyte.
Potassium chloride must not be employed due to promote the precipitation of platinum chloride in the sludge and small amount in the cathode. Basically, the formation of chlorides by the impurities in the anode bullion and considering that gold is precipitated at the cathode, must gradually render the electrolyte weaker and weaker in auric chloride. The reduction of the amount of auric chloride present is improved from time to time by the addition of small quantities of this salt in solution.