Refining processes are employed to improve the gold product quality. There are several types of refining procedures employed, but some of them are selected according to the daily production. It is important to mention that the gold purity is variable according to the process employed and the worker’s experience. If the final content is important, some assays must be performed. This point is valid when there is a previous commitment on gold quality. Other aspect very important to mention is about impurities, if they are difficult to remove, the process selected must be changed. Ensuring that most of gold is recovered is a very important aspect so that the production cost can justify the operation. Finally, there are aspects related to health, safety and contamination that have certain influence in the process.
The techniques employed are Cupellation, Inquartation and Parting, Miller Chlorination Process, Wohwill Electrolytic Process, Fizzer Cell, and Aqua Regia Process. The final election is based on the initial gold content. For example, the Whohwill Process is selected if the gold content is not less than 98%, the Aqua Regia process is a good option when the silver content is less than 5%. It the charge to be refined contains platinum group metals such as platinum and palladium, the Inquartation and Parting Process can be selected. In general the final selection is not easy and is good practice to equip a facility to perform more than one process.
The Cupellation process is employed in very small scale and is based on the fire assay process. The final product can collect gold and platinum group metals. Perhaps, small miners can considers this option when there are economical restrictions and the final product quality is not very important. The Inquartation and Parting Process is based on the solubility between gold, silver and copper. Initially there is an impure product obtained my smelting that must be grained to improve the surface area so that the metallic product can be attacked with nitric acid in order to dissolve copper and silver and the leaching residue is metallic gold. During the acid attack, platinum and palladium forms part of the rich solution. However, Iridium, Ruthenium, Rhodium and Osmium are not dissolved. The gold purity is close to 99.99% when the platinum group metals content is very low.
The Miller chlorination process is well know and has been practiced for a long time in the gold refining industry. The process consists of chlorine addition into the molten Dore by using an immersed tubing system. Initially there is a slow reaction of chlorine gas with base metals forming volatile compounds. Once is complete the initial reaction, there is a fast generation of non-volatile compound called Miller Salt that must be removed from the surface of the molten charge. It is important to avoid the formation of gold chlorides in order to avoid losses. One solution is to the problem is to cover the molten charge with mixture composed by borax and silica. The end of the process is noted when appear purple fumes of chloride gold. Platinum group metals are not removed and gold purity is between 99.5 to 99.8%. Although this technique is old, the process has to be operated by skilled workers and the facilities must have an excellent exhaust system in order to avoid contamination by chlorine.
Other process used for a long time is the Wohlwill Process and is considered as the second part of the Miller process due to refine its product. This is an electrolytic process based on the dissolution of gold in an acid bath prepared with chlorine gas and hydrochloric acid. The final product is a gold cathode with 99.99% purity. Although the process is efficient, there is an issue related to the cost due to it is necessary to heat and apply a specific current density such as 110 amperes per square foot. Platinum group metals and silver are reported on anode slimes and the remaining base metals are in solution.
Impure Gold from Electrowinning Process Wohlwill Process
The Fizzer Cell Process is an improved Wohlwill Process whose main change is the cathode design. There is a porous ceramic container that works like a membrane whose purpose is to avoid gold losses and adhesion of some impurities on cathode surface. The electrolytic cell is drained and filtered. A very important aspect of this process is referred to the possibility of treating impure anodes, basically when the silver content is as high as 10%.
The Aqua Regia Process is appropriate to be employed in small scale and the metallic gold to be obtained is as pure as 99.9%. The main steps of the process are to dissolve the precious metals and some impurities. Since the Aqua Regia is a mixture between nitric acid and hydrochloric acid, part of the silver will remain in solution and other part will form a silver chloride precipitate. Then, it is important to be careful with the silver content. It must be mentioned that the silver chloride formation has a negative effect on gold dissolution. Once the dissolution is performed, the solution and the precipitate are separate by filtration operations. The solution contains gold and is treated with reducing agents such as ferrous sulphate, sulphur dioxide or sodium bisulphate. Gold powder of high purity is obtained and can be melted later. When gold precipitates, platinum group metals remain in solution and have to be recovered by other procedure.
Gold Refining Steps | Effect of Market and Environment on Gold Refining | Pre-Refining Processes | Silver Recovery | Gold Electrorefining | Gold Chemical Refining | The Wohlwill process | Effect of Impurities on the Wohlwill Process | Wohlwill Process Operation | Miller Chlorination Process | Effect of Impurities on the Miller Chlorination Process | Transference of Gold and Silver into the Slag | Operation of the Miller Chlorination Process | Refining of Precious Metals with Sulphuric Acid | Recovery of Gold from the Sulphuric Acid Residue | Recovery of Silver by the Sulphuric Acid Method | Parting with Nitric Acid | Recovery of Gold from the Parting Residue | Recovery of Silver from the Parting Solution | Gold Refining by Cupellation | Cupellation Process Operation | Gold parting by using the Dry Method | Gold Parting Process adding Sulphur and Litharge | Gold Parting Process by adding Sodium Chloride | Introduction of the Miller Chlorination Process | Operation of the Miller Chlorination Process | Parting of Gold Alloys by using Nitric Acid | Parting Process using Nitric and Sulphuric Acid | Parting Process using Sulphuric Acid | Effect of Impurities in the Parting Process with Sulphuric Acid | Removal of Copper from Dore Bars | Inquartation of Gold and Silver | Treatment of the Silver of the Refining Alloy | Operation of the Dore Refining Process | Treatment of the Refining Residue to Recover Gold | Refining Process of Granulated Dore Bars | Treatment of the Silver Sulphate Solution | Operation of the Silver Sulphate Treatment