the smelting process treats Merrill-Crowe precipitates or cathodes from electrolytic cells to produce a Dore bar. Once performed the retorting process, the product obtained is heated in a furnace in the presence of slag forming fluxes at temperatures over the melting point of all the components of the change, commonly between 1150 to 1450 oC. This operation is performed for around two hours to ensure complete separation of impurities from the precious metals. Gold and silver form an alloy that is heavier than the slag and sinks to the bottom of the furnace. Once the smelting is complete, the slag is removed and the precious metals are casted in ingots. The slag contains gold and silver and must be reprocessed in order to minimize the losses of precious metals.
The flux is a very important element of the smelting process and is added to the charge in order to remove base metals and other impurities from the Dore bar. The mixing of charge and fluxes has to be done in ventilated areas. A good practice is to vacuum the flux and charge from the retort pans either directly into the furnace or into a hopper that will feed the mixture into the crucible. In this way, personnel will not be in direct contact with the flux or any residual mercury vapor. In general, the flux and the retorting product must be mixed with safety.
The flux is a mixture of several reagent chemical such as borax, silica, sodium nitrate, sodium carbonate and fluorspar. Borax is a white crystalline mineral used in glass and ceramic enamel mixes. In the smelting process, borax helps to reduce the smelting point of the charge and capture metallic oxides. The addition of this reagent must be controlled; otherwise the slag will be extremely hard and non-homogeneous. Other negative effect is to avoid the separation of phases into the molten charge. Silica is an acid flux and reacts with metallic oxides forming stable silicates. Excessive addition creates slags with high viscosity and metallic oxides are no trapped with efficiency. Normally, the borax/silica ratio is 2/1. Sodium nitrate is an oxidant for iron, copper and zinc. When the charge is near 500 oC, sodium nitrate produces oxygen. Sodium carbonate is an alkaline flux whose melting point is 850 oC and reacts with silica forming sodium silicate. The latter reacts with oxides and produces other oxidized compounds. Sodium carbonate must be added with caution due to an excessive addition creates sticky slags that are difficult to remove. Fluorspar modifies slag viscosity because modify the silicates formed during the smelting process. An excessive addition attacks the crucible and reduces borax action on metallic oxides.
When the flux and the charge are mixed in the right proportion, the slag will take the right smelting point, density and viscosity are low, fluidity is good, metallic oxides are eliminated easily; precious metals losses are reduced at minimum and can be broken easily. Separation efficiency between precious metals and slag is evaluated by taken slag samples and perform assays for gold and silver. The performance depends on the charge properties, gold and silver content and fluxes efficiency.During the smelting process, any residual mercury contained in the concentrate will be eliminated in the first moments. An exhaust hood must be placed over the furnace to extract mercury and other fumes released during the heating process. In the market exists two types of hood, one considers and annular hood mounted on the top of the furnace, and the other design considers an enclosure around the furnace. Depending on the silver content, the process can produce silver fumes. Then the Dore bar should be allowed to cool in the molds to facilitate removal of the bars after they have cooled. An exhaust hood must be considered in case silver fumes can be a problem.
The molten charge has to be sampled by using a glass vacuum tube or a simple ladle to take a deep sample. Dore bars are cooled after they are poured and then chipped to remove slag. This step can be done by using a pneumatic chipping gun or automatic descaler. Probably, considering the daily production, the best option must be chosen.Once the slags were removed and cooled, will be stored for further processing. Precious metals can be recovered by gravimetric devices and cyanidation. A common practice is to crush the material until 100% passing 20 mesh. The material is processed in a shaking table and the concentrate is smelted again and the tails is crushed again in order to have a finer size, which must be appropriate for cyanidation. Intensive cyanidation is a good option, but obviously, the material can be sent to the leaching process employed to treat the raw ore. Gold and silver recoveries are between 99.5 to 99.7%. Although slag production weight is variable, a typical operation can generate 10 kilos of slag per 1000 oz of gold.
The precipitate obtained in the Merril-Crowe process is very sensitive during the smelting process due to the process employ diatomaceous earth as a filtration medium. This material contains silica. Then, it is important to know the silica content in the cement so that the fluxes can be added in the right proportion in order to avoid unexpected problems in the process. Also the final product quality is influenced because the silver/gold proportion is modified. Other very interesting point is the degree of oxidation generated by the flux. If the oxygen liberated is high, there will be produced a foaming effect during the smelting process. The foam contains water vapor, metallic oxides fumes and combustion products and creates an appropriate environment to trap gold and silver into the slag. When this happen, it is necessary to observe the molten charge so that we can be sure the foam disappeared. This operation can takes time, but the losses are minimized.
Pouring molten charge into molds
The recipe to be used in variable and is based on the charge composition. For example, 1 ton of concentrate or cement can be mixed with 300 kg borax, 100 kg silica, 10 kilos sodium carbonate, 5 kilos sodium nitrate and 0.5 kg fluorspar. As was mentioned before, the flux has influence on the slag properties and Dore bar composition. Precious metals contents are between 98 to 99%.There is an alternative process that employs fluxes, carbon and lead oxide (litharge). In this case carbon helps to reduce oxides and distribute the temperature correctly in the crucible. Flour is the most known source of carbon. When the charge is taken high temperatures lead oxide is reduced and metallic lead is produced. This metallic lead in liquid state traps gold and silver. As slag and metallic charge have different densities, lead and precious metals go to the bottom of the crucible and the slag floats. In other words, lead reduces the losses of precious metals into the slag. The mixture of heat, fluxes, carbon and lead assure an appropriate reducing environment. When the smelting process is complete, the charge is poured into a mold. Once the slag is cool, this can be separate from lead and precious metals by using a hammer or any device than can break the slag. The metallic product is named button and have to be processed in the other smelting process called cupellation. By this second stage, lead and precious metals are separated. The button is placed in cupels which are porous containers. These empty spaces will absorb lead oxide and the Dore metal is formed. Cupellation is performed under oxidation environment in order o oxide metallic lead to PbO.