Gold concentrates, precipitates, and Merrill-Crowe product can have mercury is variable proportion. If we smelt the charge directly at 1100 oC, mercury will be released to the environment and the smelting place can be a dangerous place. In order to avoid this situation, is necessary to eliminate mercury by retorting. The same considerations taken in the amalgamation process are valid for this stage. The small amount of elemental mercury that is extracted from the host rock can affect workers health. Considering the fact that cyanidation is the main gold recovery process and indirectly mercury by a factor of 2,000 – 5,000. The last one needs a small volume of high grade gold solution that is suitable for final recovery. Elemental mercury vapor is released when the pregnant leach solution undergoes the recovery process required to produce dore bars.
The hazards associated with mercury exposure include inhalation, absorption and ingestion. As an air contaminant, the main concern is inhalation of vapors as results of releases. Acute exposure to high levels can lead to severe lung damage and death. This poisoning has three phases, initial exposure results in flu like symptoms lasting between one to three days, followed by signs and symptoms of severe pulmonary toxicity; the final phase is mouth sores and ulcers with memory loss, depression and insomnia. It must be mentioned that these symptoms are characteristic of high exposures. Then, this step is very important. OSHA has established a permissible exposure level of 0.05 mg/m3 based on eight hours average time.
Retort
There are several types of retorts and the final selection is based on gold production, mercury recovery system, and safety conditions. The charge is heated slowly until boiling point of mercury is reached. At this point the mercury is eliminated and condensed in cooling tubes passed through water. Obviously, some losses can be produces and this can be estimated as 0.065 grams per 255 grams of mercury. The process takes two to three hours. The final product is composed by gold, silver, sulphides and base metals.
Although the process is simple, the elimination of mercury is not totally perfect. The problem has two causes; first, flouring or tiny mechanical reduction of size during grinding. The second cause is due to chemical reactions that forms coatings or layers of some impurities. These coatings can be oxides, sulphides, or sulphates of some base metals that present in the ore. The employment of mercury which contains with base metals improves mercury elimination. Oxides are formed by weathering and they are not soluble in mercury, and then float on its surface. Gangue minerals are other possible source of coatings due to recovery and concentration processes are no totally efficient.
Retorting Process
The mercury retort must be located in a special place. Basically, the room should be kept under a negative pressure with respect to the rest of the building, which can be accomplished by using either fans or local exhaust ventilation. The makeup air vents for the room exhaust ventilation system must be located on the wall opposite the refiner fresh air supply plenum. The retort has to be operated and maintained in accordance with the vendor recommendations. Test must be performed to know the optimum operating parameters prior to start its service. This will minimize the potential of sending mercury contaminated to the flux mixing station and furnace section. During the operation the retort must work under vacuum and the air exiting the water condensation system has to be scrubbed with a mercury vapor removal system such as a filer with activated carbon. Then, the flow can be introduced into the vacuum system. The scrubber system is required due to the water condensation system is not efficient.The condensed mercury vapor is trapped in a condensation pot. Normally, liquid mercury can be removed from the container by opening a valve and draining into other container. The mercury is then poured into a flask. This procedure must be done carefully.
Mercury Bottles