On the thirteeth of November Michelle McAnn spoke before our Chemical Engineering class about her career and the processes that go on at the mine she works for in the Elko, Neveda area. Michellle works for a company named Newmont. Their main mining operation is involved in the extraction of gold from ore.
Michelle explained to us a little bit about her history. She is originally form Butte Montana. She has worked for Newmont for five years and her first job out of college was for J.C.I. is South Africa.
Many aspects of Newmont were explained. Some of the more interesting processes that their facilities perform were oxide milling, a roasting plant, and two processes that they developed for gold extraction, Bio oxidation and ammonium thiosulfate refraction of ore. The later processes have given new life to mines that would have been abandoned if these processes did not exist. They enable the mines to extract from raw materials which normally would be waste.
Biooxidation converts sulfuric minerals to soluble sulfates, which can then be used to extract the gold from. Michelle gave a very good analogy to the process by explaining the ore as a chocolate chip cookie. The material you want to get is the chips but you have to get them away from the rest of the cookie. This biooxidation process utilizes two or more strains of bacteria, thiobacillus ferrooxidants and moderate-thermophillic ferrooxidants. These bacteria oxidize, hence the name, the gold ore which enables the refinery to extract the gold in solution. Being an exothermic process, heat is a good sign during the oxidizing stage.
There are three types of ore that must be treated in three different ways; they are Bio Cyanide, Bio Thio, and Direct Thio. As I understand it the process goes as follows:
The first step is to grow bacteria that will be used. These bacteria need a pH value that is between 1.5 and 2.0. They, the bacteria, must have food and nutrients, which come in the form of ferrosulfate, ammonium sulfate, oxygen, and a couple others that I missed. The next step is to crush the ore that comes for the pits. This is done in two steps. First, the ore is ent through the primary crusher which leaves it is pieces averaging three inches. Then, this 3" ore is sent through the secondary crusher, which crushes it to about 3/4 of an inch. This final crush is introduced to the bacteria and set on a leach pad for approximately 270 days. One of the byproducts during this period is sulfuric acid, which also helps keep the pH below the 2.5 barrier. After the 270 day period the pH is brought up by adding lime to the ore as it is fed into a hopper (about 20-25 lbs. per ton of ore). The pH is brought up to about 10. If there is no organic carbon in the ore it goes directly to the leach pad (cyanide treatment). If, in the other case, organic carbon does exist, the ore pile is set on a leach pad and treated with ammonium thiosulfate. Once the leaching is finished, the solution is drained through the pile and sent to the pregnant pond (the solution is considered 'pregnant' because if contains the gold ore). This solution is clarified and sent to the operating plant to extract it by use of a cyanide solution. This cyanide solution is treated with a copper slurry, which precipitates the god. The gold is filtered through a press and sent to the refining plant. The 'barron' solution is then reused in the process.
There are many different tasks that must be accomplished in the process of mining other than the act of digging and extraction. Michelle explained some of the things that go on. The ore must be picked with extreme scrutiny. Clay values have to average around 5-15% or the bacteria cannot bet the ore or have oxygen get to it. The gold grade is usually around 1.055 once per ton. The sulfide values should be between 1.5 and 2.5%. The carbonate value usually should be below .007%.
The process of finding the ore has many different aspects, but the basic idea is you pick an area. A drill sample is taken and analyzed for the ore content among other things previosly mentioned. Once the area is found that the ore extraction is to take place the pit is blasted and dug. Finally, as the pit is being dug, the ore is sampled to decide where it will go and which process will be used to extract the gold.
Some interesting facts that Michelle shared with us included the recovery rates for the different processes. The rates were as follows: Bio cyanide- 60%, Bio Thio-55%, Direct Thio-50%. The current facility that is runnig has produced 1.2 million tons of ore. The gold produces from this ore is 44,000 ounces. The average grade of the ore dug is .040502 per ton.
The talk by Mrs. McAnn was very informative and interesting. The fact that new processes for extraction are still being invented and discoved is rather exciting and leaves many possibilities for those of us going into the field in the future.