Newmont Mining Corporation

We use alternative gold recovery methods in some processing plants to accommodate different ore characteristics or other requirements. For example, ore that has a high level of sulfide minerals or carbon (or both) is called refractory ore. Refractory ore resists normal processing methods as the high sulfide minerals trap gold particles, making it difficult for the cyanide to reach the gold and leach it.

Heating

To leach gold from refractory ore, it must be subjected to high temperature, high pressure and/or oxygen. Newmont treats refractory ore in two ways: by using an autoclave or a roaster.

An autoclave is used before leaching occurs. First, the slurry is heated and fed into an autoclave, where high-pressure steam, water and oxygen are applied to oxidize the sulfide material by a chemical reaction. The slurry is then cooled and sent back into the process to be leached.

An alternative to an autoclave is a roaster, a very high temperature oven that is often used instead of an autoclave if the ore to process contains a large amount of organic carbon. Roasting uses heat and air to burn the organic carbon into fuel and to burn the sulfur off ore, which we heat to 932 to 1,202 degrees F.

Heap leaching

In heap leaching, we dump crushed ore into piles called heaps, where we apply a weak cyanide solution, using drip feeders, to the ore. The gold dissolves into the cyanide solution. The entire heap leach area is lined with heavy duty liners to ensure no solution leaks into the environment. Next, we collect the gold-cyanide solution in ditches and ponds, and then transport it to a recovery plant.

Flotation

Flotation is a method of separating minerals depending on their ability to attach to air bubbles. 

Flotation can be used for a number of materials by adjusting the chemicals. At Newmont, it is used for copper recovery and, in a very limited number of cases, for gold processing.

We introduce air bubbles to the slurry while it is in small tanks, called flotation cells. We add some chemicals to the slurry to assist the process. The desired minerals stick to the bubbles and rise to the top, resulting in froth. The froth overflows from the tank, and is removed and sent to the next step in processing.

Gravity circuit

A gravity circuit recovers course gold before it is leached. Gravity circuits use the same principles as gold panning: course gold is heavier than other material and will settle to the bottom so that it can be removed (gold is 19.3 times heavier than an equal volume of water).