This series of posts is an attempt to use the extensive mining knowledge we have obtained through years of helping set up mining operations to help you avoid the mistakes others have made.
As an engineer for GWP, I’ve had the opportunity to work at numerous placer mining operations around the world. Some succeeded, and some failed. Some had their equipment seized by hostile government agencies. Some were unable to establish an economical supply chain due to inadequate infrastructure, Ebola, and a civil war. Meanwhile, some seemed to do everything right. However, the vast majority of failures were the result of a single issue: inadequate and inaccurate sampling.
Extensive prospecting and geological analysis are critical pieces to a complicated puzzle when deciding where to mine. It’s important to know exactly where the gold is, why it’s there, the composition of the surrounding head ore, and the geology of the vicinity. The sampling process may take several months (or years depending on the permitting process!). If this is the stage you’re at, that’s exciting! Don’t skimp! That’s the biggest mistake you could make. If you need expert advice or boots on the ground to help guide the process, we have in-house resources available.
Another common mistake is choosing the wrong machine size or type to process the ore. For a placer operation, there are two major machine types: vibratory gold wash plants and gold trommels. Although they are both designed to sort material and extract gold, they have significant differences that require consideration. For our purposes, this series of articles will compare a GWP vibratory gold wash plant with a generic trommel.
One of the major differences is machine size. The footprint for a GWP 50 ton/hour unit is less than half the size of the footprint of a comparable heavy-duty trommel. In my opinion, this difference in size is due to the frequency of the scrubbing action on the respective machines. Even the fastest-spinning trommel can only rotate at 60-100 rpm, compared with the 600-1000 vibrations per minute for a vibrating grizzly deck. The increased size of the trommel may or may not be an issue, depending on the specific area to be mined, particularly if the head ore NEEDS to be trucked in for processing at a central location where the wash plant or trommel is located. That being said, think very carefully about all the options before committing to an approach that involves trucking material to a central site. There are significant overhead and equipment costs associated with handling the head ore two or even three times before processing. Increased expenses mean that the ore needs to be richer or more ore needs to be processed in order to remain profitable.
Another major difference between the two machines is the type of material that can be processed effectively. When faced with the heavy red clay found in northern South America and elsewhere, the capacity of a vibratory wash plant is reduced to as little as 1/3 of the rated throughput. This means that in a worst-case scenario, the grizzly deck of a GWP 200T is only able to process around 70 tons per hour when dealing with heavy clay and organics. The tumbling action of a trommel will often result in a smaller reduction in throughput for the machine, and a trommel nominally rated at 200 tons/hour may be able to achieve 50% of the rated throughput. On the other hand, most trommels limit the maximum size of material they can process to around 8 inches or less because the weight of the stones can damage the screen as the drum rotates. If the head ore contains rocks that are larger than this, preprocessing through stationary grizzly screens becomes necessary, which increases gold losses and reduces throughput. In contrast, the vibrating grizzly deck of a GWP 50T unit can handle 12-inch stones, and the larger 100T and 200T machines can easily process up to 20-inch boulders because they are supported by heavy-duty 1” thick steel grizzly plates.
In the next article, we will address differences in water needs and machine maintenance.