Part one of our improved practices series covers the liberation of gold from hard rock. We’ll take a look at current practices used by artisanal small-scale gold miners (ASGM) and examine improved methods that mitigate the use of harmful chemicals while boosting productivity and yield.
The state of naturally existing gold varies, and so does the way humans retrieve it. The two primary types of gold deposits are known as lode and placer deposits.
Lode deposits are characterized by veins of gold trapped in hard rock.
In placer deposits, gold exists in tiny flakes sometimes fully liberated and other times attached to bits of rock.
Oftentimes, the gold found in various types of placers is already fully liberated and only requires concentration.
When the gold is attached to other minerals, it’s known as ore and must be milled in order to separate out the gold and concentrate it. Here, we’ll focus on lode deposits and how gold is liberated through the process of crushing and milling.
Current ASGM Practices
Primary Crushing: Ore needs to be reduced to the ideal size required for milling. After gold-bearing ore is extracted from mines, it is carried to the surface to be broken down. This is often done by hand with a hammer or mallet.
The miner will sit for hours, processing the rock manually until it is reduced to gravel. A more advanced but less used method is the stamp mill, which uses heavy weighted objects to pound ore into smaller sizes for further processing.
Milling: The goal with milling ore is to crush it into as fine a particle as possible, thus liberating the gold from anything it might be attached to. Ore is milled in two ways; with and without water.
In the case of dry milling, the additional step of laying the gravel-sized ore out on tarps in the sun is added. Hammer mills and ball mills are then utilized to pulverize the ore, though often inefficiently.
Conducive milling should yield a product with an even grain size, preferably 30 mesh (0.6 mm) or smaller. In a vast majority of small-scale milling operations, the ore is only milled to 10 mesh, which results in gold that is unliberated and unrecoverable. Miners endure unsafe working conditions, back-breaking manual labor and exposure to harmful dust and chemicals only to recover thirty percent of the gold in their ore due to inefficient processes and equipment.
Improved Practices
Primary Crushing: Jaw crushers are highly efficient at primary crushing. MBMM offers a full range of jaw crushers that are optimally sized for small-scale processing needs.
Our crushers feature cast manganese jaw plates and can be powered via electric, gas or diesel motor. The processing capabilities range between one and sixty tons of ore an hour, and the discharge size of gravel is uniform and can be adjusted from 4” to ¾” in size. They are an excellent example of how improved technology can elevate the potential of a grassroots industry and eliminate inefficient processes.
Milling: Some upgraded technology has been widely adopted by the ASGM already. Tanzanian ball mills are widely used, for example, but samples of tailings show a wealth of unliberated gold. The difference lies in the quality and design of machinery, and all that missed gold goes to show that present tools and techniques are in need of change.
Our improved technology hammer and ball mills deliver a product that is uniform in particulate size and small enough to liberate the gold from its surrounding ore.
The hammer mills produced at MBMM can run wet or dry material and discharge between 1-5 tons an hour with 70% of processed ore passing 30 mesh.
Our ball mills represent the ultimate in processing capability and can run continuously, 24 hours a day, 7 days a week, 365 days of the year.
The size of discharge can be customized to specification, ranging between 65 and 300 mesh, and the throughput varies from 0.3 and 11.0 tons/hour depending on mill and target discharge size.
If you’d like to see our equipment in action, have a look at the demo videos below or visit our website.
Crushing gold-bearing quartz ore with a mobile jaw crusher