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Recovering Diamonds

The main equipment types for diamond recovery in exploration from smallest (and cheapest) to largest (and more expensive) are jigs, diamond pans and heavy media separation plants.

Jigs are typically used for small (± 1 tonne) bulk samples and are the lowest cost option for sample processing provided the diamond source rock can be broken down effectively to release (or "liberate") the contained diamonds. The feed must be closely sized and the feed rate is very low. Jigs can also be used for the reduction of concentrates from diamond pans prior to hand sorting.

The diamond pan was developed on the South African diamond fields and was designed to treat soft "yellow ground" (weathered kimberlite) with minimal water consumption. The "pan" can treat un-sized feed at a feed rate of about 1 tonnes per hour. An experienced operator is required as the separation medium is mud derived from the clays in the sample. The correct viscosity of the medium (mud) is essential for the efficient operation of the pan. Pans can be used in areas of limited infrastructure as they are simple in construction and easy to maintain. They are also useful when dealing with a small sampling program where only a few samples require treatment and the program cannot justify the establishment of a larger heavy medium separation plant (see below).

The heavy media separation (HMS) plant has been used in diamond exploration and commercial diamond plants for many years and is the main process used in the Western World for commercial diamond recovery. Exploration applications use a small HMS plant capable of processing about 7 to 10 tph. These types of plants have been described by Abate (1993) and Popplewell G and Smith J. (1992). The feed material is prepared by crushing (if necessary), prior to scrubbing (small autogenous mill), and recrushing if necessary, before being fed to a heavy media cyclone for mineral separation. Operation of the plant is complex and experienced personnel are required. Access to a large water supply is a pre-requisite and some programs in the more arid climates have required the establishment of a water supply borefield before processing can begin. Water consumption can be high (approx 20,000 l/h) and recycling facilities (e.g. settling dams) are almost essential. The advantage of the HMS plant is that it can treat large samples with usually good reduction to concentrates of less than 0.1% of the original feed tonnage, depending upon the heavy mineral content of the samples. The disadvantage is that reasonable logistical support is usually required and its operation can be expensive.

Final diamond recovery in the field from the concentrates produced by jigs, pans or HMS plants are usually by either hand sorting, jigging, grease tables or X-ray sorters, or a combination of one or more of these methods. Hand sorting is slow, particularly in the small (less than 2 mm) sizes, and requires a degree of skill or training for the sorters. The diamonds that are present in the larger sizes are usually easily and quickly identified. The smaller sizes are more difficult and need to be checked by further processing by grease tables or x-ray sorting machines.

The material is now mixed with ferrosilicon and passed through a gravity separating process which separates the diamonds and other heavy minerals from the lighter waste minerals.

The diamonds are separated from the heavy mineral mixture by an ingenious machine that uses the X-ray fluorescence properties of minerals. The concentrate passes through an X-ray beam, causing the diamonds to fluoresce (glow). A photo electric cell detects this fluorescence and triggers off an air jet, blowing the diamonds into a bin for collection. This method is very fast and 99% effective.

The Argyle processing plant is one of the most efficient in the world. It was recently expanded to process up to 10 million tonnes of ore per year - a significant increase from the mine's first full year production of 3.3 million tonnes.

The diamonds are cleaned in an acid solution which dissolves any other minerals which are stuck to the diamonds, then transported to Perth.

In Perth, the diamonds are sorted according to size, shape, colour and quality, before being offered for sale on the world markets. The three main classes are industrial grade, low quality gems and high quality gems. The large, clear gem stones are categorized according to colour, clarity and size.

Very small and poor quality stones (very dark coloured or containing numerous inclusions of other minerals) are classed as industrial diamonds. These are used as the abrasive material on cutting wheels and drill bits used for mineral and oil exploration. Other uses include machine bearings, glass cutting implements, and as grinding and polishing powder.

Gem quality diamonds are cut into a variety of shapes and polished to reveal their true brilliance. Usually 50%, sometimes as much as 80%, of a rough diamond is lost during cutting and polishing