Final Recovery Methods

Final Recovery Methods

After processing the diamond bearing material in to concentrated form, final recovery of the diamonds is done by taking advantage of two further unique physical properties of diamonds – that they are hydrophobic, and that they fluoresce when exposed to x-rays.

Grease Tables

By being hydrophobic, the surface of diamond resists being wetted by water, but sticks readily to grease. This unique feature allows diamond miners to utilize "grease technology" which generally comes in two forms: vibrating grease tables and automated grease belts. Both of these methods apply the same principle but are differentiated in the amount of automation and security they offer as well as by costs. The most common design for grease tables is that of a small, stepped table which special "collector" grease is spread onto by hand. Water is applied through sprayer bars over the vibrating table as the concentrate passes. The diamonds stick to the grease while wetted garnets, ilmenites, and other heavy minerals commonly found in kimberlites travel over the grease and are carried down the table into the waste stream. A thin layer of the grease containing the diamonds is then scraped off and deposited into a "boiler" in order to separate the diamonds from the grease.

Grease Belts

Automated grease belts function in a similar way by replicating the process without requiring the manual application of the collector grease or the hand scraping of the surface area which the diamonds have adhered to. The wetted concentrate is fed across the belt continuously rotating perpendicular to the flow of the concentrate. At one end the belt is coated with a thin layer of grease prior to it encountering the concentrate. A blade at the other end automatically scrapes clean the entire surface of the belt and deposits the grease and diamonds into a boiler below. The entire system is enclosed and thus "hands-off" adding a security feature to the process which can not be understated. Of course, the automation of grease belts come at a price and are generally twenty to thirty times more expensive then an average vibrating grease table.

X-Ray Fluorescence Separator

The second unique physical property of diamonds, that they fluoresce when exposed to x-rays has allowed for the development of x-ray sorter technology. An x-ray fluorescence separator works on the basis of diamond emitting light (fluoresces) when exposed to x-rays. A thin stream of particles from the concentrate falls through the air under an intense beam of x-rays. When a diamond fluoresces, it activates a photodetector that triggers a jet of air which deflects the diamond in to a collector box. This system is also completely enclosed or hands-off and provides significantly increased security in during the final recovery process.

Care must be taken to accurately analyze the type of diamonds produced from each deposit as they will have an effect on the final recovery and determine which technology is most suitable. Certain diamonds may be coated with material that is "wettable", requiring treatment before the recovery process. Other diamonds, particularly the more valuable type II stones, do not respond well to x-rays, requiring the use of grease technology to be used to recover the diamonds.

Two-Stage Recovery

While either final recovery technology may be used as a "stand alone" process, more miners are beginning to understand the benefits of implementing a two-stage final recovery system utilizing both x-ray and grease technology. In this system, an x-ray sorter machine would have a grease belt or grease table inline following it so that all concentrate would pass through both systems. Any diamonds thus not recovered by the x-ray machine should then be captured by the grease technology thus maximizing recoveries.

The last step in the diamond recovery process is to collect the material from the boilers or x-ray boxes. The strictest of security procedures at this stage is essential as the small amount of material which comes out is often made up of 50% or more of rough diamonds. This highly concentrated material containing the diamonds is then washed and dried before the final hand sorting can take place.