Constructive cyclones
Often the ancillary elements of a larger system provide some of the more interesting engineering solutions, as Dan Bailey discovers when investigating mining in southern Africa.
There are two inevitabilities about mining for certain types of minerals such as gold, platinum and chrome: they consume vast amounts of water and produce equally large quantities of waste. For example, one source reckons that for each kilogram of gold recovered, 240 kg of cyanide and 250,000 litres of water have been consumed and over 3,000 tonnes of solid waste and 25 tonnes of CO2 produced.
It follows then that not only is tailing management an expensive part of the process, itÔÇÖs also one that needs to be handled with considerable responsibility. Environmentalists in the remotest parts of the world are there to pressurize mine operators. In the deserts of southern Africa, there is also the practical need to recover scarce water resources with utmost efficiency.
The mineral extraction process, in essence, requires rock to be crushed and ground small enough so that the minute ore particles can be extracted either by chemical or mechanical means. Depending on the particle size, the tailings left behind are referred to as slurry or slime: the former contains a proportion of grit where the grains can be as large as 1.6mm while the latter is predominantly composed of much smaller particles of less than 300 microns.
Historically, tailings were simply discharged directly into streams and rivers. In the west this form of disposal has been outlawed since the 1930s but in some parts of the world it still continues, particularly where the terrain is difficult or the rainfall high. Such blatant disregard for the environment at mines in Papua New Guinea and Indonesia have gone a long way to creating hostility for the mining industry at large as well as burdening the community with enormous environmental liabilities and costs to remedy the situation.
More efficient extraction techniques are now allowing lower grade ores to be mined, creating greater quantities of waste in the process. Tens or even hundreds of thousands of tonnes of tailings a month are not untypical and if storage can only be done on the surface, they call for huge dams to be employed that can stretch several hundred hectares with walls rising to 30 metres or more in order to store them. As the tailings have no commercial value, they are a burden to the mine operator, but they do have the potential to cause problems so their management is not something that can be taken lightly.
The exact nature of the dam design and construction depends to a large extent on the geography in question, but since 1998, Cyclone Projects & Consulting of Krugersdorp in the Gauteng province of South Africa has been offering a process of dam construction known as cycloning. The business was established by a South African engineer, G. J. van Ryssen, who had a quarter of a century of experience in dam and tailings management across a wide range of minerals such as phosphate/copper, gold, platinum, magnetite, chrome, manganese, gypsum, fluorspar and kimberlite. 
Cycloning is a process that eliminates the need for large footprints, which can be both expensive and impractical in many situations. At the same time, it extracts more water from the tailings for reuse.
Van Ryssen identified the need to construct tailings dams by utilizing cycloned slimes/tailings disposal techniques on minerals other than just phosphate. Over the intervening years it has been responsible for much development work in perfecting the cycloning techniques for various types of ore. In fact, since CP&C was established it has carried out the majority of contracts in the design, supply and commissioning of cyclone systems for cycloned dams in Africa.
A cyclone works on the principle that in a centrifuge the heavier particles suspended in a slurry will gravitate to the outside of the flow while the lighter materialÔÇöthe waterÔÇöremains on the centreline through the vortex finder of the cyclone. As a result, tailings pumped through a cyclone will be separated and the solid particles can be ÔÇ£dischargedÔÇØ directly where they are needed as the building material for a tailings dam wall, while more of the slurry bearing water is discharged in the basin of the dam with water being recovered in the process.┬á
In reality, itÔÇÖs not always that simple. Some materials donÔÇÖt lend themselves to dam wall construction. They lack the fundamental stability necessary to build a safe structure without the need for uneconomically wide wall zones. In the past, haphazard design has been a costly exercise for the industry. As such, CP&C recommends that on-site cyclone test work be conducted on the tailings before a custom built cycloned system is designed. Often initial trials are carried out on the companyÔÇÖs in-house cyclone test rig before on-site tests are conducted. CP&C utilizes a mobile test rig to determine the geotechnical properties of the different cyclone streams, creating data which can then be used for dam capacity calculations, the required drain design for that specific tailings product and the design of the cyclone system suitable for the material.
Providing the cyclones is only a small part of the turnkey service CP&C provides. First there needs to be a thorough assessment of the proposal, evaluating the feasibility of building what the client wants. Once the system has been designed to ensure the best cyclone underflow, material is utilized for a safe and stable wall zone and the individual units produced. CP&C sources the entire package of pipe work, valves and instrumentation before completing installation and commissioning of the final system and handing over to the client with operating manuals and maintenance schedules.
Cyclone Projects & Consulting has created any number of industry innovations. For example, at a diamond mine in Botswana, slurry from the kimberlite diamond bearing rock (high in slimes) was being deposited within expensive earth constructed impoundments and very little water was being recovered. CP&C, in the first such plant in the world, showed that by leaving the grit in the slimes stream at the scrubbing plant, cyclones could be used to produce a stable wall and more than double the amount of water being reclaimed.
Again, at a gold mine in the Republic of Mali, West Africa, CP&C demonstrated that cyclone walls were possible with tailings made up of 10 percent laterite and 90 percent saproliteÔÇöa material considered unsuitable for cyclone construction. CP&C was set the challenge of devising a system capable of building a safe and stable slimes dam main wall through an 1800 metre valley by utilizing the laterite available in the slimes stream. On-site tests proved the concept was feasible and the mine now has a cycloned wall over 15 metres high with a vertical freeboard of 7 metres or moreÔÇöwell within international standards.
Tailings management might be the dirty, uninteresting side of mining, but it is a vital component in the overall profitability of a plant and a key component when it comes to safety and environmental issues. Having established itself as the leading consultant and supplier in southern Africa, CP&C is steadily being invited to lend its expertise in other mining parts of the world.
Cyclone Projects & Consulting has established a sister company which operates the dams on behalf of the client under management contracts. The company, Cyclone Engineering Projects (Pty) Ltd is managed by a dynamic team from the offices of Cyclone Projects & Consulting in South Africa.
