The aggregate industry has seen a gradual evolution in the treatment of tailings and process water. While the original aim was generally to clean and retrieve the water for reuse as quickly as possible, over time the need to comply with environmental regulations and site land use issues has led to ever more intricate processing methods. In this article we will look at one style of thickener that is used in a number of applications in the mining industry and, while it has been considered, appears to have been bypassed by the aggregates industry.
Sand is generally washed for construction aggregate to remove deleterious material and to comply with specifications. The process water contains the material removed, which the industry generically calls ?silt?. Silt ponds or tailings ponds have been used for decades across the country with varying degrees of success. Many were undersized or, through various upgrades in production, became too small.
Eventually these had to be dug out or pumped out, to be reused. If not cleaned out, land area eventually becomes a problem. If incorrectly positioned, portions of the deposit become sterilised. Silt is frequently rehandled multiple times on-site. Flocculants have been used in material going to a silt pond to agglomerate silt and get it to ?drop? out of the water more quickly.
{{image2-a:r-w:300}}A good deal of work has been done since thickeners were first introduced to the aggregates industry. Initially, thickeners allowed water to be cleaned and recovered for reuse in a timely fashion. However, the silt underflow still had to be dealt with and ?stored? somehow, usually back into a pond. Secondary processing allowed more water to be removed and a range of belt filter presses were trialled.
Some were reasonably successful and some were eventually shut off and not used. More recently, centrifuges were looked at, with the odd installation. Again, there were successful applications of this technology.
In the past five years or so, the plate filter press (with some variations around that name) has been promoted as the next possible solution to tailings/silt treatment. It still requires a thickener to carry out the primary process on the silt-laden water. As a rule, the press will produce a stackable material or one that can be conveyed and hence trucked. However, there are much larger capital costs involved, as well as the potential for additional chemicals (flocculants) increasing operating costs.
PASTE-THICKENED TAILINGS
An alternative that has been around for some time is the use of paste-thickened tailings. Paste-thickened tailings or paste technology (it is known by a variety of terms) has had a successful history in the mining industries, treating a range of tailings materials. Interestingly, this was first ?discussed? at the 2005 annual IQA conference in Adelaide, and had been put into practice by Rocla at its Golden Grove, South Australia, operation.
It appears minimal work has been followed up by the rest of the industry. Some attempts have been made by chemical manufacturers to thicken the underflow of existing thickeners, although it would appear this did not achieve consistent results.
Thickeners were invented more than 100 years ago.1, 2 John Dorr patented the continuous thickener in about 1906.1 Coagulants changed from starches and guar in the early days to synthetic flocculants in the 1960s.1 High rate thickeners were developed in the 1980s. High density and paste thickeners have a history stretching back to the 1970s, with three different organisations developing thickeners to produce as high a suspended solids concentration as possible in the underflow slurry. Some of this development took place in the Australian alumina industry.
{{image4-a:l-w:250}}A paste thickener today has numerous design features that differentiate it from the high rate thickeners that are typically used in the Australian aggregates market. These are illustrated in Figure 1 and include a much deeper compression depth, a much steeper tank bottom and pickets on the rake arm.
A paste thickener will require more torque to turn the rake arm and contain more steel than a high rate thickener.
Good flocculation remains the key to any successful thickener operation.1, 3 In addition to this, for paste technology, the thickener would require relatively deep pulp depth, a steep floor slope (as previously mentioned) and mechanical devices to work the compacting slurry.
Most major mineral processing equipment manufacturers have a division covering thickened tailings and the appropriate thickener design. These include descriptions such as high density, paste and high compression. There are at least 20 thickener suppliers worldwide.2 The reality is the density of the underflow/discharge material (see side bar on rheology) should be specified before equipment selection takes place. A complete system can then be designed around a predicted output.
LIMITS TO DENSITY
There is a limit to the concentration or density that can be achieved with any slurry. As thickened mud approaches its limiting concentration, it behaves less like a Newtonian fluid and has little tendency to flow.1 While concentrations in excess of 50 per cent solids by weight are possible on some mineral tailings (60 to 70 per cent solids are now achieved in certain copper tailings operations4), it is anticipated that levels approaching 40 to 45 per cent solids by weight may be achieved with the clays and silt that are typically washed from sand. Tailings treatment is an industry in itself.
Hydrometallurgy is a specialised field of research, covering material in the underflow and the overflow of thickeners. Within the sand and aggregate industry, much less work has been done on handling what is essentially a waste material. Most operations will continue to use existing silt/tailings ponds until there is no alternative.
This is merely a review of some of the developments in the mining industry (in rheology and hydrometallurgy, actually) with regards to tailings treatment, with some appropriate illustrations and explanations. It should not be taken as an endorsement or recommendation of paste technology over the range of primary and secondary processing techniques.
An adequate comparison of tailings treatments, primary or secondary, would require a good deal more effort and space for reporting. In most cases, an individual solution may work on one site and not be appropriate on another.
Initially, thickeners were used to retrieve and recycle water as quickly as possible in the aggregates industry. They were used to overcome limitations on land area and poor design of silt ponds. Now, more and more, environmental concerns are coming into play with the treatment of tailings in the sand and aggregate industry. Thickeners are inevitably part of the initial process of treating process water in sand washing.
Secondary treatment may be required in certain applications. However, paste-thickened tailings may offer an alternative that cuts one process out and potentially lowers initial capital, ongoing wear of filters and operating costs.
{{image3-a:c-w:600}}
REFERENCES
1. Schoenbrunn F. Dewatering to higher densities ? An industry review. In: Paste 2011, Perth, Australia.
2. Klepper R. The evolution of thickeners. Paper presented at the ALTA Metallurgical Services conference, 26 May, 2009.
3. Fawell PD, Farrow JB, Heath AR, Nguyen TV, Owen AT, Paterson D, et al. 20 years of AMIRA P266 ?Improved Thickener Technology?: How has it changed the understanding of thickener performance? In: Paste 2009, Vi?a del Mar, Chile.
4. Cengel YA, Cimbala JM. Fluid mechanics: Fundamentals and applications. McGraw-Hill Higher Education, 2010.
5. Doraiswamy D. The origins of rheology: A short historical excursion. DuPont iTechnologies, Experimental Station Wilmington, DE 19880-0334 I. Prelude to rheology. www.rheology.org/sor/publications/Rheology_B/Jan02/Origin_of_Rheology.pdf