Thickeners have various uses in aggregate and mineral applications. They can be used in the production of a material, such as in the concentration of slurries for minerals extraction or in the creation of a solids-free liquid to extract dissolved metals. They can also be used on the waste handling side of the process to recycle water from the wash plant for immediate reuse and to reduce the tailings footprint.
McLanahan points out that regardless of whether a thickener is used for material processing or waste handling, its main function is still to separate solids from liquids in a slurry. It achieves this through rapid settling of the solids aided by chemicals that allow the lightweight solids particles to stick together, forming larger, heavier particles called ‘flocs.’ These particles settle to the bottom of the thickener tank. The solids-free liquid then overflows the weirs at the top of the thickener tank, while the concentrated solids are pumped out to waste or to the next part of the process.
There are several different types of thickeners, including high-rate thickeners; ultra rakeless thickeners; high-density thickeners and paste thickeners. McLanahan states: “High-rate thickeners are characterised by lower side walls and lower underflow sludge densities. High-density thickeners are characterised by longer mud residence time, higher side walls and denser underflows requiring higher torque drives. Paste thickeners are characterised by maximum sludge retention time, tall tanks and dense underflows with little to no further free-water drainage. Ultra rakeless thickeners are simple devices characterised by good mud retention time and compaction, tall tanks and dense underflows, but not to the density of the paste thickener.”
McLanahan also states that selecting the right thickener for an application is critical for optimum performance. “Besides the thickener type, the size of the thickener, drive type, pumping requirements, and chemical type and dosage play a key role in the thickener’s operation; thickeners are configured differently for each application. A common misconception is that the selection process varies for different types of thickeners, but all thickeners are selected based on the same parameters.”
These parameters include optimum solids flux, which is dependent on the feed solids concentration; rise rate, which is typically 1.5 to 2 times the settling rate; desired underflow solids, which will set the required mud residence time that dictates the side wall height; and mud yield stress, which dictates drive size, structural requirements and underflow pump selection.
“These parameters are determined by tests performed on samples of the application’s material. These five tests include particle size analysis, cylinder test and chemical screening, static settling, dynamic settling and yield stress. Since the settling rate is heavily influenced by the particle size, the first test in thickener selection is a particle size analysis. The size distribution – particle size x content – is a lot more valuable than a single measurement. For mineral or aggregate samples of less than 100 microns in top size, this analysis is commonly performed by laser technology.”
The cylinder test is used in conjunction with chemical screening to understand the optimal solids flux and chemical dosage rates to achieve satisfactory settling rates. Flocculant is the main type of chemical used, but sometimes a coagulant and/or cationic polymer is necessary in conjunction with the flocculant to achieve optimum settling. “After the chemicals are selected, a simple cylinder test is performed to evaluate the settling rate, compaction of the underflow mud and clarity of the liquid. The settling rate provides some insight into the size of thickener needed, and the compaction indicates what the underflow density might look like.”
Simple cylinder test to evaluate settling rate
When submitting a sample of material for testing to determine thickener selection, McLanahan stresses the importance of using a proper container to store and ship the sample. Five-gallon (20 litre) buckets with lids are a good and appropriate vessel for transporting the sample to the lab. Also be sure to submit a representative sample. The thickener will be designed specifically for processing the material in the sample. If the sample is not representative, the thickener may not work as designed if the material changes when mined from deeper or elsewhere on the site.”
The company adds: “Along with the sample, it is important to provide the application’s objectives and current circumstance. For example, what are the downstream considerations? Is the thickener an addition to an existing plant or part of a greenfield project? Also consider the nature of the material, any contaminants that may be present in the material, the water source and quantity, as well as the discharge requirements. With proper material testing ahead of time, the right thickener can be chosen for an application, one that will perform optimally with the lowest chemical dosage.”