In the past, dry screening was very common and accepted. Today, to meet the needs of advancing world markets, there are now better options. For mining Companies using washing and scrubbing for materials, they can ensure they are producing a silt free product.
There are a a variety of reasons why mining companies employ washing systems to clean their materials. For the most part the goal is the same: to improve the quality of the target product by reducing and eliminating contaminants. Contaminants like these are considered undesirable because of their chemical/mineral content.
Washing is also employed to accurately separate based on particle size. Fine screening at small apertures is more efficient and therefore higher capacity when using washing screens. The water helps separate particles and acts as a carrier for the finest material through the screen. Dry separation systems rely entirely on gravity or air currents, neither of which offers the same ability to select precise cut-points as water-based systems.
Dry separation, (for example standard sizing screens in the case of coarse fractions or air classifiers in the case of sand / fines), cannot offer the same efficiency of separation and the same quality of final product as imparted by a modern washing process. When mining companies wash materials in this manner, latent moisture content, sticky or agglomerated materials, the presence of low mass but variable size contaminants, can all be much better addressed via a wet system.
The target product(s) and the contaminants can vary tremendously according to the application. Some examples include:
• Excess fines (<75μm / 200#) in the case of sand for construction purposes
• Excess sea minerals in the case of marine dredged sand
• Sulphates found in some sand deposits, particularly in areas of the G.C.C.
• Surface contaminants such as iron (FeO2) in the case of high-quality silica sand (SiO2)
• Clays adhering to aggregates or minerals; for example, alumina Al2O3 / silicate clays bound to lump iron ore
• Silicate clays adhering to bauxite ore
• ‘Trash’ in recycled materials; for example, paper, plastic, grass in construction and demolition material
• Oils and heavy metals in excavated soils or tar sands
Each of these above examples (any many more applications) needs a correctly considered solution to provide the customer with the most efficient and effective system.
Perhaps the most frequent requests for washing systems come from the construction materials sector, especially for sand used in concrete manufacture. Most international standards for concrete sand state clear sizing parameters that sand should fall within. One aspect of this is the limit on ‘fines’.
Excess fines in concrete sand can cause a number of issues for the end user:
• Inconsistent internal coherence, potential cracks
• Inconsistent drying time leading to cracks / blemishes
• Excess cement consumption (small particles have a higher surface to volume ratio)
Specialist sands applications are typically considered as those which require more intervention than screening and removal of fines, as would be typical for concrete and other construction sands production.
Specialist sands can be generally characterized as having very precise and narrow particle size distribution requirements and / or the need to address aspects relating to the material’s chemical composition.
A frequent example of this is silica (SiO2) sand used in glass manufacture. Here, it is important to the end user that the sand has a very narrow range of granular size so that the particles melt evenly and at consistent rates. There is also often the need to reduce chemical contaminants such as heavy metals content including FeO2. The presence of metals would affect the final quality of the manufactured glass, in particular its color. (Iron is added to glass to give it a brown color). The purer the final SiO2 product, the higher the commercial value and potential revenue for the client. Such systems therefore typically include more complex combinations of sizing and scrubbing equipment compared to a construction sand application. Highly efficient high-frequency screens capable of accurate separations <1mm, upward flow classifiers, carefully selected hydrocyclone circuits and attrition cells often feature in these projects. They may also feature vertical spiral classifiers and magnetic separation. The additional capital investment for such systems compared with more typical ‘sand and gravel’ operations is recovered through the much higher per ton value of the product generated.
Additional examples of specialist sands applications include frac sand processing, where the target PSD is extremely narrow, special sports sands such as USPGA golf sand, sand for water treatment filter beds, contaminated soils where oils and heavy metals may be present, oils sands and many others.
Three main approaches are used by Matec for cleaning and sizing aggregates: screening and scrubbing. We will look at each of these in the following sections.
To divide the aggregate / mineral feed into the desired size fractions the universally accepted approach is to use ScreenTec. Most screens across the mining and construction materials industry are dry, meaning that no water is added to the material.
For virtually dry, particularly crushed material, this offers adequate quality product. It is also preferable to employ dry systems upstream of crushers as these operate best with low moisture content feed. If however, there is a high residual moisture content, high fines content, sticky clay, soil or other contamination then bringing water into the equation increases quality and efficiency significantly.
AggreTec is a true game changer within the materials handling and washing sector. Conventional systems require a series of stand-alone units employed in series; for example, hopper – feed conveyor – washing screen – sand washing unit – product stockpilers. As individual units these all need to be correctly deployed and integrated on site to ensure efficiency and performance. AggreTec brings all these elements together into a single, easily delivered, installed / relocated platform. It includes a feed hopper with various grid / live head options; robust feed conveyor; purpose designed washing screen of 2 or 3 decks; sand washing system for one or two sand products; product stockpilers (up to 5 output grades); complete support structure; electrical controls with optional remote management; complete pre-wire and factory-fit pipework.
ChemREADY is your go-to partner for wastewater treatment management. Along with offering the best chemicals and products for your wastewater pretreatment process, our team has the expertise to make sure that you’re getting the perfect solution for your unique needs.
If you want to learn more about how we can help with the wastewater treatment process, or you’d like to discuss your needs in detail, contact the ChemREADY team today.
