To “Wash” can be defined as “to remove a substance by or as by the action of water”. In a mineral processing sense, this commonly refers to the removal of loosely attached particles- often fine clays, from the surface of more competent ore.
In this definition, it is distinct from classification which would be a subsequent processing step that separates the different sized fractions.
Despite increasing concerns about the value and availability of water, ore washing remains a common and cost-effective means of beneficiation, as water usage is enhanced by the greater use of water recovery.
Advantages that accrue from the scrubbing of ore can range from a reduction of gangue to process further, improved product quality, and higher throughput of downstream equipment.
Although generally thought of as removing waste material, in highly weathered deposits the inverse may apply and the valuable mineral be contained in the fines. Examples occur in alumina, nickel laterites, uranium and occasionally gold.
The over-riding principle in ore washing is to provide sufficient energy and time to detach the fine clays but not to cause comminution of the intrinsic particles to smaller components.
Care also needs to be exercised to provide appropriate energy and flow to maintain the fine particles in suspension and not allow them to agglomerate and grow as ‘clay balls’. Thus defining a suitable ore washing process should consider the following:
- Energy input: Expressed in kWh/t
- Particle Size Distribution (mm)
Equipment sizing for ore washing has been largely empirically based on scale-up of laboratory or pilot tests, subsequently benchmarked off operating installations.
Miller (2009) created a mathematical model for sizing rotary drum scrubbers that calculated input power and residence times based off physical geometry however this still relied on test work to establish the range of acceptable criteria.
The high degree of variability in clay mineralogy leads a preference to do site-specific testing. Standards such as ASTM D4318 – 17, ASTM D4221 – 17, ASTM D3744-03 exist to help define the “plasticity” and/or the “dispersive” characteristic of clay materials and “aggregate durability” – but to date, a measureable correlation between these and energy input has not be published.
Test work is commonly carried out in either commercial testing laboratories or by vendors. McLanahan Corporation has facilities in Australia (NSW), USA (Pa) and UK
Equipment used can vary from scaled down vendor equipment, adapted commercially available substitutes (concrete mixer), equipment designed for alternative testing (ISO tumble drum ISO 3271:1995) to purpose built pilot plant capable of taking full size range.
A major limitation with all but the last, is the ability to process a representative sample size range particularly for those duties with a feed size of up to 250mm (10”).
Most small-scale test work is done on a batch basis. This can create some interpretation issues. It is generally acknowledged the batch testing provides an indication of the required solids residence time however most equipment is operated in a continuous mode.
Subject to the geometry and flow patterns within the equipment the fluid phase will traverse at a different rate. As such, residence time distributions of both the liquid and solid phases must be accounted for.
Miller (2009) suggested 0.3-0.5mm as a guideline for the coarse/fine (approximating solids/liquids) split however it was also acknowledged this is application specific. Industry standards use a range of 2.0 to 2.5 to compare the solids residence time to an average.
Common design specifications refer only to an average residence time hence evaluation of the equipment used for testing is critical for establishing design parameters.
Discrete Element Model (DEM) using computer simulation has been successfully used to approximate both residence time and power draw on a comparative scale when calibrated against known installation.
Key Equipment Descriptions
Once energy requirement, particle size, residence time and throughput are established the following matrix can provide a guideline for equipment selection.
Husaini H, and Cahyono S “Upgrading of Indonesia’s Bauxite by Washing Method” XXV International Mineral Processing Congress (IMPC), Brisbane, 2010
Preston M, & Tatarzyn J “Optimising Plant Efficiency With Attrition Scrubbers” Mining Engineering, October 2013.
Miller G M “Drum Scrubber Design and Selection” Metallurgical Plant Design and Operating Strategies, Perth, AusIMM, September 2004.
Slige S, “Improving Iron Ore Quality” AusIMM Bulletin February 2016