Well Scrubbed

The selection of washing equipment for processing of clay-contaminated rock and aggregate

By Mike Burton, managing director, Sepro Mineral Processing Int’l Ltd

Efficient clay removal from ‘as dug’ gravel and sand is of increasing importance in maximizing product value and mineral resource utilization. Similar incentives are driving the reprocessing of high clay content scalpings stockpiles in hard rock quarrying operations.

Four types of process equipment are typically used for clay dispersal:

Ore scrubbers: drums that run at relatively high-speed with heavy-duty high-lift liners. Their intensive washing action is characterized by vigorous turbulence and cataracting of the charge under the influence of the speed of rotation and the form of the lifters, which are custom designed for each application.

Washing barrels: drums that run at relatively low-to-medium speed, generally with thinner liners and lower lifters than scrubbers. Their washing action is characterized by tumbling and rolling of the material in the toe and body of the charge.

Log-washers: usually comprise twin contra-rotating shafts fitted with a series of intermeshing angled blades. Their washing action is generated by inter-particle attrition between the blades, which also transport material uphill from the feed point to the discharge point.

Attrition cells: multi-stage tanks, each fitted with a high-powered agitator with opposing-flow blades. Their washing action involves vigorous inter-particle attrition at high pulp density.

Attrition cells are only suitable for sand-size feeds, leaving three process options to be considered for new or retrofit aggregate washing plants. Of these, traditionally only washing barrels and log-washers have been considered for clay removal applications in UK operations – until recently. Washing barrels have been widely used historically but their relatively low speed can tend to agglomerate, or fail to disperse clay masses that occur in some deposits, leading to the misconception that rotary-drum washing equipment is inferior to log-washers in handling feeds with high clay contents. Consequently, log-washers have sometimes been specified in the expectation of better performance in applications with high clay contents, despite limitations on feed size and higher maintenance costs.

Overseas experience with rotary ore scrubbers in metallurgical ore washing plants has led to very different conclusions. These plants typically process ores with very high clay contents. The viability of such operations depends on the efficient dispersal of this clay content for efficient recovery of the metal ore. The rock and sand fractions that are rejected must be super-clean to avoid the loss of high-value ore minerals, which typically represent only 0.01–1.0% of the plant feed. By using high-speed scrubbers in these applications, it has been possible to obtain a metal recovery rate of 99%, compared with only 60–70% when using washer barrels or log-washers, which do not fully disperse the clays or completely remove clay coatings from the rock and sand particles.

High-speed scrubbers operating in washing plants for alluvial precious metals, nickel/cobalt, tropical laterites, diamonds and similar ores, with clay contents up to 90% in some cases, have demonstrated that rotary scrubbing, when properly applied, can totally disperse the clays to allow liberation of the very fine mineral values. Without complete dispersion, metal recovery levels rapidly decline, which has driven the optimization of the washing equipment used. Performance in this type of washing plant, in various countries, has clearly demonstrated that high-speed rotary scrubbers offer an effective solution to efficient clay removal from rock and sand.

Beneficial features of modern, well-designed, heavy-duty rotary scrubbers that are applicable to the processing of gravel, crushed rock and sand with significant clay contamination include:

• an ability to efficiently handle much coarser feeds than log-washers or washer barrels, with plants running successfully that are processing run-of-mine or primary crushed feeds up to 500mm. This eliminates the need to scalp the feed ahead of the washing process and removes the risks and operational problems of feeding unwashed scalper oversize to secondary or tertiary crushers
• no de-sanding of feed or countercurrent operation is required. Scrubbers operate efficiently with a simple co-current flow of solids and water, irrespective of feed, clay and fines content
• tolerance of wide fluctuations in feed rate and quality
• very low maintenance costs
• very high availability, typically in excess of 95% on 24 hours-per-day, 365 days-per-year mining operations
• disintegration of weathered mineral, soft siltstone and similar deleterious contaminants
• very clean product surfaces due to the vigorous washing action and absence of dead areas, which are a particular characteristic of log-washers.

Two examples of successful ore scrubber applications are summarized below:

Bonte Gold Mines, Ghana

Bonte Gold Mines previously operated a 3.0m diameter x 8.0m long washer barrel to process 350 tonnes/h of terrace alluvial gold ore containing up to 60% clay interspersed with quartz boulders up to 600mm in size. The gold content of approximately 0.5g/tonne was intimately associated with the clay fines. Gold recovery was less than 70% due to a lack of full clay dispersion and consequent loss of gold particles tied up in clay ball agglomerates and residual clay coatings. This machine was replaced with a Sepro 3.0m diameter x 8.0m long heavy-duty ore scrubber, which immediately increased the gold recovery rate to over 95% and subsequently maintained this level at an increased feed rate of 700 tonnes/h (figs. 1a and 1b).

Moa Nickel SA, Cuba

Moa Nickel operated four 72in log-washers handling up to 700 tonnes/h of extremely sticky and fine lateritic nickel/cobalt ore with an inherent as-mined moisture content of 35–40% and 5–15% serpentine rock content. The feed was scalped at 120mm on a wet vibrating grizzly to protect the log-washers from oversize rock, leading to metal losses in lump clay and coated rock in the grizzly oversize. The log-washer product was screened at 0.85mm with large volumes of sprayed water but undispersed clays in this material carried 30% of the nickel content in the ore to reject.

After several years of fruitless experimenting with changes to the log-washer operation, and tests by various Canadian manufacturers and consultants to improve metal recovery, a 1.2m diameter x 2.1m long Sepro scrubber was purchased and used for test work on reprocessing the log-washer product. These tests showed that intensive scrubbing increased recovery from 70% to 98% and, as a result, a 2.5m diameter x 6.0m long Sepro scrubber was installed as part of a 300 tonnes/h log-washer reject reprocessing facility. This plant was commissioned in 2003 and metal recovery has been maintained above a guaranteed 98% level ever since (figs. 2a and 2b).

This demonstration of the benefits of high-speed intensive scrubbing has led to a current project to replace the log-washers and expand plant capacity with the installation of two 3.0m diameter x 10.0m long scrubbers.

Aggregate washing

The effectiveness of intensive scrubbing on typical UK aggregates has been demonstrated by two installations where 2.1m diameter x 5.0m long high-speed scrubbers have been installed in preference to larger conventional washer barrels. The first scrubber processes heavily contaminated as-dug gravel and handles up to 500 tonnes/h, producing a clean washed product. A similar scrubber installed in 2004 processes 300 tonnes/h of granite primary scalpings with substantial clay contamination. In both installations the machines were selected in preference to larger conventional washer barrels, and both have fully vindicated this decision by their performance, reliability, spare capacity and very low maintenance costs.

Conclusion

High-speed intensive ore scrubbers have been shown to be extremely effective in fully dispersing clay coatings and agglomerates to deliver clean washed rock products, and will fully repay their consideration as the equipment of choice for new or retrofit stone-washing applications.