Case Studies

By-products: waste or waste-not?

Commonly plant is designed to process high value products demanded by our customer base. Not enough attention is paid to the effect this singular pursuit has on achieving the total extraction of all the various materials in the resource deposit. The commercial benefit from the sale of a low value product outweighs the cost of accumulating what may become a contaminated waste product.

Over the last 50 years, every quarry that I have seen closed down as a depleted resource still contained volumes of good quality stone. It was competent resource material sterilised by negligence.

The pursuit of the extraction and processing of high grade stone, with disregard for the need to remove overburden, weathered rim material and poorer quality low value material on an annual basis, has prompted many a premature closure.

This situation is brought about by constraints on capital and insufficient attention to the ?value adding? requirements of the lesser quality materials which are stockpiled, contaminated or dumped. The importance of addressing these aspects is often overlooked. The eventual financial impact of this oversight is not immediately obvious and creates an accumulating financial burden not fully appreciated until the latter stages of the resource extraction.

Prevention of this predicament must be addressed in the initial planning. A comprehensive drilling exploration of the resource site will reveal the volumes of the various quality materials in the total resource area.

The data collected then provides for the establishment of a management plan that identifies with the removal, processing and/or placement of a range of all materials on an annual basis to achieve the ultimate total extraction of the resource.

A budget for the sales department must be created to market an annual balanced range of total quarry products for the future wellbeing of the quarry operation. There is a demand in the construction industry for these lower profit line materials and the future wellbeing and profitability of the resource operations requires that balance.

It is important that these low value products are marketed with the same energy and emphasis as higher quality, more profitable materials.
Today?s economy cannot tolerate a management plan, or a plant design, that generates a waste product or a by-product. We need to add value to every product generated by the plant.

The volume of overburden material in a basalt resource in south east Queensland has been dramatically reduced with the introduction of a large mobile screening plant.

A scalped road base material has been recovered from what was originally considered overburden. Oversize weathered stone has also been suitable as a construction material or for further processing.

While this is a dry season operation, it contributes to the operating efficiency of the site that will be further appreciated as the overburden placement areas become increasingly occupied.

The most common by-product in a quarrying operation is scalped road base. Most plants scalp off weathered or dirtier materials at the primary screen.

Depending on the amount or frequency of this material in the resource, this is a -25mm,type 2.5 road base material or a larger lump size that is necessary to be taken out of the circuit in order to maintain the quality of the material processed in the tertiary end of the plant.
In the latter case, a screen installed in a closed circuit with a small impactor type crusher is capable of producing a good roadbase and a range of drainage materials. Value can also be added to that scalped road base by correcting the particle size distribution (PSD) or grading envelope of the material with added crusher dust to create a higher performing, more acceptable product.

Blue dust will assist to decrease the plastic-ity of this road base product, where necessary.

Water added to the product will enhance its workability and reduce placement costs for a potential customer. A 30 tonne load of roadbase can contain 1.5 to two tonnes of water. Road base prices for water is good business!

Road base containing water treated with a wetting agent will improve the optimum moisture content (OMC), will increase the maximum dry density (MDD) and, in recent laboratory trials, has increased the California Bearing Ratio (CBR) value of a given scalped road base material by a remarkable 20 percent! The cost is $0.005c per litre (or $0.30c to $0.35c per tonne).

It is a waste of competent resource material to create drainage materials or under slab fill from high quality fresh stone processed through the tertiary end of the plant because of an inferior particle shape or because it constitutes unprocessed ?crusher dust?.

Misshapen particles from the tertiary plant should not exist – a correctly adjusted, appropriately fed tertiary cone crusher will produce an ?acceptable? particle shape. Misshapen particles in the past were the result of a poorly adjusted and selected crusher circuit, or the product of deflating screens, both of which are totally unnecessary with today?s modern crushers and the VSI (eg the Barmac).

Crusher dust derives from the cone crusher?s pressure crushing action. While the PSD of this cone crusher material may be similar to that of the Barmac, the particles are sharper and have a higher voids count.

Voids in sand are measured in a ?flow cone? where a measured volume of crusher dust is timed as it flows through the cone. The same measured volume of ?Barmac product? flows through the cone at a much faster rate, identifying a reduced voids and superior particle shape! Voids in concrete materials require filling with a cementations paste.

Laboratory tests and concrete trials in the past 40 years have identified the improved performance of the Barmac type sand compared to ?crusher dust?. In recent laboratory trials, aggregates and sand processed through a Barmac circuit have dramatically decreased the water cement demand with elevated strength values of all concrete mixes beyond what was achieved employing an alluvial sand. This manufactured sand has also reduced the volume of purchased imported fine sand required in all mix designs. Cement volumes can still be further reduced from the present revised mix designs.

At a large basalt quarry in Indonesia, crushed aggregate from a HP cone crusher circuit was an acceptable shape for the local concrete industry. The crusher dust was washed through a cyclone laundering plant and employed as concrete sand. The problems with the slimes generated in this process were immense and created massive disposal and containment problems.

Trials were carried out processing this dust through a laboratory Barmac and creating pan mixes of various concrete mix designs. As a result, a Barmac was installed to process only the -7mm crusher dust and the launder plant was closed down. This sand was blended with 30 per cent of sand from the tin mining activity on Bunker Island and an increase in the concrete strengths across the range was experienced.

An ultimate particle shape will optimise the performance of any construction material.

A modern cone crusher may provide product with an ?acceptable? particle shape.

Here, the particle size closest to the nip or closed side setting (CSS) of the crusher will have the best shape. Material smaller than this nip point will contain more misshapen particles.

Material processed through the Barmac relies on particle impact, where the larger particles destroy smaller misshapen material and attrition in those multi-particle collisions provide an optimum particle shape to every size range in the feed material down to the smallest sand fraction. Over the past several decades, I have fed a -35mm material into the Barmac situated in a closed circuit, even in a wet sand and gravel operation.

Twenty years ago, I was required to provide the concrete aggregates for the construction of sections for the Sydney Harbour Tunnel. This was nominated to be processed from ?the Bumbo basalt deposit? of a quarry at Dunmore in southern NSW. Initial aggregate samples failed to meet the performance requirements of the ridged contract specifications. A Barmac circuit improved the particle shape of the aggregate which then passed the stringent ?sand equivalent wet dry strength variation criterion? required for that harsh marine environment.
While this presentation offers solutions to common waste products or by-products in the quarry industry, a large generator of this type of material is our closely associated concrete industry.

Concrete wash-out from returned concrete may represent five per cent of material out the gate in some of the larger operations. In recent times, a ?Jadair Rediwash? recovery plant has been installed to recover all of the sand and separated gravel from the wash-out material, leaving only a lime rich, spade-able waste product to be disposed of.

The product would be ideal for neutralising acid sulphate soils that are a common problem in subdivision development in the coastal regions.

The return on investment on this plant is excellent when you take into account the cost of an alternative disposal off the site. This includes the replacement cost of the material, the transport costs to supply the initial delivery, to take away the waste and then replace that volume
of material.

A war on waste and a focus on value adding should be a common objective of all operations.

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