How waste material can replace depleting sand

Natural sand deposits are a depleting resource in many parts of the United States, prompting quarry owners and operators to take a second look at a waste product many aggregate producers already have on site in massive stockpiles.

The process of dry screening and crushing produces a screenings byproduct that contains high levels of fine material, silts and clays. This screenings product has a very limited market but, as a result, producers could accumulate hundreds of thousands of tons of this “waste” over time.

These stockpiles take up valuable space on the quarry floor and cost money to maintain rather than generate a revenue.

Using new, advanced washing systems, screenings can now be reprocessed to create spec manufactured asphalt and concrete sands that can be used in place of natural sand.

Introducing washing into a completely dry process in a quarry was traditionally challenging. The permitting required to get wet processing permits was costly and time consuming – if it was possible at all. Settling ponds used to recirculate water are also costly to maintain and take up valuable real estate.

These issues have pushed a number of quarry operators to look at closed-loop water and silt management systems. They have many benefits, including the elimination of tailings ponds and the associated cleanout costs, recycling up to 95 percent of the process system water and reducing waste by 75 percent.

These benefits make the permitting process much easier, with no concerns over groundwater contamination and much less fresh top-up water required.

The process

The first stage of the process is the washing of the screenings to reduce the percentage of silts to meet asphalt and concrete sand specifications (i.e., ASTM C33 requires less than 3 percent passing the number 200 sieve). One efficient method of doing this is using hydrocyclone technology that maximizes the recovery of in-spec sand and minimizes the amount of waste material going to the silt management system.

The addition of a dewatering screen after the hydrocyclone reduces the percentage moisture in the finished product and maximizes the recovery of water to be reused in the system.

The second stage of the process is a closed-loop water and silt management system to clean the wastewater and recycle it back to the wash plant. The water management system will maximize the process water return, reducing fresh top-up water demand to a minimum.

The system comprises a thickener tank that scalps off most of the water over a weir system and produces a slurry of milkshake consistency. A high-pressure pump then transfers the material to a plate press that filters the mud through the plate chambers, producing a perfectly dry, dehydrated, low-moisture mud cake.

Because the plate press is fully automated and does not require a flocculant/coagulation additive to achieve the dry cakes results, running costs are kept to a minimum.

The environmental and economic benefits of the process are a reduced demand for natural sand while turning a waste product into revenue and profit for the quarry owner.

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Moira Brady is sales and marketing manager at Emerald Equipment Systems.