Engineered Sands Production
Experience with Dry Engineered Sand
Luck Stone’s Experience at the Leesburg Plant
Luck Stone submitted test samples for evaluation with the gravitational inertial classifier in the autumn of 1992. Based on the initial tests, as well as an on-site visit of the testing facilities, the design process was initiated to include a classifier at Luck Stone’s Leesburg, Virginia plant.
During the design process a structure to support a dual AC22.5GI was built. A single unit, installed in 1993, absorbed the existing 60 t/h of the 2 mm (10 mesh) screenings produced by a Nordberg 5 1/2 foot short head cone. With the addition of a new HP300 to the circuit in 1994 the increased feed tonnages required the installation of the second AC22.5GI unit to give a combined throughput of 120 t/h.
A single conveyor from the circuit feeds both units by use of a splitter at the head pulley discharge. After passing through the classifiers, the finished asphalt sand product falls through a mixer where water is added for dust control and keeps the product homogenous. This air-scrubbed sand product is then conveyed to a stacker for outside storage.
The Sand Plant Fines (SPF) is the minus 75 μm (200 mesh) that has been removed from the classifiers and is separated from the air by the dust collector, and then stored in a 350 ton silo. Draft for both the dust collector and the dual AC22.5GI classifiers is provided by a single 75 kW (100 HP) system fan. The system operates under negative pressure so fugitive dust is not a problem.
Product Characteristics and Uses
The final classifier product (air-scrubbed screenings) provides a more desirable material for the asphalt producer. Limiting the moisture in this dry process limits the amount of energy required by the asphalt producer to remove the moisture. This results in cost savings for the asphalt producer. Removing water also leads to added savings because these plants become more productive as they retain stone for less time. This allows the full design tons per hour to run through the plant.
An important question Luck Stone asked themselves when considering this process was what could be done with the generated ultrafines. They determined that they could be blended back into the base product without any detrimental effects. A handling system to do so was designed into the plant layout. Although these fines range over 90 % passing the 75 μm (200 mesh), their impact upon the base quality is limited due to the differences in comparative volumes. Because these fines are easy to handle, a system for loading them directly into trucks was included in the design.
A New Challenge – Engineered Concrete Sand
Today Luck Stone employs air classification technology at the majority of their plants and have also adapted its use to make engineered concrete sand.
Several years after the successful production of asphalt sand with their air classifiers, Luck Stone began to investigate a replacement crushed stone product for natural sand in the concrete or ready-mix market. This pursuit continues after over a decade of effort and has resulted in the production of high-performing crushed fine aggregate for use in concrete.
Production of engineered concrete sand is much more difficult in the USA compared with other parts of the world due to the tight regulatory specifications from ASTM. The ASTM C33 specifications are designed for natural sands with little accommodation for manufactured sands in the envelope. For this reason, achieving specification with engineered concrete sand in the USA is a difficult task. It is also the case that most concrete customers in the US are only experienced with natural sands and thus very resistant to engineered concrete sand technologies.
Luck Stone started their research into this area with the International Center for Aggregates Research (ICAR), a joint operation of The University of Texas at Austin and Texas A&M University dedicated to aggregate research. The goal of one of ICAR’s projects – ICAR Project 102 – was to prove that engineered sands can and do compete technically with natural sands and demonstrate that high percentages of minus 75 μm (200 mesh) are feasible in Portland Cement Concrete (PCC).
The results from this research proved that higher percentages of minus 75 μm (200 mesh) are not detrimental to the quality of engineered sand and does in fact have advantages over natural sand. The results showed that processing with a Barmac VSI produced excellent texture and shape in the engineered sand.