More than twenty years ago Luck Stone Corporation, one of the largest family-owned and operated aggregates companies in the USA, embarked on a sequenced investigation to satisfy demand for a dry asphalt sand product that would avoid the environmental concerns inherent in wet systems. The solution also led them to applications for the production of their own design for dry engineered concrete sand.
In 1989, Luck Stone’s Northern Virginia sales staff requested a manufactured sand product to provide to their asphalt customers. Engineered sand specifications for concrete in the USA are controlled by the American Society for Testing and Materials (ASTM) through C33 and are significantly different to most other countries in the world. Manufactured sand for use in asphalt in Virginia, Luck Stone’s primary market, is generally considered to be a material passing 6.3 mm (1/4 inch) screen. However the real asphalt issue was, and is, the excess minus 75 μm (200 mesh) which was desired to be less than 7 % passing 75 μm (200 mesh).
Because most “screenings” have significantly more material passing 75 μm (200 mesh), a process to remove this excess material was undertaken. Luck Stone had routinely employed a wet process to remove this excess 75 μm (200 mesh) material, but concerns with space and the environmental aspects of this process pointed to the desirability of a dry processing system.
Although consideration was given to the wet process, Luck Stone did not believe it would be the best solution. As a result, in 1990 they began an investigation into dry alternatives to de-watering with a classifying screw.
The Search Begins
As with many aggregate producers, Luck Stone was faced with a dilemma. How could they produce asphalt sand without the associated problems of a wet process? They didn’t want to sacrifice aggregate stockpile space to locate de-watering ponds nor did they want to process or produce wet material during freezing weather. They also didn’t want the environmental concerns that are inherent with ponds, wet fines containment and water or slimes disposal.
The challenge of finding a dry solution was given to Luck Stone’s Chief Engineer, Bob Stansell, since deceased, who considered and tested several options.
Dry Fine Screening
Luck Stone began their investigation with a suggestion from their Leesburg plant manager. He suggested they investigate a multi-deck screen which had previously been demonstrated to him. The desired result with this type of screen would be realized if it could remove a sufficient amount of minus 75 μm (200 mesh) material by either screening or air sweeping to meet a gradation of less than 5 % passing the 75 μm (200 mesh).
After several days of testing and subsequent evaluation, Luck Stone concluded that this particular screen would not be suited to dry separation at a cut point below 300 μm (50 mesh). They determined that this screening unit would not be effective in the production of a final product due to its moisture sensitivity and their inability to control the weather.
Luck Stone next investigated a fluid-bed style dryer. This process would have the advantage of being able to remove moisture at the same time as the ultrafines were being removed. The equipment included a slightly inclined vibrating pan with many small holes throughout. This arrangement allows heated air to pass through the material, both drying and removing the ultrafines.
This process offered excellent control regarding the cut-point and eliminated the moisture problem associated with damp feed. The results were favorable with good levels of control and predicted consistency.
However, several areas of concern preempted Luck Stone’s favorable view of this equipment’s performance. Even though it provided excellent results they determined that the capital required to install such a system would be excessive. Their concern extended to the higher maintenance considerations of this process due to abrasion and complex burner requirements. The added processing cost to provide the necessary energy for drying was also a detriment for this process. Their final concern was the safety of such a system to operate unattended with the balance of the automated circuit.
Luck Stone determined to continue their search for a better process, although the fluid-bed dryer was superior in providing a good, clean and consistent product.
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