Much is written when new equipment is installed, or a new plant is purchased for an upgrade. However, not all upgrades in the quarry industry involve new or “purpose bought” equipment. As the old saying goes, “Necessity is the mother of invention.”
The selection of plant in the extractive industry frequently involves the use of second-hand or used equipment. This often means making do with existing equipment and using it in inventive ways. Smaller operations will use equipment they have retained over the years, overhaul or repair any deficiencies and install a newer and hopefully more efficient plant or process.
Tegra Australia’s operations in southern New South Wales were covered in Quarry in 2012. As noted in the 2012 article, Tegra was severely affected by the 2010 floods of southern NSW. The company lost a fine sand plant consisting of a sump or bin, a pump and a cyclone tower.
In looking for a replacement to process this material, it moved into an area of topical discussion. Can you wash fine sand with a standard fine material washer or a sand screw? This question is frequently posed by operators of sand screws.
After the devastation of the floods, replacement of the fine sand plant was not an immediate priority. The general deposit profile on the site consists of overburden of variable depth, the fine sand with high silt/clay content (30 per cent plus passing the 75 micron sieve), a distinct clay layer, followed by an extensive sand and river rock layer. Hence, this fine sand/high silt material needed to be mined to expose the rest of the sand and gravel. {{image2-a:l-w:640}}
Archimedes Helix
Sand screws are somewhat “old school” technology; they have been around for decades. The basic principle is based on the Archimedes spiral (which is actually a helix). The spiral augers the material that is coarse enough, or heavy enough, to sink out of the tub of water, with the silt overflowing with the water. It is a simple operation. When you know how to control the operation, it is the most efficient way to wash sand, with low energy consumption, low wear rates and excellent product. However, with a large number of suppliers in the market, there is some confusion as to how to operate
them efficiently.
{{image3-a:r-w:300}}Tegra had a sand screw that was planned for use elsewhere. Combining this with a 30-year-old feeder/screen and an associated radial stacker it also owned, it set up a simple processing operation. This allowed Tegra to wash both the fine sand with high silt, and reduce the minus 75 micron portion to acceptable levels (see Table 1 on page 24). It also has the flexibility to run a coarser material through the sand screw, to make a blended concrete sand when required. This consists of a blend of three sands, pluscrusher dust (see Table 2).
Running a sand screw efficiently required matching the unit’s speed (spiral or helix revolutions per minute) to the amount of fine material in the feed, while balancing the amount of water used to remove the silt or minus 75 micron material. According to Mick Cross, Tegra’s quarry manager, the company worked closely with a local hydraulics expert and developed a hydraulic drive for the sand screw, based on a load-sensing piston pump. This gave the company flexibility in adjusting the screw speed. The drive runs a diesel engine with the capacity to expand, should it be needed.
{{image4-a:r-w:300}}By eliminating the higher power requirement for the pump and cyclone system it lost in the flood, Tegra significantly lowered its energy costs. As Cross said, the plant made use of existing equipment to get it back into operation, while increasing efficiency and product recovery, and lowering operating costs. The plant now uses less water, retains more fine product, produces a cleaner, drier product, has greater flexibility and saves money on production and maintenance.
Well-worn items renewed
It has been said equipment has four or five lives in the quarry industry. With majors purchasing new equipment, middle-sized operations may buy good quality used or reconditioned equipment. Smaller operations frequently obtain bargain pre-owned equipment, while even smaller operations take well-worn items and bring them back to life over a period of time. Subsidiary or ancillary processing plants are frequently put together from a range of used or pre- owned equipment. Tegra has shown that when you know your equipment and you know what you’re doing, you can develop an effective process.
So, back to that age old question: can you wash fine sand in a sand screw? If you understand the operation of a sand screw and can effectively control and operate it, you can indeed wash fine sand with it!
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