- Created: Tuesday, 10 February 2009 07:53
- Published: Tuesday, 10 February 2009 07:53
- Written by Rock Products News
Economic crisis continues to pummel the aggregates industry and corresponding construction materials markets. Housing and construction companies are dropping like flies. Limestone, hard rock, sand and gravel—all are feeling the pain. A U.S. Silica operation in Ottawa, Ill., however, seems immune to this phenomenon.
Economic crisis continues to pummel the aggregates industry and corresponding construction materials markets. Housing and construction companies are dropping like flies. Limestone, hard rock, sand and gravelóall are feeling the pain. A U.S. Silica operation in Ottawa, Ill., however, seems immune to this phenomenon.
Sales remain constant and there havenít been any layoffs since at least the 1970s, says Administrative Manager Robert L. Krepps. There are 92 hourly employees and about 30 salary employees. Although, he says the operation can go for long periods without having to hire because of a low turnover rate. Recently, the company posted five hourly positions and received 480 applications. Krepps has been with the company for 31 years.
îIt is a very good place to work,î Krepps says. ìI think people, for the most part, enjoy coming to work, and itís a team effort.î
A Diverse Market
U.S. Silica benefits most from a diverse market because so many products can be produced from its high-quality silica, explains Krepps. For at least 10 years, the Ottawa operation has produced industrial silica at full capacity, which is around 2 million tons per year. The operation produces more than 200 unique products between 20 and 120 mesh for a plethora of different markets; so if one market is down, another will be up.
For example, about 30 percent of the silica is used as a melting sand for glass products. The automotive industry consumes much of this product, but recent demand has been low because of the economy. But, the same sand also is used in making glass containers, a market that is up because the beer-bottle market is up.
U.S. Silica supplies about 20 grades for molding and coremaking applications. Ground silica also is used in plastics, rubber and polishes. It is an ideal pool-filtration media and filters drinking water on a municipal level. The white color also benefits decorative grouts and mortar mixes.
The very first product produced at the Ottawa site, called Sawing Sand, was a cutting agent for the limestone business. Krepps says the original owners were a family from the quarry business in Bedford, Ind. They were in search of a more efficient means to cut limestone blocks. And in 1900, they founded this site to create this specific product.
The silica was stronger than the limestone and aided the cutting process, as it worked between the blade of the saw and the limestone ore. The blocks of limestone typically were used for buildings such as the Ottawa courthouse and many government buildings in Washington, D.C. Krepps says the company still produces the same product, but it is not necessarily used for sawing.
ìMany times we are not even aware of what the end uses are,î Krepps says. ìCustomers can be very secretive.î
U.S. Silica has 13 silica operations in the United States, but the Ottawa operation is by far the largest. It is spread across 1,900 acres and should yield 50 years worth of reserves at full-face mining with a constant production volume of 2 million tons per year. Krepps says it is part of the St. Peterís sandstone deposit that runs through Illinois, from southern Minnesota to southern Wisconsin. It also touches parts of Missouri and Kentucky. The operation also neighbors four competing silica operations.
ìLaSalle County is where most of the industrial sand is mined in the world,î Krepps says. ìThis is quite possibly the largest industrial silica operation in the world.î
Stripping and Blasting
The operation relies on one of the most cost-efficient forms of miningóhydraulic miningóKrepps says. There is a small overburden ranging from a few inches to several feet. He says the competition can have up to 70 feet of overburden. U.S. Silica relies on an outside contractor for stripping at the Ottawa site. The contractor uses an excavator and haul trucks. It is a sandy soil that is not saleable, but serves well enough for reclamation purposes.
Other contractors are charged with drilling and blasting. They drill 6.5-inch bore holes that are loaded with ANFO and blasted. The state regulation is 1 inch per second, but U.S. Silica tries to limit vibrations to 1?2 inch per second. This is done out of respect for its neighbors.
Krepps says that the company prefers to contract stripping because it is performed in campaigns. Otherwise, U.S. Silica would have idle equipment on the field and might have to periodically lay off operators. Blasting is contracted to avoid the burden of obtaining needed certification. Also, it is a highly specialized field, and Krepps likes to defer to the experts.
The blast renders the weakly bonded silica-sandstone ore to a pile of sand with only a few rocks, Krepps explains. Those are easily crushed with a bulldozer, so no primary crusher is required. No wheel loaders, haul trucks or excavators are needed for the extraction process. Instead everything is moved by high-pressured streams of water.
Typically two miners stand on the pit floor wielding high-pressure water monitors powered by vertical turbine pumps. They supply up to 6,000 gallons per minute at under 250 psi through 2-inch monitor nozzles. The water is delivered from more than 10,000 feet from a reservoir on the opposite side of the property through a 24-inch waterline. It takes several pumps to do this, and the final booster pump provides enough power to disintegrate the shot material into a slurry.
The stream is strong enough to punch holes in many things, Krepps says. All monitor operators have a deep respect for this equipment and have had extensive training. Krepps cannot recall a single accident with a monitor during his 31 years.
On a hot August day, operating the monitor is an ideal job. January is a different story. There is a fine mist emitted from the stream that leaves everything covered with a layer of ice. After an hour, the operators could be frozen solid and need a mallet to break the ice off of their heavy coats. Krepps has never seen the slurry freeze, but pumping rates drop when the temperatures fall below 10∞ F. On these days, the plant will rely on a winter stockpile of material and load the plant with a wheel loader.
U.S. Silica ties to maintain a feed rate of about 500 tons per hour. The material first settles in the low point of the quarry where material is collected by a pump and moved to a screen house. The screen scalps off the large rocks. Undersize is pumped to the processing plant through a 16-inch, high-density polyethylene slurry line. A back-up line runs right next to it, under the Illinois River. Krepps believes that the two lines, combined, will move material for more than 40 years.
He says a standard steel pipe will be worn through in as little as two months if it is not rotated regularly. The new slurry lines, as well as a 24-inch steel waterline, electrical conduit and fiber optics, stretch 3,300 feet under the Illinois River that separates the north side from the south side. The slurry travels nearly 13,000 feet to the plant.
The Processing Plant
The slurry line discharges into a washer that sprays water from the bottom up. This further liberates clay and ultrafine sand particles. And an overflow weir allows the ultra-fine material to be removed as waste. Good material moves on to a hydrosizer that separates the material into coarse and fine products that are stored in separate 5,000-ton concrete silos.
This surge point offers feed for a series of Derrick scalping screens for further removal of organic material or oversized grains. The filter belts then reduce the material from 50 percent to about 6 percent moisture. Moisture is pulled through each of the three filter belts by vacuum pumps. Material then falls into vibrating feed hoppers that feed the three fluid-bed dryers operating with hot air.
Krepps says that many operations would use a dryer surfactant to reduce moisture further. However, these surfactants leave a residue on the sand, which would affect foundry sands and ASTM sand.
When the material is fully dried, several things happen. The coarse sand receives additional screening with dry Derrick screens that create two products. The fine sand is introduced to several air sizers. Basically, a curtain of sand drops in front of recirculating air within the unit. The air segregates the sand grains by size. The sand grains fall into several pockets, fine grains traveling farther than coarse grains. The sized material is conveyed by hoses from each air sizer to a vibrating conveyor capable of transporting multiple grades simultaneously. Each graded product is stored in a bin.
The Blending Station
Those 12 unique sizes are blended into about 70 different products by a single employee in a blending station, which is the crux of the operation. This employee manipulates all blending and loading with two computers, using Wonderware software. Krepps says the blender has two truck loadout spouts, two rail loadout spouts, and a loadout spout for trucks hauling material for barges. He also sends material to either of two bagging stations and to the grinding plant. All of this is accomplished with four conveyor belts running beneath the blending bins.
The position of blending operator is the highest-paid production position in the plant. The operator often has more than 20 years experience.
ìThey tend to have a feel for the sand and know what will blend into what products,î Krepps says. And if one bin is empty, they often can utilize other bins to make an equivalent product. Occasionally, they will miss a blend, Krepps say. In this instance, they dump the product (maybe 300-400 tons per month). However, they produce 160,000 to 180,000 per month.
Rodney Gerard has been with the company for 30 years and has been working as a blender for 17 years. He says the automation technology that has been incorporated over the past 10 years has made the current volume of production possible. ìIt is way better than it used to be,î he says. ìTechnology is an amazing thing.î
Meeting the Specs
Another essential piece of technology at the loadout station is the cross-cut sampler. This takes a teaspoon-sized sample from the finished product, as it is being loaded into a truck or rail car. The sample is dropped into a hose that fills a can outside of the blending station door. Gerard will dump that sample into a bag and leave it for the driver to collect with the paperwork. It is the driverís responsibility to carry this sample to the lab for testing.
Itís just a short wait as the sample undergoes a series of tests to ensure the product meets the desired spec. Tests vary depending on the specs provided. X-ray analysis is performed to monitor for elemental contamination including iron and aluminum, which poses problems for melting sands. The color of the sand is closely monitored, especially for decorative products such as tiles or counter tops. Krepps says that if the color is off, the company will contact the customer and possibly stop shipments until the product returns to the preferred white.
Rotaps are used to determine particle size distribution of whole grain products such as tiles or counter tops. Sedigraphs are used to analyze particle size of ground silica, which typically is finer than 100 micron. Samples of each product are temporarily retained and numbered. If a problem arises, staff can consult the sample and locate the source of the contaminant. Krepps says more times than not, the problem is the carrier.
In the Bag
What doesnít leave the site as bulk via truck, rail or barge is bagged and shipped off on pallets. ìAll of the bags weigh 50 to 100 pounds,î Krepps says. ìNone of them are hand carried or hand stacked. The machinery does all of the work.î
There are two large bagging sites at the Ottawa operation. One is controlled through the blending house and another operates independently. Material is conveyor driven and stored in bins above the bagging stations. Only one operator is required to fill bags. As he pneumatically fills the bag, it also is weighed and stops automatically at 100 pounds. The filled bag is ejected from the saddle and onto a conveyor belt. It moves in front of an ink-jet printer that marks the grade of the material, a lot number and any other markings requested by the customer.
From there the bag moves to an automatic palletizer, which patterns one ton of bags onto a skid. Krepps says 20 bags can be positioned in about 44 seconds. Once filled, the pallet drops to the main floor and awaits a forklift driver.
The blender also sends material to the grinding plant. Krepps says that material can be ground to a consistency of wheat flour. It can be used as extenders in paints, drywall compounds and makeup buffing compounds. Some of it is even melted.
There are six grinding mills of various sizes. Grinding is achieved with ceramic grinding balls, measuring 1.25 inches in diameter. These diminish in size as material is crushed, and the company must add media on a weekly basis. Liners are made of flint, which is much more durable than steel, Krepps says. Liners also are replaced in sections, as wear occurs. The ground material is air classified and is either loaded into trucks, rail cars, or bagged.
These products, whether they are a fine dust or a granular product used for drilling, all keep U.S. Silica operating at full capacity.