Adding a new 550 tpd rotary kiln more than doubled lime production capacity at Linwood Mining and Minerals Corporation's Buffalo Plant, located near Davenport, IA...but it also raised the problem of what to do with a greatly increased output of fines. Realizing that landfilling its plant property with waste fines at rates of up to 100 tpd would not be a good long-term solution, the company not only dodged that alternative but also gained a way to keep new fines in the product stream at full value with the help of an automated roll press briquetter.
Considered one of the country's largest underground limestone mines, producing aggregate and chemical lime products for more than 60 years, the Buffalo Plant currently mines approximately two million tons per year, with enough proven reserves to continue at that rate well into the 22nd century. Located along the west bank of the Mississippi River, straddling both a railroad line and a state highway, the company enjoys three handy alternatives for shipping to customers across its Iowa-Illinois-Wisconsin marketplace.
Blessed with two different high-quality deposits, the Buffalo plant supplies an unusually wide range of products. From the Davenport Seam, about 120 feet (36 m) below the surface, Linwood brings up stone with approximately 97% calcium and low iron/silica content. This results in chemical lime products with high appeal for steelmaking, water treatment, and production of glass and asphalt shingles as well as animal feed-grade granules and agricultural lime.
Working the Otis Seam as far as 200 feet (61 m) down yields a different stone that makes excellent aggregate for concrete and asphalt paving. This bodes well for Linwood sister-firm McCarthy Improvement Company, which for more than 103 years has been a major supplier to the highway and heavy construction markets of the Midwest. McCarthy Improvement was the founding member of what has become the parent McCarthy Bush Corporation of Bettendorf, IA, a diversified, family-owned enterprise that includes Linwood Mining & Minerals as well as various operations in dirt excavation, steel fabrication, residential & commercial real estate and other subsidiaries.
As with most aggregate quarries, Linwood's mine output is crushed and screened to produce various sizes of stone...but here, only about 40% of it ends up as construction aggregate products. The larger portion, screened to -1¾' +3' (-44.4 +6.4mm), is conveyed over to the chemical side of the business. Once there, about half of it gets crushed to produce 16-mesh granules for making glass and animal feed, or dustlike 200-mesh particles for use as filler in asphalt roofing shingles. The other half goes into the lime kilns.
Trouble Seen Coming
According to Assistant Lime Plant Manager Dennis Jones, growing market demand and the appeal of more efficient production led Linwood to add the new kiln in 1998, raising the 350 tpd combined capacity of the plant's three earlier kilns to a total of 900 tpd. 'The minute we started planning the expansion,' he says, 'we knew we'd get into trouble with fines.'
He explains that calcined lime (calcium oxide) coming out of the kilns is screened to -7/8' +3/16' (-22.2 +4.8mm) to derive pebble lime as specified by the steel and water treatment industries. Oversize material is crushed to -7/8' and returned to the screen. In this process, about 20% of the material screens out as -3/16' fines. 'Nearly all of the fines produced by the earlier three kilns has been hydrated on-site and sold in powder form for such uses as stack purification (desulfurizing), or making asphalt for paving,' Jones points out, 'but our fines market just isn't big enough to consume the additional load that would come from the new kiln.'
Briquetting the fines for sale as the equivalent of pebble had been considered earlier, recalls Plant Manager Jeff Dahl, but that never became a viable option because of the relatively small amount left over after satisfying buyer demand for hydrated lime. Plans to add the fourth kiln finally made it feasible. 'Briquetting always looked like the only way to reclaim fines in this quantity, and the only company we saw that could supply the kind of equipment we needed was K.R. Komarek, Inc., located near Chicago.'
'Our main concern,' Dahl says, 'was that the hardness of our lime might prevent it from briquetting well. Most pebble lime breaks easily by hand, like chalk, but with our pebble, you need a hammer to break it.' Addressing that concern first, Linwood sent several barrels of calcined fines to K.R. Komarek Research, Inc., a subsidiary headquartered in Anniston, AL, for testing on specially instrumented laboratory versions of Komarek's production machines. Tests there confirmed that under about 50,000 psi (345 MPa) pressure, the material forms good briquets at a density of about 2.25 g/cm, with no need for binders.
Show of Strength
Although Anniston data showed that briquet hardness could be increased a bit more with the addition of Calcium Stearate as an internal lubricant, it wasn't seen as necessary. In a crushing-strength test, in which briquets are crushed between parallel platens, the force needed to crush Linwood's briquets ran up to 290 lbs (1290 N), compared to the 156-246 lbs range typical of lime briquets.
Guided by the results of Anniston's evaluation, plus screen-fines volume projections from Linwood and design considerations generally associated with calcium oxide briquetting operations, Komarek proposed a roll-press briquetter engineered to produce an oval-shaped 0.43 in3 / 6oz (7.1cc / 17g) briquet approximately 1-2/5' long, 7/8' wide and 9/16' thick (34.9 x 22.2 x 15.9mm), at throughputs of up to 10,000 lbs (4,500 Kg) per hour.
The proposed mill, a variation of Komarek's Model B400 design, forms briquets with two counter-rotating rolls, vertically opposed and fed from one side by a horizontal infeed screw. Capable of roll separation forces up to 120 tons, the B400 is a cantilevered design with rolls mounted outside one end of the machine. This cantilevered design allows quicker, easier roll changes, which Komarek recommended because the abrasiveness of lime will require periodic replacement of worn-out rolls.
In Linwood's briquetter, the rolls are 18' (457.2mm) in diameter and 6' (152.4 mm) wide, with half-briquet cavities (pockets) machined into their circumference faces. Each roll has three rows of pockets across its face arranged in an offset, or staggered pattern, with each row containing 60 pockets around the roll circumference. Both rolls are synchronously driven by a single 100 hp / 75 KW electric motor through a dual-output reducing gearbox. The rolls are indexed so the pockets on opposing rolls match as the rolls turn, with upper and lower halves closing to form whole briquet-shaped cavities as they pass through the vertical centerline between rolls.
The horizontal infeed screw, independently driven by a 15 hp /11.25 KW motor delivers fines between the rolls at their 'nip' region (where the rolls come together), filling the roll pockets just prior to closure. Horizontal infeed provides more consistent material flow into the pockets than conventional gravity-feed systems, and the Linwood machine's infeed screw drive has a variable-speed control to allow fine-tuning of briquet density.
In this application, hydraulic pressure at 2,000 psi (13.8 MPa) applies a roll separation force of 80 tons to achieve the prescribed 50,000 psi briquetting pressure as the pockets close while passing through roll centerline to compress the material into a solid mass. As each pocket advances beyond roll centerline, its two halves naturally retreat, allowing the formed briquettes to drop out of the press and slide down a chute onto a belt conveyor that delivers them to a nearby 35-ton storage tank.
Needs Complete Enclosure
Because the briquetter is positioned outdoors, midway up an open-frame 104-ft. (32 m) tower, all parts of the system carrying either fines or briquets are completely enclosed to protect the lime from being ruined by environmental moisture.
Fines arrive at the briquetter's infeed hopper by gravity chute from a 30-ton tank overhead, which is kept full by an inclined screw (auger) conveyor that brings up a continuous flow of fines from the kiln-output screen, and drops them into the briquetting supply tank through an opening in the bottom of the conveyor trough. Like the kilns, the conveyor runs continuously, so it is designed with a simple but effective mechanism for automatic 'overflow' protection against the possibility that the briquetter might need to stop. If the briquetter supply tank fills to the point where the fines can no longer drop through, the fines simply continue moving up to the end of the screw conveyor to a fines storage tank, where they are held for sale to the various fines markets.
Standing next to the fines storage tank is a 500-ton silo-like tank that receives pebble lime at the top, separately conveyed up from the kiln screen, and empties at the bottom for drive-through truck load-out.
The 35-ton storage tank that receives the briquets empties onto a weigh-belt conveyor that feeds a 'super-sack' loader on ground level below, adjacent to the silo structure. Four other storage tanks feed weigh-belt conveyors on the same deck that also converge at the sack loader drop-chute, permitting measured amounts of various customer-specified additives to be blended into 2-ton, nylon-mesh bags along with the lime briquets. Mixtures are metered according to blending programs stored in a proprietary computerized control panel at the sack-loading station. 'The bagged mixtures are primarily for steelmaking customers,' Jones says. 'The mills save time and money by simply dropping the entire bag into the molten metal, instead of metering in the various ingredients separately.'
System Runs Itself
Linwood's briquetting operation is fully automated, Electrician Mike Truninger explains. 'The system starts and stops itself by PLC control, triggered by briquet-level indicators in the bagging storage tank. When the supply gets low, the PLC will start the equipment in a backward sequence, first the transfer conveyor, then the briquetter rolls, then the briquetter infeed screw and the vibrators in the drop chutes that bring fines down to the briquetter. A control panel on the briquetter deck allows manual override, but that's rarely needed except for maintenance purposes.'
Likewise, he adds, sensors in the briquetter supply tank will stop the system if the level of fines gets too low, then start the system again when the supply of fines is replenished.
'The entire lime plant operation is supervised from a central control room,' he continues. 'There, four PCs display various plant operations graphically, alert operators to any problems and allow them to take appropriate actions. As long as everything is running normally, no one has to worry about the briquetting system,' he says. 'It runs itself.'
Although Linwood's briquetter has the capacity to process more fines than the new kiln typically produces, the machine currently runs only about half of the time, says Plant Manager Jeff Dahl. 'This was intended to give us plenty of room for growth potential in bagged orders. Right now, we're bagging only five days a week, but production is running 'round the clock, seven days a week, with crews alternating three- and four-day schedules of 12-hour shifts.'
'Growth potential aside,' he continues, 'our primary goal was to reduce the cost of waste. If 20 percent of our material comes through as fines and gets dumped, then we're throwing away 20 percent of the cost of mining the stone, crushing, screening, handling and calcining it. That makes those fines too valuable to end up on a landfill. While capturing their value with briquetting required a significant initial investment, we figure this system paid for itself within a year or so.'