Methods of controlling the density and thermal properties of...

Fuel and related compositions – Particulate – Mixed with particles of different size

Reexamination Certificate

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C044S592000, C044S620000, C209S010000

Reexamination Certificate

active

06786941

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods of controlling the density, permeability, moisture retention and thermal properties of bulk materials and to compositions produced by the methods.
BACKGROUND OF THE INVENTION
Efficient, low cost transportation and storage of bulk materials from mines and/or factories to markets are vital to certain industries because the costs of transporting and storing bulk material are often major components of the total cost of the delivered product.
Coal is one of the world's largest bulk commodities moved by rail, truck, inland barges and ocean-going vessels to utilities and steel mills. The cost of transporting coal plays a critical role in expanding markets for coal. Changes in environmental laws in the United States have created a demand for low-sulfur, premium quality steam coal. Before 1975, underground mines in West Virginia and eastern Kentucky supplied most of the premium quality coal needed to meet environmental requirements at coal-fired utilities. Although vast low-cost, strippable reserves of low sulfur coal resided in the West, distance and associated high transportation costs excluded them from serious consideration for Midwestern and Eastern markets. This situation changed as railroads recognized the opportunity for new markets and began investing in unit trains and improved ways and structures to haul large tonnage shipments. As a result, large productive mines were developed in the Powder River Basin (PRB) in Wyoming. Production of PRB coal has risen steadily since 1980 replacing higher cost eastern coal. Production is expected to rise to 400 million tons per year in the near future. Since transportation can account for up to 75% of the total delivered price, it continues to play the critical role in expanding the market for western coal. The increased demand for PRB and other western coal will not be realized unless the railroad companies continue to find ways to reduce costs and improve efficiency.
The market for metallurgical coal is also dependent on the cost of transportation. For example, steel mills are extremely competitive and are constantly looking for lower cost coal to fuel their blast furnaces. Although the best quality metallurgical coal in the world reside in the eastern United States, Australian and South African producers often win contracts because of lower costs. The high cost of transporting coal by rail from eastern mines to port facilities often makes American suppliers non-competitive.
Coal has a low bulk density compared to many other common bulk materials, such as limestone, aggregates, iron ore and fertilizers. Since coal is hauled in the same rail cars, trucks, barges and ocean-going vessels as the more dense bulk materials, less weight can be carried for a given volume of cargo hold. The full weight carrying capacity of many vessels cannot be reached before the volumetric capacity is reached. As a result, costs are increased since the weight capacity of the vessel is underutilized. Consequently, a coal producer is penalized because a rail car cannot be loaded to full weight carrying capacity. One PRB mine operator reported that underweight penalties cost about $100,000 per month, totaling over $1 million ina recent year.
Storage and handling costs are also affected by bulk density similar to transportation costs. As bulk density increases, less storage volume is required to hold the same amount of coal. Smaller stockpiles require less area to hold coal resulting in lower storage costs. Likewise, the smaller volume also requires less loading and unloading time and labor.
When bulk materials are hauled in conveyances such as rail car, barges, and trucks during cold weather, moisture contained in the material may form ice that can adhere to the conveyance. Frozen material, accounting for up to 10 percent of the net payload, may not discharge from the conveyance at the point of delivery. The added weight increases transportation costs by reducing the useful carrying capacity of the conveyance and increasing the weight of the conveyance returned to the producer.
Sub-zero temperatures and long transit times can cause the payload to freeze creating large lumps of aggregated material, particularly when water goes through the material and pools at the bottom of the conveyance before freezing. As a result, special equipment is required to break the frozen lumps into manageable sizes that are compatible with material handling and storage equipment.
Two principal methods are typically used to mitigate the adverse effects of frozen material. The first method involves adding a chemical such as a salt compound or liquid glycol antifreeze to the bulk material to depress the freezing point of water or weaken the ice that binds the solid particles together as described, for example, in U.S. Pat. No. 5,079,036 entitled “Method of Inhibiting Freezing and Improving Flow and Handleability Characteristics of Solid, Particulate Materials” and in U.S. Pat. No. 4,290,810 entitled “Method for Facilitating Transportation of Particulate on a Conveyor Belt in a Cold Environment.” The second principal method involves heating the walls of the conveyance to thaw the frozen layer of material adhering to the walls as described, for example, in U.S. Pat. No. 4,585,178 entitled “Coal Car Thawing System” and in U.S. Pat. No. 4,221,521 entitled “Apparatus for Loosening Frozen Coal in Hopper Cars.” Several manufacturers offer electric and gas-fired radiant heaters to warm the bottom and sides of a conveyance to melt the frozen layer of material. The choices of chemical or thermal methods depend on the type of conveyance, cost constraints, and material compatibility. Treating frozen materials has become more expensive because many rail cars are fabricated from aluminum, a thermally sensitive material that can corrode when it comes in contact with low-cost salt compounds.
Thawing and chemical treatment methods are time consuming and expensive. Thawing costs range between $0.20 and $0.50 per ton, depending on the source of energy. Chemical treatment costs range between $0.20 and $1.00 per ton, depending on the type of chemical and dose rate.
Most bulk materials that are crushed to a specified topsize for commercial reasons have a naturally occurring particle size distribution that, when plotted, fit under a typical single gaussian curve. Such naturally occurring size distribution does not have the optimum particle size distribution to produce sufficiently high bulk densities to effectively lower transportation and storage costs or to mitigate the effects of freezing. In addition, known methods of altering the thermal properties of bulk materials, such as lowering permeability and increasing moisture retention, result in decreasing the bulk density since the materials are simply crushed into a smaller size in an attempt to increase the surface area of the bulk material.
Compacting or vibrating is commonly used to increase bulk densities by many industrial applications that handle relatively small volumes of high-value fine powders (0.5 mm and smaller). Examples include pharmaceuticals, cosmetics, ceramics, sintered metals, plastic fillers and nuclear fuel elements. However, many applications that involve large volumes of coarse bulk materials (up to 150 mm) cannot effectively use compaction or vibration to control bulk density. If the coarse material is of relatively high value, expensive oil or other chemical additives that modify the particle surface characteristics can be applied to modify bulk density. For example, steel mills typically control bulk density of metallurgical coal feeding cooking ovens by applying additives as described in U.S. Pat. No. 4,957,596 entitled “Process for Producing Coke.”
Accordingly, a need exists for low cost methods of controlling the density, permeability and moisture retention of bulk materials. The present invention satisfies this need and provides related advantages.
SUMMARY OF THE INVENTION
The present invention relates to methods of controlling the density, permeability, moistu

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