1. Field of the Invention
This invention relates to apparatus for and a method of producing pellets from a water containing feed material, and more particularly to such apparatus and method for dewatering and pelletizing feed materials containing an excess of water.
2. Description of the Prior Art
It is well known to form finely divided solid particles into pellets to facilitate handling and use of the products. Charcoal is formed into briquettes for use in backyard barbecues and metal ores are formed into pellets to be fed into smelting furnaces. Animal feeds are formed into pellets to minimize waste, and waste materials are pelletized to facilitate handling and disposal. Other pelletizing operations too numerous to mention are well known.
In many commercial pelletizing operations, the feed material is a product of a controlled manufacturing process and the composition is consistent so that pelletizing is easily controlled; in others, the feed composition varies so greatly that pelletizing must be carefully monitored and controlled. For example, it is well known that variations in the amount of water or liquid in a feed material will have a marked effect both on the ability to form the material into pellets and on the strength and integrity of the pellets formed. Commercial pelletizers may have feed blenders and mixers including means for the addition of water or other liquids as necessary to enable the formation of competent pellets; however, such devices do not include dewatering systems for the removal of excess moisture.
Substantial effort has been and continues to be expended to economically recover and use products such as coal fines, biomass and waste products as fuels, but these efforts have met with limited success. As a result, industry is compelled to spend substantial sums to dispose of such waste products, and the disposal procedures have not always been satisfactory. For example, the large volumes of coal fines produced in the washing of mined coal have an excessively high water content for economic recovery and use despite their potential fuel value. Such coal fines, which normally have a particle size of about 1 mm or less, are routinely collected in tailing ponds as waste. This practice is not only expensive but also harmful to the environment.
Similar problems plague other industries such as the paper and plastics industries where the substantial waste products are not easily disposed of. While furnaces have been designed to burn many such waste products as fuels, such furnaces are expensive both to design and to operate. specialized materials handling equipment may be required and operation of the furnaces is erratic and difficult to control.
U.S. Pat. No. 5,743,924 discloses a pelletized fuel made from a mixture of finely divided coal particles, paper making sludge waste, waste plastics and waste paper, which products are mixed together and pelletized without the use of adhesives or binders. A commercial pelletized fuel plant constructed in accordance with this patent utilizes sludge from a paper making facility, in combination with finely divided coal and shredded plastic materials, and is utilized by the paper making facility as fuel. This commercial facility not only realizes substantial savings on the cost of fuel over the previously used coal, but also saves the cost of disposal of the waste sludge. Further, substantial reductions in SO.sub.x, NO.sub.x, and particulate emissions are realized. Since the present invention is particularly useful in pelletized materials of the type disclosed in U.S. Pat. No. 5,743,924, the disclosure of this patent is incorporated herein by reference.
U.S. Pat. No. 5,658,357 discloses a process for producing coal logs or compacts from finely divided coal particles, utilizing laboratory procedures, but no commercial facility utilizing the process is believed to be in operation. The described process includes accurate control of the water content, accurate control of the zeta potential of the product, and the use of very high pressures over an extended time during which excess water may be expressed. The manner of expressing the water is not disclosed.
Slurry and sludge dewatering devices are also known in which excess water is extracted from a slurry or sludge pumped or forced through the devices. Examples of such dewatering devices may be found in U.S. Pat. Nos. 3,938,434, 4,798,194, and 5,173,196. These devices generally employ an elongated filtering chamber through which the solids/liquid mixture passes, with excess liquid being permitted to escape through filtered openings in the sidewalls of the chamber to produce an increased solids concentration or compact discharged from the end of the chamber. The problem of solid particles tending to plug the filter openings is discussed, for example, in U.S. Pat. No. 3,938,434.
While the pelletized fuel disclosed in the above- mentioned U.S. Pat. No. 5,743,924 represents a significant advance in the use of biomass and waste as a fuel, the processing costs and the moisture content of the pellets have been relatively high. These factors affect the strength and abrasion resistance of the pellets and limit the distance which the fuel can be economically transported. Further, the water content of the biomass and waste components must be monitored since excessive water content can result in the inability to form a satisfactory pellet. These factors make it difficult for such fuels to be price competitive with coal in the steam production and other markets. Similarly, excess liquid in the feed material can adversely affect other commercial pelletizing operations. Accordingly, it is a primary object of the present invention to produce a pelletized material which may be produced at a lower cost and which can be processed into pellets having a relatively low moisture content and high strength.
Another object of the invention is to produce such a pelletized material which may incorporate a wide variety of biomass and/or waste materials.
Another object of the invention is to provide a method of producing a pelletized product in which the material being pelletized is effectively dewatered during the pelletizing process.
Another object is to provide such a method in which the materials to be dewatered and pelletized are processed in a pelletizing die having water escape openings in the die walls to permit excess water to escape during the pressurization and pelletizing process.
Another object is to provide an improved pelletizing apparatus including a pelletizing die which permits the escape of excess moisture during the pelletizing process.
Another object is to provide an improved pelletizing die which may be installed in new or existing commercial pelletizing apparatus.