Abstract

Calcined petroleum coke arrives at the graphite manufacturer's plant in particle sizes ranging typically from dust to 50–80 mm diameter. In the first step of artificial graphite production, the run-of-kiln coke is crushed, sized, and milled to prepare it for the subsequent processing steps. The size of the largest particle is generally set by application requirements. The manufacturing process begins with the mixing operation. The purpose of mixing is to blend the coke filler materials and distribute the pitch binder over the surfaces. The intergranular bond ultimately determines the properties and structural integrity of the graphite. Thus the more uniform the binder distribution throughout the filler components, the greater the likelihood for a structurally sound product. After the forming operation, the purpose of which is to compress the mix so that pitch-coated filler particles and flour are in intimate contact, in the next stage, the baking operation, the product is fired to 800–1000°C. One function of this step is to convert the thermoplastic pitch binder to solid coke. Another is to remove most of the shrinkage in the product associated with pyrolysis of the pitch binder at a slow heating rate. This procedure avoids cracking during subsequent graphitization. A variety of baking furnaces are in use. One common baking facility is the pit furnace, so named because it is positioned totally or partially below ground level to facilitate improved insulation. Another common baking facility is the so-called ring furnace. Two equal rows of pit furnaces are arranged in a rectangular ring. A more recent development is the carbottom furnace, which is mounted on wheels and movable. The method used to improve those properties necessary for many high performance applications of graphite is impregnation with coal-tar or petroleum pitches. Graphitization is an electrical heat treatment of the product to ca 3000°C. The purpose of this step is to cause the carbon atoms in the petroleum coke filler and pitch coke binder to orient into the graphite lattice configuration, producing graphite with intermetallic properties useful in many applications. In the temperature range of 1500–2000°C, most petroleum cokes and coal-tar pitch cokes undergo an irreversible volume increase known as puffing, associated with thermal removal of sulfur. Depending on particle size and on the product size, heating rates must be adjusted in the puffing range to avoid splitting the product. Fortunately, the use of puffing inhibitors has eased the problem.