A new process is being developed to produce graphite from prime coking coal. Coal is dissolved in dimethylformamide (DMF), on addition of sodium hydroxide. The minerals and undissolved coal are separated by centrifugation and filtration to give a solution (referred to as Refcoal solution or RCS). Over 90 wt % of the organic part of a flotation product, from the Tshikondeni mine, can be dissolved at temperatures ranging from room temperature to 135°C. The purified coal (referred to as Refcoal) and DMF need to be separated. the Refcoal to be coked and the DMF to be purified and recycled. This process should be as efficient as possible, whilst both products should be low in water content to minimise drying costs. The addition of water to the Refcoal solution causes precipitation to take place, forming a gel (referred to as Refcoal gel) liquid system. This mixture can be either centrifuged or filtered to give a denser gel, containing water, DMF and coal solids, and supernatant or filtrate, containing water and DMF. Different techniques and processes can be used to improve the separation of the DMF from the Refcoal by achieving a denser Refcoal gel: • Longer centrifugation times improve the density and therefore the separation, but this technique has its limits. • The use of low-temperature water improves the separation. • The use of syneresis could improve separation at a lower cost: heated standing tanks are used to expel the supernatant and therefore increase the density of the gel, thereby decreasing the required number of washing stages. • The addition of toluene at the beginning of a wash improved the removal of DMF by 20%, using centrifugation as separation method. • Pressure filtration gave a 20% improvement on centrifugation, with no additives. • The addition of toluene to the pressure filtration process gave another improvement of 15%, and after three stages the percentage of solids in the gel was 28%, the highest so far achieved. This method also resulted in the highest removal of DMF in the first stage (73% of the original DMF in the RCS was removed). Counter-current washing shows the greatest potential, using the least amount of water. The concentration of DMF in the wash solution, to gel the Refcoal solution, is a limitation of this process. If the concentration is too high, no gelling and therefore no separation can take place in the first stage. It is recommended that counter-current washing using pressure filtration should be investigated; however, this will be difficult on a laboratory scale due to the mass losses during transfers.
Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2009.