Improvements in preparation of a charge for a calcium carbide furnace

摘要

A charge for a calcium carbide furnace is prepared by firing a pelleted mixture composed of at least one calcinable compound containing lime and coal containing volatile constituents by moving the said mixture co-currently and in direct contact with a firing atmosphere containing oxygen in an amount sufficient to burn the major portion of said volatile constituents and insufficient to burn all of them, and controlling the firing atmosphere so that it is highly oxidizing and at a relatively low temperature at the mixture inlet end of the firing zone and is non-oxidizing and at a relatively high temperature at the exit end of the firing zone and there still contains a small amount of unburned volatile. The so formed pellets of carbon and calcium oxide are then suitable for the production of calcium carbide in an electric carbide furnace. In Fig. 1, coal, limestone and calcium hydrate, suitably the by-product from acetylene generation, are fed from hoppers 11, 12 PICT:0723455/III/1 PICT:0723455/III/2 and 13 respectively to a conveyer 17 and passed to a pug mill 18 where they are intimately mixed with water. The mixture then passes to an extruder 20 where it is converted to cylindrical pellets of about 1 1/4 inch diameter and length, which pellets then pass to a drier 25, maintained at about 400 DEG F. The dried pellets are then conveyed by elevator 26 and a line 24 through a sieve 27 to a sealed rotary kiln 28 where the coal is devolatilized and coked and the calcium oxide material is calcined. The calcined pellets then pass to a water-cooled rotary cooler 29 and from thence to storage. Combustion air for the kiln 28 is supplied by line 35 which passes through a heat exchanger 33; this heat exchanger is heated by gases containing volatile constituents which leave the lower end of the rotary kiln at 30 and are burned with air from pipe 31 in combustion chamber 30a before passing through the heat exchanger 33. The hot combustion gases leaving the heat exchanger 33 may then be used for supplying heat to the drier 25 by passage thereto through line 30. As shewn in Fig. 2 the rotary kiln 28 is provided with stationary hoods 44 and 46 with sealing means between them and the rotating kiln; hot air for combustion enters through valved pipe 35 and the pellets entering at 56 leave at 47. The combustion products pass out through the duct 52 in the hood 46. A gas burner 60 is provided for starting the furnace in operation after which the process should be autothermal. The thermocouple 54 in the hood 36 may be used to control the valve 58 in the air conduit 35 and also to control the temperature of the combustion gases after burning in chamber 30a before entry to the heat exchanger 33; this is effected by control of an air supply line 32. The preheated air supply to the kiln is preferably at a temperature of 500 DEG to 800 DEG F. and the flue gas leaving the kiln is maintained at about 2000 DEG F., the pellets leaving the kiln having a temperature of about 1850 DEG F. The lime supplying raw material may be calcium carbonate or calcium hydrate or a mixture thereof, for example, an 85 per cent/15 per cent carbonate-hydrate mixture is suitable. The coal employed should preferably have a volatile content above 30 per cent and should have good fluidizing property i.e. the capacity to form a hard aggregate from small particles when heated. The proportion of carbon to calcium oxide in the formed product should be 1 to 1.55 but to ensure a free flowing melt in the carbide furnace the portion of calcium oxide may be increased so that the ratio of carbon to calcium oxide is about 1 to 1.75.