GB793730A - Apparatus and process for the production of boron - Google Patents

Abstract

In a process for producing boron, a mixture of hydrogen and gaseous boron trichloride is passed over an electrically heated rod with intermittent reversal of the direction of gas flow to deposit boron evenly on the rod, and the <PICT:0793730/III/1> <PICT:0793730/III/2> <PICT:0793730/III/3> <PICT:0793730/III/4> effluent gases are treated in succession, to condense unreacted BCl3, to bleed off a portion of the remaining gases to prevent build-up of boron hydrides in the cycle, to separate HCl, and to recycle remaining H2, mixed with condensed BCl3, to the process. Chlorine from 80 (Fig. 1) passes by pipe 81, into preheater 45, which is lined with boron carbide or carbon, is filled with porous coke 72 (Fig. 2) and has electrically heated graphite rods 75 to maintain a temperature of 900 DEG or 1000 DEG C. The preheated chlorine passes through channel 70 into boron carbide or carbon lined chamber 10 containing a bed of boron carbide lumps 76 on grate 65. The temperature in chamber 10 is 300-2000 DEG C., preferably 1200 DEG C., and B4C is fed in through double valves 18, 19 which act as an air-lock. The boron trichloride vapour passes through condenser 90 to remove entrained impurities and is condensed in bottle 98 immersed in ice-salt mixture. BCl3 from bottle 98 passes into mixer 110 and is mixed with hydrogen, from bottle 112 or recirculated by pump 113, in a mol. ratio H2 : BCl3 2 : 1 to 10 : 1. The reaction mixture passes into manifold 115 and is fed into a plurality of tubes 120. Tubes 120 contain graphite or carbon rods 121 (Fig. 4), which are in two parts joined by graphite nipples 126 and have high strength graphite reinforcing pins 128 to reduce breakage. The rods 121 are mounted to allow for expansion in bore 140 and counterbore 141 in graphite electrodes 125, which have bores 131 for passage of gases and are secured in furnace heads 135 closing, and insulated from, tube 120, and having inlets 139. Power cables 159 connected to a number of binding posts 158 in watercooled brass clamp 165 supply current at 2000 amps. and 30 volts to heat the rods 121 DEG to 1000-1600 DEG C. Sighting tubes 180 are provided for temperature measurement, and the outside of tube 120 is cooled by water sprays from 190 and 191. The ratio of the internal diameter of the tube 120 to the diameter of the rods 121 should be 2 : 1 to 7 : 1. Alternatively, rods 121 may be of solid graphite with short graphite, or boron carbide sleeves, or of graphite coated with B4C or silicon carbide. Since the B is deposited mostly near the entrance to tube 120 and gas flow in the tubes is periodically reversed. The effluent gases pass from tubes 120 through manifold 195 and water-cooled heat-exchanger 200, where gases are cooled to 25 DEG C. to condenser 220. Through pipe 305 the gases enter condenser 220 (Fig. 3) which has thermally insulated walls, and carbon or graphite Raschig ring packing 290. The gases are cooled to - 40 DEG to - 107 DEG C. by flowing up through the packing countercurrent to liquid BCl3 entering through spray 289, and the BCl3 flows through grate 260 and collects in accumulator tank 227, whence it is withdrawn by pump 280 and returned to sprayhead 289 through coil 283 in heat exchanger 240, which has thermally insulated walls and contains a methyl alcohol-solid CO2 mixture to cool the BCl3 to - 100 DEG C. The overflow from tank 227 passes to tank 225 and may be recycled to the mixer. The effluent gas from condenser 220 at 255 has 3-20 per cent bled off by metering valve 231, and the remainder passes to lime beds 235 to remove HCl and remaining H2 is recirculated through pump 113. The tube 120 may be removed from the cycle as necessary, dismantled and the layers of B and B4C removed from rods 121. The B and B4C are segregated and the B4C may be reused for BCl3 preparation, or alternatively the B4C layer may be left on the rod 121 to reduce C diffusion. Specification 716,083, [Group V], is referred to.