power from some small central station plant may be compelled to select the two-phase Rennerfelt arc furnace instead of the single- phase Rocking Detroit furnace, because the latter type of current cannot be economically provided for one power user alone.
The figures in the accompanying table are given by Gillett for the output and power consumption of the various furnaces named above, when melting brass and bronze.
The electrolyte contains 30 % free HC1 and 80 to 85 grammes of gold per litre, with varying amounts of platinum and palladium. When the amount of these latter two metals in solution is sufficient to render it worth the expenditure of time and chemicals, a portion is withdrawn, and the platinum and palladium are then separated and can be refined by the appropriate chemical and metallurgical methods of treatment.
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Type of Furnace
Power required K.W.
Charge of metal in Ib.
Output per day in tuns
Power consumption in K.W.-hrs. per tori
10 hrs. 24 hrs.
i. Ajax-Wyatt
30
300 l-ll 3-32
325 267
(Yellow brass)
60
600 2^-3 6-7
275 218
2..Baily ....
105
800 to 1500 2j3i 6-10
475 338
(Yellow and red brass)
3. Snyder ....
100
600
3
1 J
380
(Leaded bearing bronze)
300
2000
12-18
290
4. Rennerfelt . . . 100
500
1 J
475
(Red brass and bronze 125
IOOO
2-2! 7-10
400 350
and bearing metal) . 300
2OOO
10-16
325
5. Detroit rocking
40
125
J
400
(Yellow and red brass) .
225
1300
3i 8|
332 262
300
2000
6-7 16-20
287 237
Nos. 3, 4 and 5 use from 2 J Ib. to 6 Ib. of graphite electrodes per ton of metal charged.
Bullion. " Bullion " is the technical term for the alloys of the precious metals silver and gold; the name is also applied to the bars or ingots in which these metals are sold (for coinage purposes) to the mint authorities of the various countries of the world. Although chemical and metallurgical methods are still employed for separating or " parting " the silver and gold in bullion from one another, and from the baser metals with which they are often associated, since the year 1895 electrolytic methods have been making steady progress, and at the present time a very large proportion of the silver output of the world is electrolytically refined by aid of the Moebius process. As regards gold, the electrolytic method is also making progress; and electrolytic refineries for treatment of gold bullion by the Wohlwill process have been operated at Frankfort, Hamburg, Paris, New York, Philadelphia and San Francisco.
The chief disadvantages of the electrolytic methods of bullion- refining are those resulting from the value of the gold and silver locked up by the processes; and the latest improvements, there- fore, are directed chiefly towards reducing the volume of the solutions in the tanks, and also the time required for the refining.
The Moebius process of silver-refining has already been fully described (see 25.1 15). The following description of one of the latest plants erected is of interest, since it shows that the horizontal system of travelling belts which act as cathodes has been dropped, and that the vertical type of anode, enclosed in bags, has teen rein- troduced. The plant is attached to the Amboy Refinery of the American Smelting & Refining Co., and consists of 144 stoneware tanks, grouped in 2.1 sections of 6 tanks each. The anodes weigh 100 oz. and are cast by hand in the lead refinery. The cathodes are made from cold-rolled silver sheets 3^ in. thick. Each tank contains
4 anode bags and 5 cathodes, the bags each holding 4 anode bars. The electrolyte is a neutral nitrate solution, containing, per litre, 1 5 to 20 grammes silver and 30 to 40 grammes copper ; 75 %of the silver in the anode can be deposited in 24 hours. The deposit upon the cathodes is continuously removed by means of wooden sticks attached to a frame which has a reciprocating motion. A current density of 40 amperes per sq. ft. can be maintained under these conditions since no treeing of the silver can occur, owing to the continuous removal of the crystalline deposit.
The bags which surround the anodes receive all the slime, and are removed at regular intervals for recovery of the gold and other precious metals. The slimes are first boiled with sulphuric acid of 1-842 Sp. Gr. in order to remove copper and silver, the residue is then washed, dried, and cast into anodes for treatment by the Wohlwill process.
The gold-recovery installation at Perth Amboy is equipped with
5 earthenware cells, and a current density of 150 amperes per sq. ft. is employed. This density is considerably higher than that used in the early trials of the Wohlwill process, the object of the increase being, of course, to reduce the standing charges for interest per unit of output. One special feature of this installation is the use of mercury cups on the ends of the copper bus-bars. By aid of these cups, and cross bars of copper with bent ends to fit into these cups, any unit can be quickly cut in or cut out of the circuit. The cathodes are thin sheets, rolled from electrolytic gold, and are connected to the contact bars by bending one end of the sheet round them, and by fastening this down with a clip.
Cadmium. This metal was produced during the World War period in America, by the electrolysis of acid solutions of the sulphate, freed from all impurities by chemical treatment.
The electrolysis was carried out in semi-circular lead-lined tanks, provided with rotating disc-shaped cathodes of aluminium sheet, J in. in thickness. Under these conditions, smooth coherent deposits of metallic cadmium could be obtained, when using a current density of 15 amperes per sq. ft. of immersed cathode area. The average weight of cadmium deposited per 24 hours in the plant referred to above was 113 ib. per tank, and the current efficiency was 85%.
As regards output in 1914, 91,000 Ib. of metallic cadmium were produced in the United States, and the total had increased to 207,000 Ib. in 1917. No figures were yet available in 1921 for the German output during the war years, but it was known that they also pro- duced large amounts of cadmium and used it as a substitute for tin, in the manufacture of solders. The low melting-point of cadmium renders it useful also in the manufacture of fusible plugs in sprinkler systems of fire protection; and it has also been employed in con- junction with lead for the manufacture of bearing metals.
Calcium. The method of Rathenau and Suter for production of metallic calcium upon a commercial scale, by electrolysis of the fused chloride, has already been described (see 4.971).
Up to 1921 no important uses had been found for calcium; consequently there was little demand and the price of the metal remained comparatively high. Before the war calcium was quoted in Germany at M.S. 50 per kgm., equivalent to a price of 35. to 35. gd. per pound. It could still be produced at this price, if a large demand for the metal were created.
The only application so far suggested for calcium is as an absorbent for the occluded and trapped gases in molten metals; and in this direction it comes into competition with the cheaper and lighter metal aluminium. Soddy, in a Royal Society paper of 1906, referred to the use of calcium for removing the last traces of O. and N. from rarefied gases, and stated that by its use very high vacua could be obtained, but no practical industrial appli- cation of this suggestion appears to have been made.
Calcium Carbide and Calcium Cyanamide. The manufacture and application of these two products of the electric furnace have been described (see 1.138). Whereas the use of calcium carbide for generating acetylene for domestic or public lighting purposes is not extending, its manufacture and use as an absorbent for nitrogen increased rapidly after 1910. The cyanamide process, in fact, when operated in favourable localities, seems likely to remain one of the chief competitors of the Haber and electric-arc processes for the fixation of atmospheric nitrogen. According to reliable authorities, the relative power consumption for the three processes is as follows:
Haber (taken as unity) . . . . . . I
Cyanamide 8 to 10
Arc processes ... .... 25 " 30
Regarding power considerations alone, therefore, and ignoring technical questions, it is clear that the Haber process is of universal application while the cyanamide process will only
