about this time, the weak liquor of the reservoir A is generally all used up, and is replaced by pure water introduced by the tube d. All the solutions which filtered through, marking over 3 to 4° B., are put together, and form the strong liquor which marks about 12° B. This extraction of the aluminate being completed by the pure water, the residue on the filter is taken out, and a new operation may be commenced.

II. Preparation of the Alumina.

The strong liquor is introduced into a vessel having an agitator, where a strong current of CO2 may precipitate the Al2O3 from it. The gas is produced by small streams of hydrochloric acid continuously falling on some limestone contained in a series of earthenware jars. The precipitation vessel is called a baratte. The CO2 after having passed through a washing flask, is directed to a battery of three barattes, where the precipitation is worked methodically, so as to precipitate completely the alumina of each baratte, and utilize at the same time all the carbon dioxide produced. In order to do this, the gas always enters first into a baratte in which the precipitation is nearest completion, and arrives at last to that in which the solution is freshest. When the gas is not all absorbed in the last baratte, the first is emptied, for the precipitation in it is then completed, and it is

made the last of the series, the current being now directed first into the baratte which was previously second, while the newly charged one is made the last of the series. The process is thus kept on

Fig. 9.

B

a. Charging pipe.
b. Steam pipe.

c. Steam drip.

d. CO2 enters.

f. Discharge pipe.

A. Agitator, made of iron rods.

C. Tank in which the precipitate settles.

B. Baratte body.

D. Steam jacket.

continuously. The apparatus used is shown in Fig. 9.

1

Each baratte holds about 1200 litres of solution, and the complete precipitation of all the alumina in it takes five to six hours. A mechanical agitator stirs the contents continually, and a current of steam is let into the double bottom so as to keep the temperature of the solution about 70°. The precipitated alumina and the solution of Na2CO3 which remains are received in a vat placed beneath each baratte. The solution is decanted off clear, after standing, and then evaporated down to dryness, regenerating the Na2CO3 used in treating the beauxite to make the aluminate, less the inevitable losses inseparable from all industrial operations. The deposit of alumina is put into a conical strainer to drain, or else into a centrifugal drying machine, which rapidly drives out of the hydrated alumina the solution of Na2CO3 which impregnates it; a washing with pure water in the drier itself terminates the preparation of the alumina. At the works at Salindres, a part of this alumina is converted into sulphate of alumina, which is sold, the remainder being used for the aluminium manufacture. After washing in the dryer, the alumina presents this composition:

III. Preparation of the AlCl 2 NaCl.

When a current of chlorine is passed through a mixture of anhydrous alumina and carbon, AlCI is obtained. This simple chloride may be employed for obtaining aluminium; it was first so employed by Deville; but it is deliquescent, its preservation is difficult, and its employment very inconvenient. Industrially, as indicated by Deville, the double chloride is always used, as it does not present these inconveniences to so large a degree. The double chloride may be obtained in the same manner as the simple chloride; it is sufficient to put some common salt, NaCl, into a mixture of alumina and carbon, and, on heating this mixture strongly, there is formed, by the action of the chlorine, Al2C16.2NaCl, which distils at a red heat and condenses in a crystalline mass at about 200°. The hydrated alumina obtained in the preceding operation is mixed with salt and finely pulverized charcoal, in proper proportions, the whole is sifted, and a mixture produced as homogeneous as possible; then it is agglomerated with water and made into balls the size of the fist. These balls are first dried in a drying stove, at about 150°, then calcined at redness in retorts, where the double chloride should commence to be produced just as the balls are completely dried. These retorts are vertical cylinders of refractory earth, each one is furnished with a tube in its lower

part for the introduction of chlorine, and with another towards its upper end for the exit of the vapor of double chloride. (See Fig. 10.) A lid

carefully luted during the operation with a mixture of fine clay and horse dung serves for the charging and discharging of the retort. The double chloride is condensed in earthen pots like flower pots, made of ordinary clay, and closed by a well-luted cover, into which passes a pipe of clay to conduct the gas resulting from the operation into flues connected with the main chimney. Each retort is heated by a fire, the flame of which circulates all round it, and permits keeping it at a bright red heat. An operation is conducted as follows: