Pigments, Paint and Painting: A practical book for practical men

by George Terry

It is to similar action of electricity as that to which we last refer that we ascribe the formation of the crust on the gratings of lead in the non-adherent and disintegrated condition in which it is produced, and by reason of which it is so easily detached from the lead and broken up to powder. No edge runner grinding, such as is required by the stack process, is in this case necessary.

The crude white lead and crushed material, whether in a dry state, or incorporated with oil, is finished and ground in a granite roller paint mill, from which it issues as dry white lead, or as white lead in oil.

Paint made from this electric white lead has been sent to America, to France, to Belgium, to Germany for trial, and has also been largely tested in this country by painters, engineers, and others unacquainted with its precise nature, and it has been productive of good results.

Of its density, body, and covering power, there can be no doubt, and never once have these qualities been called in question.

The cost of the manufacture of white lead by the stack process is about 3l. to 3l. 10s. per ton. By the German method, the cost is about the same as by the stack. By Gardner’s electric process, the cost of conversion is 10s. a ton. To this 10s. must be added the cost of labour expended in preparation, an item which cannot be well estimated on the present limited scale of manufacture; it could not exceed an additional 15s. a ton. This would bring the cost of manufacture of electric white lead to 25s. a ton.

In this electric process inferior lead can be operated upon with success. Brands of that metal such as white lead makers by the stack method dare not employ, may be successfully converted in an electric chamber, and with fair results as to the quantity and quality of the white lead produced.

By the use of Gardner’s electric process it would appear that we not only preserve health, but save lives; we not only save time, but interest on large capitals, which lie idle for long periods at a time; and we can economise and simplify the whole manufacture and preparation of white lead, divesting it of all its present cumbersome and unhealthy stages. Gardner’s process, we believe, must take a prominent position as one of the most necessary, valuable, and scientific inventions of modern times.

Hannay’s Process.—Mr. J. B. Hannay, whose name is well known in connection with various chemical and engineering inventions and processes, has recently brought out a process for the manufacture of white lead. The old method of producing white lead or carbonate of lead is one involving much time and labour, together with no small risk to the health of the workpeople. By the new process brought out by Mr. Hannay, sulphate of lead is manufactured direct from galena or lead ore, without the necessity of the intermediate process of the reduction of the ore and the extraction of metallic lead. It is said that the sulphate of lead produced by the new process is whiter and more permanent than the carbonate.

The process is described as follows:—A furnace, 36 in. by 30 in., and 48 in. deep, contracting to a narrow chamber about 36 in. long by 14 in. wide, communicating with the main flue, is charged with coke and brought to a red heat. The bed of coke is so thick as to be almost up to the level of the sill of the furnace door. The lead ore is not charged in large quantity and left for an indefinite time, but is thrown in in quantities of a few shovelfuls at a time, the object being to effect extremely rapid volatilisation and consequent oxidation. The proportion of ore to fuel is 1 ton to 1 ton, and the result is said to be the conversion of 95 per cent. of the charge into its equivalent in white lead. In the first or wide chamber, the coke is oxidised, carbonic oxide being the resulting gas, the rapid volatilisation of the galena or sulphide of lead also taking place in the same chamber. On entering the inner portion of the furnace, the volatilised sulphide of lead and the carbonic oxide are converted into carbonic acid and sulphate of lead.

Forced blast has been employed to cause the high temperature necessary for the volatilisation of the ore; but it has been found unnecessary, admission of air at atmospheric pressure through tuyere holes being quite adequate. After leaving the inner chamber of the furnace, the gases pass into a flue, level with the furnace, and about 40 feet long by 16 square feet in sectional area. From this flue the gases pass into a tower about 20 feet high, and from thence into wrought-iron flues 3 feet in diameter. These flues terminate in a wrought-iron chest, in which are fitted two steam injectors. The gases are forced by these injectors into the central chambers of the condensers. These condensers are two in number, and contain a central chamber, 16 feet by 12 inches, into which the gases are forced as described. The gases escape through interstices into the outer chambers of the condenser, and are there condensed, a continuous stream of water occupying the lower part of the condenser. The waste gases escape by a downcast leading to a tall chimney. The temperature of the gases as they enter the condenser is about 840° F.