The following is an outline of the process when hydrochloric acid is used: Starch ("` green" starch in America) is made into a "milk" with water, and the milk pumped into boiling dilute acid contained in a closed "converter," generally made of copper or cast iron; steam is led in at the same time, and the pressure is kept up to about 25 lb to the sq.
When the converter is full the pressure is raised somewhat, and the heating continued until the conversion is complete.
On its way from the blast furnace to the converter or open hearth furnace the pig iron is often passed through a great reservoir called a " mixer," which acts also as an equalizer, to lessen the variation in composition of the cast iron, and as a purifier, removing part of the sulphur and silicon.
There are two distinct varieties of this process, the original undephosphorizing or " acid " Bessemer process, so called because the converter is lined with acid materials, i.e.
Siliceous; and the dephosphorizing or " Thomas " or " basic Bessemer " process, so called because the converter is lined with basic materials, usually calcined dolomite, a mixture of lime and magnesia, bound together with tar, and because the slag is made very basic by adding much i?`- -, - .1 -?=Woi krii fr ?`?'??-«:: e i h ..
Though all this is elementary to-day, not only was it unknown, indeed unguessed, at the time of the invention of the Bessemer process, but even when, nearly a quarter of a century later, a young English metallurgical chemist, Sidney Gilchrist Thomas (1850-1885), offered to the British Iron and Steel Institute a paper describing his success in dephosphoriz ing by the Bessemer process with a basic-lined converter and a basic slag, that body rejected it.
In carrying out the acid Bessemer process, the converter, preheated to about 1200 0 C. by burning coke in it, is turned into the position shown in fig.
- Bessemer Converter, of molten pig iron, which turned down in position to receive sometimes weighs as much and discharge the molten metal.
The converter is then turned upright into the position shown in fig.
16, so that the blast, which has been let on just before this, entering through the great number of tuyere holes in the bottom, forces its way up through the relatively shallow layer of iron, throwing it up within the converter as a boiling foam, and oxidizing the foreign elements so rapidly that in some cases their removal is complete after 5 minutes.
The oxygen of the blast having been thus taken up by the molten metal, its nitrogen issues from the mouth of the converter as a pale spark-bearing cone.
To carbonic acid as it meets the outer air on escaping from the mouth of the converter, and generates a true flame which grows.
Of the flame are not so decisive as to justify them in omitting to test the steel before removing it from the converter, as a check on the accuracy of their blowing.
Adds enough carbon to give it the content desired, and then immediately pours the steel into a great claylined casting ladle by turning the converter over, and through a nozzle in the bottom of this ladle pours the steel into its ingot moulds.
The lining of the converter is made of 90% of the mixture of lime and magnesia which results from calcining dolomite, (Ca,Mg)CO i, at a very high temperature, and 10% of coal tar freed from its water by heating.
In either case such a lining is expensive, and has but a short life, in few works more than 200 charges, and in some only loo, though the silicious lining of the acid converter lasts thousands of charges.
Further objections to the presence of silicon are that the resultant silica (1) corrodes the lining of the converter, (2) makes the slag froth so that it both throws much of the charge out and blocks up the nose of the converter, and (3) leads to rephosphorization.
But Massenez and Richards, following the plan outlined by Pourcel in 1879, have found that even 3% of silicon is permissible if, by adding iron ore, the resultant silica is made into a fluid slag, and if this is removed in the early cool part of the process, when it attacks the lining of the converter but slightly.
Whatever be the form into which the steel is to be rolled, it must in general first be poured from the Bessemer converter in which it is made into a large clay-lined ladle, and thence cast in vertical pyramidal ingots.
But no part of the Bessemer converter is of a shape easily affected by the heat, no part of it is exposed to the heat on more than one side, and the converter itself is necessarily cooler than the metal within it, because the heat is generated within the metal itself by the combustion of its silicon and other calorific elements.
In it the steel heats the converter, whereas in the open-hearth and crucible processes the furnace heats the steel.
It is in large part because of this shallowness, which contrasts so strongly with the height and roominess of the Bessemer converter, that the process lasts hours where the Bessemer process lasts minutes, though there is the further difference that in the open-hearth process the transfer of heat from flame to charge through the intervening layer of slag is necessarily slow, whereas in the Bessemer process the heat, generated as it is in and by the metallic bath itself, raises the temperature very rapidly.
In the duplex process the conversion of the cast iron into steel is begun in the Bessemer converter and finished in the openhearth furnace.
In the most promising form of this process an acid converter and a basic open-hearth furnace are used.
But if the conversion is only begun in the converter and finished on the open-hearth, then there is no need of regulating the temperature in the converter closely, and variations in the silicon-content of the pig iron thus become almost harmless in this respect.
But if the silicon of the pig iron is removed by a preliminary treatment in the Bessemer converter, then its presence in the pig iron is harmless as regards the open-hearth process.
Looking at the duplex process in another way, the preliminary desilicidizing in the Bessemer converter should certainly be an advantage; but whether it is more profitable to give this treatment in the converter than in the mixer remains to be seen.
Electric furnaces are at an advantage over others as regards the removal of sulphur and of iron oxide from the molten steel, because their atmosphere is free from the sulphur always present in the flame of coal-fired furnaces, and almost free from oxygen, because this element is quickly absorbed by the carbon and silicon of the steel, and in the case of arc furnaces by the carbon of the electrodes themselves, and is replaced only very slowly by leakage, whereas through the Bessemer converter and the open-hearth furnace a torrent of air is always rushing.
Why the molten metal can be freed from mechanically suspended slag more perfectly in them than in the Bessemer converter or the open-hearth furnace.
But tranquillity is clearly imp ssib'e in the Bessemer converter, in which the metal can be kept hot only by being torn into a spray by the blast.
In short the electric furnaces can be used to improve the molten product of the Bessemer converter and open-hearth furnace, essentially because their atmosphere is free from sulphur and oxygen, and because they can therefore remove sulphur, iron oxide and mechanically suspended slag, more thoroughly than is possible in these older furnaces.
They are not likely to displace either the open-hearth furnace or the Bessemer converter, because their normal work is only to improve the product of these older furnaces.
When the former is used it is roasted with calcium sulphate or alkali waste to form a matte which is then blown in a Bessemer converter or heated in a reverberatory furnace with a siliceous flux with the object of forming a rich nickel sulphide.
In America the usual method is to roast ores or concentrates so that the matte yielded by either the reverberatory or cupola furnace will run from 45 to 50% in copper, and then to transfer to the Bessemer converter, which blows it up to 99%.
The only practical mode of doing this, as yet devised, is by lining the converter with a silicious mixture.
The other items of cost are labour, the quantity of which depends on the mechanical appliances provided for handling the converter shells and inserting the lining; and the blast, which in barrel-shaped converters is low and in vertical converters is high, and which varies therefore from 3 to is lb to the square inch.
The quantity of air consumed in a converter which will blow up about 35 tons of matte per day is about 3000 cub.
The slag is then poured and skimmed, the blast turned on and converter retilted.
With pyritic smelting a sulphuretted copper ore, fed into a cupola in the morning, can be passed directly to the converter, blown up to metal, and shipped as 99% bars by evening - an operation which formerly, with heap roasting of the ore and repeated roasting of the mattes in stalls, would have occupied not less than four months.
A large furnace and a Bessemer converter, the pair capable of making a million pounds of copper a month from a low-grade sulphuretted ore, will not occupy a space of more than 25 ft.