Slag sentence example
The slag and matte formed float upon the lead in the crucible and are tapped, usually together, at intervals into slag-pots, where the heavy matter settles on the bottom and the light slag on the top. When cold they are readily separated by a blow from a hammer.
Proctor in 1877 directed attention to the composition of the slag resulting from the burning of esparto, which they found to be strikingly similar to that of average medical bottle glass, the latter yielding on analysis 66.3% of silica and 25.1% of alkalies and alkaline.
These foliaceous heaps lie along the bank like the slag of a furnace, showing that Nature is "in full blast" within.
It was about this time that the first experiments were made (in Germany) with basic slag, a material which had hitherto been regarded as a worthless by-product of steel manufacture.
A year or two later field trials were begun in England, with the final result that basic slag has become recognized as a valuable source of phosphorus for growing crops, and is now in constant demand for application to the soil as a fertilizer.Advertisement
It is then raked out on the work-stone and divided into a very poor "grey" slag which is put aside, and a richer portion, which goes back into the furnace.
The "browse," after removal of the "grey" slag, is reintroduced, ore added, and, after a quarter of an hour's heating, the mass again placed on the work-stone, &c.
Thus, a mixture of lead sulphate (45%) and oxide (44%) with some sulphide (8%), zinc and carbonaceous matter, is agglomerated by a heap-roast and then smelted in a slag-eye furnace with grey slag from the ore-hearth.
The slag is a waste product, and the flue-dust, collected by special devices in dust-chambers, is briquetted by machinery, with lime as a bond, and then resmelted with the ore-charge.
The reason why at this level the walls must form an upright instead of an inverted cone, why the furnace must widen downward instead of narrowing, is, according to some metallurgists, that this shape is needed in order that, in spite of the pastiness of the slag in this formative period of incipient fusion, this layer may descend freely as the lower part of the column is gradually eaten away.Advertisement
The slag and metal produced are then run off and the latter is cast into bars; these are in general contaminated with iron, arsenic, copper and other impurities.
The lowest layer of the molten mass is principally metallic bismuth, the succeeding layers are a bismuth copper matte, which is subsequently worked up, and a slag.
The intense heat generated tends to liberate many impurities which are carried away in the slag.
But the best class of steel, crucible steel, was freed from slag by fusion in crucibles; hence its name, " cast steel."
Between 1860 and 1870 the invention of the Bessemer and open-hearth processes introduced a new class of iron to-day called " mild " or " carbon wcarbon steel," which lacked the essential property of steel, the hardening power, yet differed from the existing forms of wrought iron in freedom from slag, and from cast iron in being very malleable.Advertisement
As a result, certain varieties, such as blister steel, are called " steel " solely because they have the hardening power, and others, such as low-carbon steel, solely because they are free from slag.
The second period, by converting the metal into the fusible cast iron and melting this, for the first time removed the gangue of the ore; the third period by giving a temperature high enough to melt the most infusible forms of iron, liberated the slag formed in deriving them from cast iron.
Thomas, who showed that, in the presence of a slag rich in lime, the whole of the phosphorus could be removed readily.
Slag or Cinder, a characteristic component of wrought iron, which usually contains from 0.20 to 2.00% of it, is essentially a silicate of iron (ferrous silicate), and is present in wrought iron simply because this product is made by welding together pasty granules of iron in a molten bath of such slag, without ever melting the resultant mass or otherwise giving the envelopes of slag thus imprisoned a chance to escape completely.
These two things are done simultaneously by heating and melting the ore in contact with coke, charcoal or anthracite, in the iron blast furnace, from which issue intermittently two molten streams, the iron now deoxidized and incidentally carburized by the fuel with which it has been in contact, and the mineral matter, now called " slag."Advertisement
Hence from this level down the only solid matter is the coke, in lumps which are burning rapidly and hence shrinking, while between them the molten iron and slag trickle, somewhat as sketched in fig.
This carburizing is an indispensable part of the process, because through it alone can the iron be made fusible enough to melt at the temperature which can be generated in the furnace, and only when liquid can it be separated readily and completely from the slag.
The fuel has, in addition to its duties of deoxidizing and carburizing the iron and yielding the heat needed for melting both the iron and slag, the further task of desulphurizing the iron, probably by the reaction FeS+CaO+C=Fe+CaS+CO.
The desulphurizing effect of this transfer of the sulphur from union with iron to union with calcium is due to the fact that, whereas iron sulphide dissolves readily in the molten metallic iron, calcium;sulphide, in the presence of a slag rich in lime, does not, but by preference enters the slag, which may thus absorb even as much as 3% of sulphur.
The duty of the limestone (CaCO 3) is to furnish enough lime to form with the gangue of the ore and the ash of the fuel a lime silicate or slag of such a composition (1) that it will melt at the temperature which it reaches at about level A, of fig.Advertisement
Of these the silica and alumina are chiefly those which the gangue of the ore and the ash of the fuel introduce, whereas the lime is that added intentionally to form with these others a slag of the needed physical properties.
In the hearth of the blast furnace the heat made latent by the fusion of the iron and slag must of course be supplied by some body which is itself at a temperature above the melting point of these bodies, which for simplicity of exposition we may call the critical temperature of the blast-furnace process, because heat will flow only from a hotter to a cooler object.
Further, an important part of the silicon may be removed in the mixer by keeping it very hot and covering the metal with a rather basic slag.
Of these the first escapes immediately as a gas, and the others unite with iron oxide, lime, or other strong base present to form a molten silicate or silica-phosphate called " cinder " or " slag," which floats on the molten or pasty metal.
Beside this their chief and easy work of oxidizing carbon, silicon and phosphorus, the conversion processes have the harder task of removing sulphur, chiefly by converting it into calcium sulphide, CaS, or manganous sulphide, MnS, which rise to the top of the molten metal and there enter the overlying slag, from which the sulphur may escape by oxidizing to the gaseous compound, sulphurous acid, S02.
In the basic Bessemer process phosphorus is readily removed by oxidation, because the product of its oxidation, phosphoric acid, P 2 O 5, in the presence of an excess of base forms stable phosphates of lime and iron which pass into the slag, making it valuable as an artificial manure.
But this dephosphorization by oxidation can be carried out only in the case slag is basic. If it is acid, i.e.
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.
The slag, in order that it may have such an excess of base that this will retain the phosphoric acid as fast as it is formed by the oxidation of the phosphorus of the pig iron, and prevent it from being re-deoxidized and re-absorbed by the iron, should, according to von Ehrenwerth's rule which is generally followed, contain enough lime to form approximately a tetra-calcic silicate, 4CaO,S10 2 with the silica which results from the oxidation of the silicon of the pig iron and tri-calcic phosphate, 3CaO,P205, with the phosphoric acid which forms. The danger of this " rephosphorization " is greatest at the end of the blow, when the recarburizing additions are made.
The usual composition of this slag is iron oxide, i o to 16%; lime, 40 to 50%; magnesia, 5%; silica, 6 to 9%; phosphoric acid, 16 to 20%.
In order that the phosphoric acid may be the more fully liberated by the humic acid, &c., of the earth, a little silicious sand is mixed with the still molten slag after it has been poured off from the molten steel.
The slag is used in agriculture with no further preparation, save very fine grinding.
The removal of the greater part of the phosphorus takes place after the carbon has been oxidized and the flame has consequently " dropped," probably because the lime, which is charged in solid lumps, is taken up by the slag so slowly that not until late in the operation does the slag become so basic as to be retentive of phosphoric acid.
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.
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.
The gangue of the ore increases the quantity of slag, which separates the metal from the source of its heat, the flame, and thus delays the rise of temperature; and the purification by " oreing," i.e.
A cold lump of ore chills the slag immediately around it, just where its oxygen, reacting on the carbon of the metal, generates carbonic oxide; the slag becomes cool, viscous, and hence easily made to froth, just where the froth-causing gas is evolved.
The acid process goes on much faster, because in it the heat insulating layer of slag is much thinner.
The two metallic masses coalesce, and the reaction between the oxygen of one and the carbon of the other is therefore extremely rapid because it occurs throughout their depth, whereas in common procedure oxidation occurs only at the upper surface of the bath of cast iron at its contact with the overlying slag.
At this time the slag is temporarily rich in iron oxide and silica, resulting from the oxidation of the iron and of its silicon as the charge slowly melts and trickles down.
Such a slag not only corrodes the furnace lining, but also impedes dephosphorization, because it is irretentive of phosphorus.
At the Carnegie works Mr Monell gets the two dephosphorizing conditions, low temperature and basicity of slag, early in the process, by pouring his molten but relatively cool cast iron upon a layer of pre-heated lime and iron oxide on the bottom of the open-hearth furnace.
The lime and iron oxide melt, and, in passing up through the overlying metal, the iron oxide very rapidly oxidizes its phosphorus and thus drags it into the slag as phosphoric acid.
The ebullition from the formation of carbonic oxide puffs up the resultant phosphoric slag enough to make most of it run out of the furnace, thus both removing the phosphorus permanently from danger of being later deoxidized and returned to the steel, and partly freeing the bath of metal from the heat-insulating blanket of slag.
Yet frothing is not excessive, because the slag is not, as in common practice, locally chilled and made viscous by cold lumps of ore.
Where the carbon, in thus diffusing inwards, meets particles of the slag, a basic ferrous silicate which is always present in wrought iron, it forms carbonic oxide, FeO+ C = Fe+CO, which puffs the pliant metal up and forms blisters.
This in turn is in part because of the greater care which can be used in making these small lots, but probably in chief part because the crucible process excludes the atmospheric nitrogen, which injures the metal, and because it gives a good opportunity for the suspended slag and iron oxide to rise to the surface.
Till Huntsman developed the crucible process in 1740, the only kinds of steel of commercial importance were blister steel made by carburizing wrought iron without fusion, and others which like it were greatly injured by the presence of particles of slag.
Huntsman showed that the mere act of freeing these slag-bearing steels from their slag by melting them in closed crucibles greatly improved them.
A pair of electric arcs play between these electrodes and the molten steel, passing through the layer of slag, G, and generating much heat.
Like the Heroult furnace, the Kjellin furnace may be lined with either magnesite or chromite, and it may be tilted for the purpose of pouring off slag and metal.
In the first stage the phosphorus is removed from the molten steel by oxidizing it to phosphoric acid, P205, by means of iron oxide contained in a molten slag very rich in lime, and hence very basic and retentive of that phosphoric acid.
This slag is formed by melting lime and iron oxide, with a little silica sand if need be.
Floating on top of the molten metal, it rapidly oxidizes its phosphorus, and the resultant phosphoric acid combines with the lime in the overlying slag as phosphate of lime.
When the removal of the phosphorus is sufficiently complete, this slag is withdrawn from the furnace.
Next the metal is covered with a very basic slag, made by melting lime with a little silica and fluor spar.
This sulphide is nearly insoluble in the metal, but is readily soluble in the overlying basic slag, into which it therefore passes.
Here sulphur may indeed be removed to a very important degree in the form of manganese sulphide, which distributes itself between metal and slag in rough accord with the laws of equilibrium.
When the desulphurization is sufficiently complete, the sulphurbearing slag is removed, the final additions needed to give the metal exactly the composition aimed at are made, and the molten steel is tapped out of the furnace into its moulds.
While the metal lies tranquilly on the bottom of the furnace, any slag mechanically suspended in it has a chance to rise to the surface and unite with the slag layer above.
The reason for this is that in it the slag, by means of which all the purification must needs be done, is not heated effectively; that hence it is not readily made thoroughly liquid; that hence the removal of the phosphoric slag made in the early dephosphorizing stage of the process is liable to be incomplete; and that hence, finally, the phosphorus of any of this slag which is left in the furnace becomes deoxidized during the second or deoxidizing stage, and is thereby returned to befoul the underlying steel.
The reason why the slag is not heated effectively is that the heat is developed only in the layer of metal itself, by its resistance to the induced current, and hence the only heat which the slag receives is that supplied to its lower surface by the metal, while its upper side is constantly radiating heat away towards the relatively cool roof above.
In order that this finely divided slag shall rise to the surface and there coalesce with the overlying layer, the metal must be tranquil.
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.
Because this pipe is due to the difference in the rates of contraction of interior and exterior, it may be lessened by retarding the cooling of the mass as a whole, and it may be prevented from stretching down deep by retarding the solidification of the upper part of the ingot, as, for instance, by preheating the top of the mould, or by covering the ingot with a mass of burning fuel or of molten slag.
The slag, which must contain a high percentage of lime, is granulated by being run while fused into abundance of water.
This granulated slag differs from the same slag allowed to cool slowly, in that a portion of the energy which it possesses while fused is retained after it has solidified.
It bears to ordinary slowly-cooled slag a similar relation to that borne by plastic sulphur to ordinary crystalline sulphur.
This potential energy becomes kinetic when the slag is brought into contact with lime in the presence of water, and causes the formation of a true hydraulic silicate of lime.
The usual proportions are three of slag to one of slaked lime by weight.
The product termed slag cement sets slowly, but ultimately attains a strength scarcely inferior to that of Portland cement.
The commonest of such substances in England are chalk and clay, but where local conditions demand it, limestone, marl, shale, slag or any similar material may be used, provided that the correct proportions of lime, silica and alumina are maintained.
It is made by granulating blast furnace slag of suitable composition and finely grinding the product, either alone or with an admixture of about To% of Portland cement clinker.
The particular method of granulating slag for Passow cement produces a material which sets per se and attains a strength comparable with that of Portland cement.
Passow cement has been successfully made from slag of different compositions in Germany, England and America.
The coarse-metal is now smelted, with coke and siliceous fluxes (in order to slag off the iron), and the product, consisting of an impure copper sulphide, is variously known as " blue-metal," when more or less iron is still present, " pimplemetal," when free copper and more or less copper oxide is present, or " fine " or " white-metal," which is a fairly pure copper sulphide, containing about 75% of the metal.
But as the slag carries on an average 46% of silica, it is only through the utmost skill that it can be made to run as low on an average as 0.3% in copper oxide.
A single section can be removed and replaced without entirely emptying the stack, as a shell of congealed slag always coats the inner surface of the jacket.
The largest furnaces are those of the Boston & Montana Company at Great Falls, Montana, which have put through soo tons of charge daily, pouring their melted slag and matte into large wells of io ft.
Practice, however, in treating copper matte differs essentially from the treatment of pig iron, inasmuch as from 20 to 30% of iron must be eliminated as slag and an equivalent quantity of silica must be supplied.
The slag is then poured and skimmed, the blast turned on and converter retilted.
But, besides removing the psychological slag which clung to Kant's ideas from their matrix and presenting reason as the active principle in the formation of a universe, his successors carried out with far more detail, and far more enthusiasm and historical scope, his principle that in reason lay the a priori or the anticipation of the world, moral and physical.
For rich ores the method of roasting the sulphide with metallic iron is sometimes employed; carbon and salt or sodium sulphate being used to slag the iron.
In the first the gold or silver is made to combine or alloy with metallic lead, the other constituents of the ore being separated from the lead as slag.
As the slag thus formed flows off to the sides of the scorifier, the assay clears and the melted metallic lead forms an "eye" in the middle.
When the "eye" has quite disappeared the door is closed and the temperature raised to make the slag very liquid.
The scorifier is taken from the muffle in a pair of tongs and the contents poured into a mould, the lead forming a button in the bottom while the slag floats on top. When cold, the contents of the mould are taken out and the lead button hammered into the form of a cube, the slag, which is glassy and brittle, separating readily from the metal, which is then ready for cupellation.
The resulting lead button hammered P into shape and carefully cleansed from slag is ready for the cupel.
The character and amount of the flux necessarily depend upon the character of the ore, the object being to concentrate in the lead button all the gold and silver while dissolving and carrying off in the slag the other constituents of the ore.
Under the most favourable conditions there is a slight loss of gold and silver in the fusion, the scorification and the cupellation, both by absorption in the slag and by actual volatilization and absorption in the cupel.
When cold, the button is hammered, cleaned carefully from slag, and weighed.
In either case we obtain a regulus of silver lying under a fused slag of chloride.
The calcium silicate remains in the furnace in the form of a liquid slag, which may be run off, so that the action is practically continuous.
The alternating current is generally used, the action not being electrolytic. One of the special advantages of the electrical over the older process is that the distilling vessels have a longer life, owing to the fact that they are not externally heated, and so subjected to a relatively high temperature when in contact with the corrosive slag formed in the process.
In Hellenic times Soli had little political importance, though it stood a five months' siege from the Persians soon after 50o B.C.; its copper mines, however, were famous, and have left copious slag heaps and traces of small scattered settlements.
In the old copper-smelting district of Arabia Petraea, clay blast-pipes dating back to the earlier dynasties of ancient Egypt have been found buried in slag heaps; and in India the native smiths and iron-workers continue to use furnaces of similar types.
The earthy matters form a fusible glass or slag melt, and collect at the lowest point of the hearth, whence they are removed by opening a hole pierced through the front wall at the bottom.
When melted the products separate on the bed (which is made of closely packed sand or other infusible substances), according to their density; the lighter earthy matters forming an upper layer of slag are drawn out by the slag hole K at the flue end into an iron wagon or bogie, while the metal subsides to the bottom of the bed, and at the termination of the operation is run out by the tap hole L into moulds or granulated into water.
In some processes of lead-smelting, where the minerals treated contain sand, the long calciner is provided with a melting bottom close to the fire-place, so that the desulphurized ore leaves the furnace as a glassy slag or silicate, which is subsequently reduced to the metallic state by fusion with fluxes in blast furnaces.
Such parts as may be subjected to extreme heat and the fretting action of molten material, as the tuyere and slag breasts of blast furnaces, and the fire bridges and bed plates of reverberatory furnaces, are often made in cast iron with double walls, a current of water or air being kept circulating through the intermediate space.
This method has received considerable extension, notably in furnace-smelting of iron ores containing manganese, where the entire hearth is often completely water-cased, and in some lead furnaces where no firebrick lining is used, the lower part of the furnace stack being a mere double iron box cooled by water sufficiently to keep a coating of slag adhering to the inner shell which prevents the metal from being acted upon.
The introduction and withdrawal of the charges in fusion furnaces is effected by gravitation, the solid masses of raw ore, fuel and flux being thrown in at the top, and flowing out of the furnace at the taphole or slag run at the bottom.
The residue contained frequent charcoal, occasional bone and some hammerscale, slag, and rare amounts of pottery.
The residue contained fragments of CBM, occasional mortar flecks and charcoal, rare pieces of slag and fragments of iron.
Remains of these smelting furnaces were found among the slag pieces.
These are small conical mounds or horseshoe shaped mounds of slag associated with bloomery furnaces geographically situated in the Highlands of Scotland.
The performance of medium grade recycled glass grit supplied by Wolverhampton Abrasives was compared with copper slag using the same blasting equipment.
The slag heaps or pit mounds of closed mines have been put to many new uses.
Physical evidence for ancient metallurgy existed in the form of large slag heaps but there was no contemporary mining.
In England on one occasion in a little town where there were iron and ore mines, they would pile up the slag.
Hammerscale, slag, iron objects and charred organic material (including nutshell) were rare, and there were several fragments of glass.
Basically it involves soil amelioration for slag waste sites, coal shale waste, builders rubble and pulverized fuel ash.
During the cleaning of these pieces, 3 small, abraded sherds of pottery were found to have adhered to the slag.
Pig iron is refined in a converter and then poured into molten iron silicate slag.
Still young, don't slag him off yet!
The impurities then float to the top of the metal forming slag.
I think some of the fillers used included slag and ash from iron works and coal fired power stations.
Even the cement was produced using powdered ground granulated blast furnace slag (GGBS ).
A hole is dug into the ground making a channel to allow molten slag to escape.
Why would I want some fat slag on TV playing with herself with a wine bottle?
Inside the cage were their feathers heaped like a black slag of tar.
The seventeenth century produced a lot of the old slag in the Belfry area.
Huge amounts of furnace slag were used in building Roman roads.
Indeed some blast furnace slag is used to make glass and cement.
The literature review is focused on glass grit and copper slag, but the scope of the work includes other key blasting media.
How about iron Age pottery from Saxon ceramics or iron slag from copper working waste?
To evaluate the temperature effects of BOS steel slag on the soil stabilization process.
Peasant scavenging for coal with his son on a state-owned coal mine slag heap.
The dried residue was rather stony and contained occasional animal bones, while pottery and slag were rare.
The earth surface above these natural furnaces has been hardened, cracked and sometimes melted into a reddish slag, called scoria, which, on account of its resemblance to lava, has given rise to an incorrect impression that the region was once the centre of volcanic disturbances.
Now in an acid-lined converter the slag is necessarily acid, because even an initially basic slag would immediately corrode away enough of the acid lining to make itself acid.
It differs from ordinary slag cement (see above) in that it is not a pozzuolanic cement depending on the interaction of granulated slag and lime.
Peasant woman scavenging for usable fuel on a state-owned coal mine slag heap that over-shadows her village.
Still young, do n't slag him off yet !
For reasons of energy saving, it is expedient to insulate the adsorber with mineral or slag wool, also as contact protection.
The size of the slag cakes is related to the size of the smelting furnaces, which must have been quite large.
Some of the slag fragments have multiple layers that may result from continuous or serial tapping.
Chemical analyzes of soil samples indicated elevated levels of arsenic, zinc, tin and copper bound into the slag deposits.
It 's not as if you have been going out of your way to slag people off 24/7 !
How about Iron Age pottery from Saxon ceramics or iron slag from copper working waste?
We then joined the supernumerary members of the expedition on an adjacent slag heap, over which the UFO was said to have hovered.
Wrought iron is an iron alloy type of metal that has a very low carbon content compared to steel and has fibers inclusions known as slag.
Other batt and roll attic insulation products are manufactured with rock and slag wool or cotton and denim.
Rock and slag is not commonly used in the industry today due to the ease of fiber glass manufacturing operations.
Similar to batt and rolls, loose fill uses fiber glass, rock and slag wool, or cellulose pieces that are blown into the attic space using a large flexible hose attached to a pump blower.
Since this type of loose fill is much denser and therefore heavier than fiber glass or slag and wool, it sometimes may not be able to be used in attics since too much weight would cause the drywall ceiling below to sag.
The ballast consists of such materials as broken stone, furnace slag, gravel, cinders or earth, the lower layers commonly consisting of coarser materials than the top ones, and its purpose is to provide a firm, well-drained foundation in which the sleepers or crossties may be embedded and held in place, and by which the weight of the track and the trains may be distributed over the road-bed.
The leading products of the blast-furnace are argentiferous lead (base bullion), matte, slag and flue-dust (fine particles of charge and volatilized metal carried out of the furnace by the ascending gas current).
For the commoner grades of dark-coloured bottles the glass mixture is cheapened by substituting common salt for part of the sulphate of soda, and by the addition of felspar, granite, granulite, furnace slag and other substances fusible at a high temperature.
When used for ore smelting, the reduced metal and the accompanying slag were to be caught, after leaving the arc and while still liquid, in a hearth fired with ordinary fuel.
When prolonged heating is required at very high temperatures it is found necessary to line the furnace-cavity with alternate layers of magnesia and carbon, taking care that the lamina next to the lime is of magnesia; if this were not done the lime in contact with the carbon crucible would form calcium carbide and would slag down, but magnesia does not yield a carbide in this way.
But whereas, from its construction, the Siemens furnace was intermittent in operation, necessitating stoppage of the current while the contents of the crucible were poured out, many of the newer forms are specially designed either to minimize the time required in effecting the withdrawal of one charge and the introduction of the next, or to ensure absolute continuity of action, raw material being constantly charged in at the top and the finished substance and by-products (slag, &c.) withdrawn either continuously or at intervals, as sufficient quantity shall have accumulated.
The material for filling may be the waste from earlier workings stored in the spoil banks at the surface; where there are blast furnaces in the neighbourhood, granulated slag mixed with earth affords excellent packing.
Other matrices are slag cement, a comparatively recent invention, and some other natural and artificial cements which find occasional advocates.
Broken bricks or tiles and broken furnace slag are sometimes used, the essential points being that the aggregate should be hard, clean and sound.
At length when the furnace was tapped a white slag was drawn off from the top, and the liquid metal beneath was received into a ladle and poured into cast-iron moulds.
In a natural state it is obtained from bones, guano and wood ashes; and in an artificial condition from basic slag or Thomas's phosphate, coprolites and superphosphate of lime.
This they did with the excuse that the new product resembled one class of steel - cast steel - in being free from slag; and, after a period of protest, all acquiesced in calling it " steel," which is now its firmly established name.
About 1740 Benjamin Huntsman introduced the " crucible process " of melting steel in small crucibles, and thus freeing it from the slag, or rich iron silicate, with which it, like wrought iron, was mechanically mixed, whether it was made in the old forge or in the puddling furnace.
Further, the more sulphur there is to remove, the greater must be the quantity of slag needed to dissolve it as calcium sulphide.
But immediately above this level the charge is relatively viscous, because here the temperature has fallen so far that it is now at the melting or formation point of the slag, which therefore is pasty, liable to weld the whole mass together es so much tar would, and thus to obstruct the descent of the charge, or in short to " scaffold."
To this objection it may in turn be answered that, though this degree of freedom of descent may suffice for a slowrunning furnace, particularly if the slag is given such a composition that it passes quickly from the solid state to one of decided fluidity, yet it is not enough for swift-running ones, especially if the composition of the slag is such that, in melting, it remains long in a very sticky condition.
Beyond this, wrought iron, and certain classes of steel which then were important, necessarily contained much slag or " cinder," because they were made by welding together pasty particles of metal in a bath of slag, without subsequent fusion.
In smelting the rich Lake Superior ores the quantity of slag made was formerly as small as 28% of that of the pig iron, whereas in smelting the Cleveland ores of Great Britain it is usually necessary to make as much.
Below this level the solid charge descends easily, because it consists of coke alone or nearly alone, and this in turn because the temperature here is so high as to melt not only the iron now deoxidized and brought to the metallic state, but also the gangue of the ore and the limestone, which here unite to form the molten slag, and run freely down between the lumps of coke.