US 3499755 A
Description (OCR text may contain errors)
March 10,. 1970 Filed Jan. 5. 1967 A 0 S FURNACE D. o. MORRIS v METHOD FOR THE PRODUCTION OF PIG IRON AND STEEL IRON RECARBONISER FIGI CARBON PERENTAGE 3 Sheets-Sheet 1 I B o s FURNACE PIG MILL
DIRECT MOULDING FOUNDRY CARBON ABSORPTION IN LIQUID IRON VERSUS TIME FIG.2
TIME IN CONTACT WITH CARBON MINUTES March 10, 1970 o. Q MQRRIs 3,
METHOD FOR THE PRODUCTION OF PIG IRON AND STEEL Filed Jan. 5, 1967 3 Sheets-Sheet 2 TEMPERATURE IN DEGREES CENTIGRADE March '10; 1970 D. o; MORRIS METHOD FOR THE PRODUCTION OF PIG IRON AND STEEL Filed Jan. 3, 1967 3 Sheets-Sheet. 8
United States Patent 3,499,755 METHOD FOR THE PRODUCTION OF PIG IRON AND STEEL David 0. Morris, 11 Turnbull SL, Merewether, New South Wales, Australia Filed Jan. 3, 1967, Ser. No. 606,915 Claims priority, applicationiAustralia, Jan. 10, 1966, 255 6 Int. Cl. CZlb 13/00; C21c /28, 1/00 U.S. C]. 7546 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method essentially involving:
(1) Supplying a charge of liquid pig iron derived from an iron carboniser unit together with added supplements of iron bearing materials and heat producing components such as carbon, silicon, aluminium, metal carbides or metal alloys to a crude oxygen steel furnace.
(2) The oxygen steel furnace may be lined with acid, basic or a neutral furnace lining, an acid oxygen steel furnace being generally preferred.
(3) Crude steel from the crude oxygen steel furnace is returned to the iron carboniser at a temperature between l50l700 C. where carbon and further supplements of iron bearing materials and heat producing components are added and the cycle repeated resulting in a continuously increased volume of pig iron.
(4) A proportion of the pig iron is periodically drawn off and utilised for conversion to high quality steel or for foundry purposes and the balance recycled again producing pig iron.
The present invention relates to a method and means for the production of pig iron and steel eliminating conventional requirements utilising blast furnaces and requiring metallurgical coke.
The production of pig iron for steel making and other purposes has traditionally involved a blast furnace charged from the top with iron ore, limestone and metallurgical coke through which large volumes of heated air is blown through tuyeres near the bottom.
To permit the free passage of the gases through the furnace all charge constituents should exceed a certain minimum size and be reasonably free of fines.
In a blast furnace the metallurgical coke, besides providing heat and reducing agent has other functions to perform and the lumps should not only conform to certain minimum and maximum dimensions but also be strong enough to support the charge without crushing or excessive loss of size by abrasion during its passage down the furnace.
By virtue of the fact that not all coals available in commercial quantity yield coke of desirable size, shatter and abrasion resistance, the location and economic employment of blast furnaces tends to be limited to areas in which suitable coking coal is available. Further, because of the ever increasing scale of consumption, known reserves of coking coals are more or less rapidly dwindling in all countries of appreciable steehnaking capacity. On the other hand however, many countries have large deposits of carbonaceous fuels which are unsuitable for conventional coke production although much thought has been given to alternate methods of coke making.
In Australia some examples of such coals include the brown coals at Morwell and Yallourn in Victoria and Leigh Creek in South Australia, the sub-bituminous coals at Collie, West Australia, and the practically noncaking bituminous Australasian and Wallarah seam coals of the Newcastle coal measures.
The development of a method of making pig iron without blast furnaces and a means of making steel without blast furnace iron, but capable of utilising either metallurgical coke or other carbonaceous material as desired or convenient, is of great value to the iron and steel industry, and would facilitate the establishment of iron and steel plants in many regions lacking coking coals. Further, such developments would prove of great benefit to many established iron and steel plants where good coking coal is expensive or in short supply compared with available noncoking or poorly coking coals.
The product of the blast furnace contains, besides iron, up to about 10% of other elements including carbon, which usually ranges between 3.0% and 4.5% depending on application and other circumstances. However, While pig iron is used in large amounts in iron foundries for making iron castings without radical alteration in its composition, its main application is for the manufacture of steel.
In the art of ron and steel making the technical difference between the two is simply that the latter contains less than about 1.7% of carbon while the former contains more than this figure, and the conversion of iron to steel consists primarily of reducing the carbon from over 3% to steel specification requirements, which in most cases do not exceed 0.50%.
As it is impossible to adequately restrict or control the absorption of large quantities of carbon in the blast furnace the conversion of iron to steel must be carried out in separate steelmaking units.
Another limitation of the blast furnace is that carbon, which is the essential reducing agent and source of heat, must be in the form of coke, low in ash and chemical impurities and also conform to special size and physical property requirements, obtainable only from caking coals of suitable coking characteristics.
These factors prevent the general utilisation of cheap noncoking coals, gas or oil as a substitute for metallurgical coke in blast furnaces and except in special circumstances restricts the location of iron and steel plants to sites Where suitable coking coals are reasonably easily available or where coal or coke transport costs are offset by other factors.
Because large quantities of carbon absorbed during iron manufacture must be removed to convert iron to steel, combined with the fact that high grade coke is essential for satisfactory operation of the blast furnace even when other cheap fuels are available, many efforts have been made to make steel direct from the ore, or to make iron for steelmaking without ablast furnace.
While several of these processes are technically practical none have proved economically successful except under special circumstances or conditions as indicated by the fact that 99% of iron and steel is still manufactured by conventional means.
Some of the more important reasons for economic failure of direct reduction processes include:
(a) Most of the suggested steelmaking procedures do not include a melting stage and the product still contains the gangue from the ore. Because of this the material produced requires melting in conventional furnaces for final purification and can therefore be used onl as a kind of scrap, which, because of its low carbon cannot satisfactorily replace the pig iron normally required in the charge.
(b) Some processes while satisfactory in small scale operation present major difficulties in scaling up to the capacities needed in conventional iron and steel manufacture.
(0) Capital cost or cost of maintenance is excessive.
(d) No process has hitherto been suggested which will concurrently make both pig iron without metallurgical coke and steel Without blast burnace iron, or in other words, constitute a completely functional iron and steel plant without coke ovens or blast furnaces.
The proposed combination method of making iron without metallurgical coke and steel without blast furnace iron has not the foregoing disadvantages and is characterised by the following:
(1) It can make all grades of pig iron normally produced in a blast furnace.
(2) As metallurgical coke is not necessary for iron production the process can satisfactorily function as an integrated iron and steel plant, even when far removed from blast furnaces and coke ovens. It can also, under similar conditions, function as an iron plant only, without the necessity to find useful applications for embarassingly large quantities of blast furnace and coke oven gas.
(3) High grade steels in ingot form and equivalent in all respects to conventional product can be manufactured.
(4) With the exeception of one item all major equipment has been used and proved in conventional operation. The item referred to is extremely simple in design and operation and capable of high production in tons per hour.
The new iron and steel process is based on the Well known fact that conventional pneumatic methods of steelmaking such as the B03 and LD, Rotor, Kaldo, Bessemer and Thomas all develop heat in excess of that needed to convert liquid pig iron to liquid steel and normally necessitate the addition of steel scrap, iron oxide, steam etc. to reduce the steel temperatures to those required for tapping and teeming the steel into ingots.
The relative consumption of steel scrap as a coolant in the several pneumatic processes mentioned approximate the following:
Percent scrap Rotor and Kaldo 30-40 BOS or LD 25-30 Bessemer (acid) 11 Thomas 8 The greater scrap absorption capacity of the rotary oxygen furnaces, compared with the B08 is primarily due to more eflicient heat utilisation. This arises from the fact that most of the CO evolved from the steel is burnt with good effect above the bath whereas in the nonrotary vessels most of the CO produced is wasted in the eflluent gases.
Unfortunately the production rate of rotary vessels in tons per hour is considerably less than for the BOS and refractory life is also much lower.
The Oxygen furnaces of course Waste less heat than the airblown processes because in the latter more calories are carried out in the effluent gases by the nitrogen of the air used.
Finally the acid Bessrner conserves more heat than the. basic Bessemer (Thomas process, mainly because of the smaller volume of the acid slag.
In all the pneumatic processes iron oxide may be used as a coolant and, provided sufiicient reducing agent is available the iron in the iron oxide is directly reduced to metal, thus increasing the quantity of steel produced. Because of the endothermic reactions involved, the cooling power of iron oxide is about 2.0-3.0 times that of steel scrap.
The new method of producing pig iron and steel essentially comprises the following steps:
(1) Liquid pig iron from an iron recarboniser, together with iron ore in various conditions and with scrap if desired, plug additional heat producing and reducing agents as necessary, is blown to crude steel in an oxygen crude steel furnace lined with nonbasic refractories, with marked gain in metal volume due to the ore and/ or scrap additions.
(2) The crude steel so produced is returned to the iron recarboniser where it absorbs carbon, yielding pig iron in considerably greater quantity than the liquid pig 8 iron originally charged into the oxygen crude steel furnace. Steps (1) and (2) are continuously repeated and constitute a cyclic process, With marked gain in metal with each cycle.
(3) The excess metal over that required for recycling is used (a) for conversion to finished steel in a conventional basic oxygen steel furnace or (b) cast into pigs for foundry or other irons, or (c) used for direct metal castings.
Very low carbon steel (ingot iron) was melted in a high frequency induction furnace and then transferred to a gas fired crucible furnace heated to approximately l400 C., in which it was kept in contact with pieces of carbonised noncoking coal by submerging the latter in the metal by a refractory shape, weighted with a heavy steel block.
At intervals a sample of the metal was taken for analysis.
A number of experiments of this nature were carried out and a curve representing the carbon content plotted against time after varying periods of contact up to 45 minutes is shown in FIGURE 2.
It will be noted that the rate of solution of carbon in the iron was very rapid in that, on the average, over 3.0% was absorbed in under two minutes and 3.75% in about ten minutes. In one experiment 3.2% of carbon was dissolved in 1 minute 15 seconds.
It is well known that the solubility of carbon in iron increases with temperature, reaching about 5.35% at 1600" C., and there is no doubt that with larger volumes of metal and carbon and higher temperatures, still faster absorption rates and higher carbon contents would be obtained. This would be a most important factor in practice in that residence times in the iron recarboniser could be extremely short, resulting in very high production rates and minimum loss of temperature.
The solubility of carbon in liquid iron in relation to temperature is illustrated in FIGURE 3.
In further experiments batches of pig iron were melted in the crucible furnace in contact with excess of carbonised coal of various types and the metal was then blown to steel with oxygen in a small top blown vessel. After the drop of the flame the steel was returned to the crucible furnace for recarbonisation and then the process of blowing was repeated.
In these experiments it was found that recycling could be achieved without difiiculty, even with the small quantities of metal involved, and that 10% of cast iron scrap could be absorbed per cycle.
STEP 1 OF THE PROCESS.THE PRODUCTION OF CRUDE STEEL Step 1 of the process is preferably carried out in a vessel of similar shape and design to that of an LD or BOS furnace. The only important difference in the vessel is that it is lined with nonbasic refractories, preferably silica or alumina. Nonbasic slags are therefore used and operation of the vessel bears a somewhat similar relationship to acid Bessemer as the LD or BOS does to basic Bessemer or Thomas.
Whether acid or neutral slags are used coolant capacity is greater than with equivalent basic practice.
While a basic lined furnace could be used for the conversion of the pig iron and solid charge to crude steel a nonbasic lined vessel is preferred for the following reasons:
(a) As it is unnecessary to remove phosphorus or sulphur at this stage, basic slags are not required.
(b) With similar charge composition slag volumes are much smaller than with basic practice, consequently much less heat is lost in the slag.
(c) Primarily because of the smaller volume and inherently lower iron content of nonbasic slags, less iron is lost in the slag.
(d) Nonbasic slags save cost of burnt lime addition.
(e) In nonbasic practice greater quantities of metallic fuels may be used for their heat producing and chemical reduction properties.
Because nitrogen would, if absorbed during the production of crude steel, be removed in the final (BOS) purification stage, comparatively impure oxygen could, if desired, be used in the crude steel furnace.
As the steel made in the oxygen crude steel furnace need not be subjected to all the finishing procedures associated with quality of steel making and for other reasons, appreciable reduction in tap to tap time is eifected.
While raw iron ore could be used in the oxygen crude steel furnace in quantities considerably greater than possible in equivalent basic oxygen furnaces, it would usually be desirable to prereduce the ore, although the optimum degree of reduction and the amount of prereduced ore used would depend on several factors, including the quantity of scrap available.
It should be noted in this connection that in conventional manufacture practically all steel is made from a mixture of steel scrap and blast furnace iron, and the scrap may be considered a kind of circulating load which facilitates the conversion process as well as cheapens the final product. This applies just as strongly to the crude steel as to the final (BOS) stage.
It is well understood that the open hearth is a much greater scrap consumer than the conventional B08 and that, as oxygen furnaces replace open hearths, more and more scrap will be available for a process of high scrap absorption capacity such as the proposed new process. It is assumed that continuous casting processes, which make less scrap than ingot processes will not appreciably affect the position because of their generally restricted application.
Complete absence of iron oxide in the charge is, of course, by no means essential, and may be economically undesirable in some instances as the oxygen in the oxide will replace gaseous oxygen and also reduce blowing time.
While prereduction might appear an undesirable feature in the process it should be noted that there is considerable current interest in USA. and elsewhere in the use of prereduced or metallised pellets etc. for the blast furnace, and also the fact that in Germany, Krupp-Renn iron granules (luppen) and in Russia, spunge iron, have for years been used with advantage to increase blast furnace production.
It may be mentioned that prereduction can be much more easily applied to the process because high physical strength is not required in the prereduced or metallised material as is the case for blast furnace feed.
If the available cheap fuel is coal the Dwight Lloyd McWane (DLM) process could be considered for prereduction although there are several others. The DLM process is attractive because it uses a sinter strand, a unit well established and provide in iron manufacture, for the reduction operation. On the DLM strand a 60% reduced agglomerated product may be made from a mixture of ore and coal, which in Australia could include South Australian or Victorian brown coal or possibly Muja open cut coal from Collie in West Australia. In New South Wales the Great Northern is one of several seams that could be considered.
As an alternative for prereduction purposes the SL-RN process jointly marketed by Stelco, Lurgi, Republic Steel and the National Lead Co. should be given particular consideration.
Prereduction could also be achieved by modification of pellet hardening methods already established for finely divided ores and the beneficiated products of taconites and jaspilites.
If the fuel is natural gas several prereduction processes are available but the Esso (FIOR) process should probably have first consideration.
It will be apparent from the foregoing that the oxygen crude steel furnace charge will consist of liquid metal from the iron carboniser plus solid coolants with contribute additional iron per cycle. The solid material may consist of iron ore of various degrees of prereduction, or scrap, which may be iron or steel. Various combinations of all of the above materials may be conveniently used.
The silicon or other metallic fuel content of the charge would depend mainly on economics as silicon or equivalent metallic fuels are at present comparatively expensive. However, the higher the quantity of metallic fuel available in the charge, the greater the iron gain obtainable per cycle, other factors being equal.
Metallic fuel usage can be reduced at the expense of percentage metal gain per cycle and the loss recouped if required, by additional recycling. In this way the proportion of carbon heating and reduction, to metallic heating and reduction, may be greatly increased.
In this connection it must be understood that, notwithstanding the great value of additive metallic fuels in increasing the gain per cycle, the new process is based primarily on the utilization of cheap carbon rendered available by recycling liquid metal between the iron carboniser and the crude steel furnace.
Because acid linings and acid slags have been used with satisfactory results in pneumatic and other steelmaking processes for over a century, silica would be the logical first choice as a nonbasic lining for the crude steel furnace.
While silicon is normally used in steelmaking in the form of a ferroalloy, silicon carbide is preferred as a metallic fuel in an acid lined crude steel furnace for the following reasons:
(1) The carbon as well as the silicon provides heat whereas with ferrosilicon the iron content is only equivalent to steel scrap.
(2) Silicon carbide suitable for the new process could undoubtedly be produced much more cheaply than available grades.
In this connection it may be mentioned that today practically all silicon carbide is made primarily for abrasives, super high grade refractories and for special purposes such as semiconductors etc., and for such applications material of high purity and specific physical properties is required.
On the other hand silicon carbide for the invention may beneficially contain excess carbon and also (if in reduced form or as carbides) appreciable aluminium, calcium, magnesium etc. which are all considered impurities in silicon carbide for most industrial applications. Because of this, cheap raw materials may be used for steelmaking silicon carbide and in addition, expensive acid, caustic and other purification and classification operations would be unnecessary.
Further, because a highly crystalline product would not be required, much lower reduction temperatures could be used rendering production in a continuous shaft furnace possible instead of the slow and expensive batch method normally used.
In addition to the above cost reducing factors, consumption of silicon carbide as a major steelmaking heat source would permit large scale production for that specific purpose, which should also tend to reduce production cost. Finally, assuming the availability of cheap thermal units and consequently cheap power, the manufacture at or near the iron or steel plant would reduce transport and related charges.
While it is obviously difiicult at present to estimate the cost for which a steelmaking grade of silicon carbide could be made it could possibly approach the unit cost of silicon in pig iron.
Incidentally, calcium carbide could, if desired, be used as an additive in an acid crude steel furnace. This may appear strange as calcium carbide is extremely basic, but if the quantity be kept to levels which will not seriously reduce the acidity of the slag, it could function not only as a chemical fuel but also as a slag conditioner. Calcium silicide or other compounds of like nature could function similarly.
Aluminium carbide forms easily in the presence of excess carbon and aluminium carbide containing appreciable silicon carbide, titanium carbide, iron carbide and appreciable free carbon would constitute an excellent fuel for the new process.
Such a material could be made in a continuous shaft furnace from crude bauxite obtainable in Australia from the Darling Ranges, Gove Peninsula or Weipa etc. without the expensive caustic alumina purification and molten electrolysis procedures essential for the production of aluminium metal.
The use of aluminium carbide as a fuel in the oxygen crude steel furnace would permit alumina in the form of burnt high alumina bricks, tarred alumina bricks, rammed alumina etc. derived from bauxite, or other suitable minerals, to be utilised as a neutral lining.
The adoption of alumina refractories would not prevent the utilisation of basic fuels such as calcium carbide, or acid materials such as silicon carbide, ferrosilicon, aluminium silicon alloys, or acid-basic combinations such as calcium silicide, so long as the neutral balance of the slag was not seriously upset, and the acidic action on the slag of the silica in prereduced ores could, if desired, be neutralised by the addition of calcium carbide or other basic material.
Alumina linings, rendered possible by the utilisation of aluminium carbide as a fuel would greatly surpass silica in refractoriness, hot strength, spalling resistance and resistance to chemical attack by iron oxide. Actually, much better lining life could be expected with an alumina lining in contact with a neutrally adjusted slag than currently obtained from the basic linings of B08 vessels.
The economic implications of using exotic fuels are of course appreciated but it may surprise how many of these materials could prove economic if specifically produced as chemical fuel for steelmaking purposes.
The use of prereduced ore as sinter, pellets, granules or briquettes has a great advantage in ease and speed of handling and charging compared with metal scrap which is normally of a miscellaneous nature, and it should be possible, if desired, to charge such prereduced material in a preheated condition and at a controlled rate while blowing is in progress, rendering possible simultaneous recarbonisation, charging and blowing.
STEP 2 OF THE PROCESS.-THE RECARBONISA- TION OF CRUDE STEEL TO PRODUCE PIG IRON The crude steel from the oxygen crude steel furnace is poured into an iron carboniser. This unit is kept filled with hot carbonaceous material which dissolves in the steel to form liquid pig iron.
Because the solution of carbon in steel absorbs heat it may be desirable to add silicon or aluminium carbide to the crude steel ladle or to the carboniser, as silicon or aluminium, on solution in steel will evolve heat and thus neutralise the cooling effect of carbon and prevent excessive loss of temperature in the carboniser.
However, as silicon and aluminium both reduce the saturation point of carbon in iron, the additions should, whenever possible be made to the crude oxygen steel furnace, thus permitting iron of the highest obtainable carbon content to be taken from the carboniser.
The iron carboniser and its charge of carbonaceous material will be maintained at a high temperature level by the repeated passage through the vessel of liquid steel at an initial temperature of about 1600 C., during the latters conversion to liquid pig iron (with a solidificathe recarboniser will be very much less than the quantity tion temperature of about 1150 C.) thus providing heat considerably in excess of that required to dissolve 4%- of carbon.
It is an important advantage that the quantity of carbonaceous material required per ton of iron produced in of metallurgical coke required per ton of blast furnace iron, and could be as low as l-cwt. per ton of metal through the carboniser compared with say, l4-cwt. per ton of iron from the blast furnace.
This does not, of course, represent the total carbon or other fuel used per ton of finished steel, which would be considerably greater depending on methods and degree of prereduction of ore, proportion of scrap used, amount of gain per cycle etc. It is quite definite however that the quantity of carbonaceous material required, would be very much less than the metallurgical coke needed per ton of iron from the blast furnace, and would probably not exceed one fifth of the blast furnace requirements.
Under these circumstances carbonaceous material could if necessary be economically transported longer distances than fuel for blast furnaces if it proved desirable to locate plants near cheap sources of natural gas for ore reduction and rolling mills.
While several types of iron carboniser (including shaft designs) are practicable a simple unit could be constructed on the principle of the teapot ladle, several types of which have for years been used in iron foundries.
In operation the ladle like vessel (illustrated in FIG- URE 4) would be filled with carbonaceous material, preferably preheated, through the opening in the removable cover, which with its refractory lining should be sufiiciently heavy to restrain the floating tendency of the carbon when in contact with the metal.
Whereas iron is normally discharged through the spout of a teapot ladle, when used as a recarboniser the liquid steel from the oxygen furnace would be poured into the spout and thence into contact with the carbon in the main chamber, discharging through the metal notch in the upper part of the vessel after passing through the mass of carbon.
The iron carboniser should be capable of tilting to control the amount of contact between metal and carbon during iron carbonisation (see FIGURES 5A and 5B) and to permit complete emptying of the vessel of carbon as required. The arc of rotation should be at least 180".
In FIGURE 4, there is shown in sectional elevation a teapot type iron carboniser having a steel charging spout 2; a removable cover 3 bearing lifting lugs 4 and having a charge hole through which carbon is fed. An iron notch 6 provides the pouring outlet for liquid pig iron.
FIGURES 5A, 5B show in sectional elevation the iron carboniser 1 in vertical and tilted positions. In the vertical position the carbon particles 7 make fullest contact with the metal achieving maximum heat exchange between the metal 8 and the carbon.
When the iron carboniser 1 is in a tilted position, the volume of the metal is reduced and the degree of contact with the carbon particles 7 lessened permitting control of the heat exchange.
The ability to tilt the iron carboniser will permit controlled heat exchange between liquid iron and carbon and thus alloy superheat to be extracted from foundry irons etc. and used for additional carbon preheat.
About four times the amount of carbon required per heat would initially be charged into the carboniser, and after each crude steel cast had been run through, the carbon equivalent to that dissolved would be replaced. In the case of a ton crude steel vessel the initial carbon charged would be about 24 tons and the replacement after each heat about six tons.
Because of the short residence time necessary in the iron carboniser for the iron to reach near carbon saturation, it is possible that the steel could be run through the spout as iron could be run out of the iron notch.
The teapot type of iron carboniser under consideration provides means and accessibility for:
(1) Charging preheated carbon and discharging residual material as required.
(2) Bottom pouring liquid steel into the vessel through the spout While holding liquid metal in the carboniser or discharging iron through the iron notch while charging steel through the spout in continuous operation, if required.
(3) Providing intimate contact between carbon and metal which has to flow up through the mass of carbon on its way to the discharge opening. The carbon is kept immersed in the metal by the weight of the heavy cover which may be clamped if necessary.
(4) Provides variation in contact of metal and carbon by angle of tilt which permits control of metal cooling and carbon heating.
(5) Provides reasonable access for relining and maintenance.
If normal care is taken to separate slag from the crude steel before pouring into the carboniser, major slag troubles would not be expected in the latter, as reasonable amounts of slag and ash from the carbon would be flushed out without difliculty with the carbonised metal.
While design for the special requirements under consideration would be necessary, operating conditions would not be particularly severe and, in view of the ease and simplicity of recarbonisation demonstrated in the pilot plant, little difficulty is anticipated with iron carboniser design and operation after teething troubles are overcome.
Refractory lining life would be appreciably assisted by the comparatively low temperatures and the reducing conditions in the vessel, together with the comparative absence of slag.
The rapidity of carbon solution in iron, as demonstrated in the initial pilot plant experiments, indicates that conversion of steel to iron in the carboniser could be much faster than the conversion of iron to steel in the oxygen furnace, which would have very important economic advantages.
STEP 3A.-RECONVERSION OF EXCESS PIG IRON TO FINISHED HIGH QUALITY STEEL The crude steel made in the oxygen crude steel furnace and the liquid pig iron resulting from its recarbonisation will normally contain higher than permissible levels of phosphorous and sulphur which need to be reduced to produce best quality steels.
This final purification would normally be conducted in a conventional basic oxygen steel furnace which would reduce phosphorus and sulphur as required to high quality specification requirements. Finishing procedures, including deoxidation, temperature control, adjustment of chemical composition and casting into ingots would be carried out in the conventional manner.
While it would usually be desirable to feed recarbonised iron to the B05 it would be possible to transfer crude steel direct to the B08 for finishing provided the crude steel carbon was sufiiciently high or other heat producing elements were available in the B05.
If suitable raw materials were available, or chemical specifications would permit, finished product of acid quality could be obtained directly from the crude steel furnace.
The utilisation of steel plant and purchased scrap, which is an important practical requirement for economic steelmaking will normally be carried out in the basic oxygen steel furnace in the usual way. As previously indicated however, scrap may also be used in the oxygen crude steel furnace as required and it should again be noted that as open hearth furnaces are gradually superceded by oxygen steel furnaces, the latter will be required to absorb greater quantities of scrap than can usually be utilised in present top blown oxygen practice.
STEP 3B.-USE OF EXCESS PIG IRON FOR FOUNDRY IRONS AND OTHER SPECIAL PUR- POSES Excess metal intended for special grades of iron will be adjusted for composition requirements by alloy or other additions at various stages including the oxygen crude steel furnace, the steel ladle, or in the iron carboniser or iron ladle before dispatch to the pig mill for casting into pigs.
Pig iron in liquid form will of course be available as needed for ingot moulds, stools or other direct metal castings.
A feature of economic importance is that foundry and other special irons when discharged from the carboniser would normally be hotter than required or desirable, unless action was taken to prevent this. Fortunately, as previously mentioned the excess heat can be extracted to preheat carbon by controlled tilting of the iron carboniser.
A more general picture of the process may be obtained by considering a hypothetical plant situated for example in Victoria at or near the brown coal deposits and designed for an annual production of about 750,000 tons of high quality steel ingots and 250,000 tons of foundry pig.
Two -ton BOS furnaces (with only one in operation) could produce 750,000 tons of ingots per annum using 600,000 tons of hot carboniser metal plus scrap.
To feed the B05 with 600,000 tons of hot metal and to make about 250,000 tons of foundry irons, two oxygen crude steel furnaces (one in operation) of about 150 tons capacity would be required, although under favourable circumstances smaller vessels could suflice. The actual size would depend on several factors including degree of prereduction of the ore, amount of scrap available, proportion of carbon fuel to metal fuel used in the process, tons per hour from the oxygen crude steel furnace and overall gain per cycle etc.
Three or four iron recarbonisers (teapot type) would possibly be required, with one or two in service together.
For each tons of crude steel fed to the iron carboniser up to about six tons of carbon in the form of carbonised lbrown coal would be required and, on the basis of an iron carboniser metal throughout of say 1,700,000 tons some 100,000 tons of carbonised brown coal would be required per year.
It would be desirable to charge the carbonised brown coal into the iron carboniser at as high a temperature as possible and a Lurgi type spulgas retort designed to give the highest practical discharge temperature would serve the purpose. It is thought that a temperature of the order of 800 C. could be obtained by dispersing with a normal cooling zone and that charging into the iron carboniser at at least 500 C. could be achieved.
Prior to treatment in the retorts the coal would need to be briquetted, preferably by roll presses, into small pillow briquettes.
It should be mentioned that the Lurgi spulgas retort was initially designed in Germany specifically for the low temperature carbonisation of brown coal, which it does very efficiently. The spulgas retort was later utilised for the low temperature carbonisation of non or weakly caking bituminous coals with excellent results and plants of over 4000 tons per day have been built.
Steel from the crude steel furnace would normally be transferred to the iron carboniser in lip poured ladles and, although the number cannot at present be calculated there is no doubt that the ladle linings will last much longer than BOS ladle linings due to (a) acid or neutral instead of basic slags, (b) the absence of stoppers and (c) much shorter time in contact with metal. Incidentally, if the iron carboniser was put on rails it might be possible to pour the crude steel from the crude steel furnace directly into it, thus dispensing with the crude steel ladles.
Although losses by radiation during the various recycling operations will of course occur, such losses are not expected to be a critical factor. In conventional practice radiation losses from a ton BOS, working on a tap to tap time of one hour should be less than 2% of the total available heat, and it is anticipated that under 11 the conditions visualised, losses during recycling could be kept 'below While the 35 odd tons of carbon initially charged into the iron carboniser will still be hot from the spulgas retort, it will be further heated and kept hot by the regular passing through it of 150 ton casts from the oxygen crude steel furnace at temperatures greatly in excess of the solidification temperatures of pig iron.
The initial casts through the iron carboniser should be for foundry iron, which needs to be cooled from about 1600" C. at the crude steel furnace to a temperature for the pig mill of about 1250 C. Assuming the radiation, carbon solution and other losses, minus a silicon solution gain bring the temperature of the iron down to about 1450 C. This would still be 200 C. higher than required for pigging and the heat equivalent of this would consequently be available for heating carbon.
Although the specific heat of carbon is somewhat higher than that for iron, the heat liberated from 150 tons of iron, while falling from 1450 C. to 1250* C. would be sufiicient to raise the temperature of the whole 35 tons of carbon in the iron carboniser by about 550 C. If in regular operation only one heat in four was for foundry iron and available for heat exchange as suggested, it could theoretically provide most of the preheat required.
To fulfill this purpose the foundry iron must of course contact the cooler, freshly charged material, which would be achieved by introducing the steel into the carboniser with the vessel in the vertical position. Under these circumstances the liquid metal would rise to the maximum height in the vessel and thus contact the greatest quantity of cool carbon. With casts intended for recycling in the crude steel furnace the iron carboniser would be tilted towards the discharge opening in order to minimise cooling by restricting contact with the cooler carbon.
With iron for the B08, which is expected to be at a higher temperature than corresponding blast furnace iron, the carboniser may or may not be tilted, or tilted in varying degree, as a means of controlling the iron superheat in relation to scrap availability, carbon preheating etc.
Prereduction of ore could be by sinter strand as in the DLM process, using brown coal as the reductant, the SL-RN process also using brown coal, or the Esso FIOR process using natural gas.
In the case of a plant built in West Australia, practice would be similar except that noncaking Collie coal could be used for iron carbonising if available at a reasonable cost, which could most likely apply to Muja opencut product. However, because of the comparatively small quantity required compared with metallurgical coke for blast furnaces it would probably be economic to transport carbonaceous material from Victoria or elsewhere.
The Esso FIOR process, using Barrow natural gas, could be used for prereduction.
It is of course well known that attempts to economically produce metallurgical fuel for blast furnaces from both brown coal and Collie coal per medium of reticulating retorts of the Lurgi type have so far proved unsuccessful in Australia. It is certain however that the outcome would be the opposite under circumstances where the physical properties of the product were of minor importance.
In the case of a conventional plant with byproduct coke ovens or where coking coal only is available for iron carboniser use, byproduct coke, gas coke or coke breeze could no doubt be used. Such fuel would be suitable, if necessary, for total preheating by heat exchange in the iron carboniser, because of the absence of volatiles. If BOS furnaces were also available, utilisation of the process on an existing plant might be simplified or development work, at least, facilitated.
In view of the large combination of manner and conditions under which the process might be applied it is very difiicult at this stage to be specific about economics, but general comparisons can be made. Assume for ex- Conventional Process New Process 1 sinter plant 2-3 blast furnaces and auxiliaries (blowing engines, stoves, gas cleaners, gas reticulation, slag plant, etc).
1 pig mill.
1-4 batterirw of byproduct coke ovens and byproduct plant to make 800,000 tons coke (including breeze) per annum.
1 oxygen plant: 160 tons (99.5%)
per ay. 2 -ton BOS furnaces and auxiliaries.
1 prereduction unit to use natural gas, petroleum or cheap carbonaceous fuel.
1 pig mill.
1 pair spulgas shafts, 100,000 tons coke per annum, with auxiliaries (gas cleaning and liquid products se aration and briquetting plant.
1 oxygen plant: 160 tons/day (99.5%); 350 tons/days (95.0%).
2 80-tou BOS furnaces and auxiliaries.
2 -ton crude steel furnaces.
2-3 iron carbonisers.
While more oxygen furnaces and greater tonnage oxygen capacity are required in the new process compared with the old, capital cost may easily favour the former because of unit size and other factors including of course the elimination of coke ovens and blast furnaces.
On the basis of thermal efliciency and raw materials cost, the new process could have considerable advantage.
It is of course, frequently claimed that the blast furnace is a most efiicient unit, but this is not so when considered as an individual metallurgical operation, mainly because of the huge amount of low B.t.u. gas evolved which carries out of the furnace enormous quantities of calories. Fortunately some of this gas can be used to heat the air blast and, in integrated iron and steel plants, use can normally be found for the remainder. It tends to be forgotton however, that not only the sensible heat carried out of the furnace in the various ways, but all the potential chemical heat in the gas is derived from expensive metallurgical coke.
An important point to remember is that the air f which four fifths is inert nitrogen) passed through the blast furnace, approximate the total weight of all solids charged, and that the air plus solids equal several times the weight of iron produced.
Byproduct ovens, in which practically all blast furnace coke is made are also inefficient, firstly because all heating of the coal is done by conduction through the brick walls of the oven chambers and secondly because enormous quantities of heat are lost in quenching the coke on removal from the ovens. Finally, a considerable quantity of breeze is made, which is not suitable for the blast furnace and must be diverted for other purposes.
The greatest advantage of the new process, is that all B.t.u.s required may be obtained from cheap fuels such as natural gas, oil, lignites, coke breeze etc., and that carbonised coal is required only in very small quantity compared with that needed for blast furnaces. Further the carbonised coal is not required to conform to stringent size and physical property requirements.
Another advantage of the new process is that plants, to be economic need not be as large as conventional plants, which, for example, normally require duplication of coke oven batteries and blast furnaces to prevent general shutdown or severe dislocation of the whole comlex during coke oven or blast furnace rebuilds or other major stoppages.
Another advantage is that iron manufacture may be conveniently divorced from steelmaking, whereas integration of iron and steel production is normally essential in conventional practice, for reasons previously mentioned.
A further advantage, under circumstances where large quantities of cheap scrap is available, is the extremely large scrap capacity of the process if required.
As in conventional practice the economics are of course affected by many circumstances and conditions and greatest benefit would be obtained by efiiciently utilising the natural advantages of the new process.
Fears have been expressed that siliceous furnace linings would fail rapidly under the rigorous conditions of top blowing with oxygen, due to high gas and metal temperatures, highly oxidising atmospheres and wet slags. There is however every reason to believe that a silica lining would prove satisfactory, particularly as procedures are available in working the crude steel furnace which would be quite impracticable in normal BOS operation.
When assessing the merits of the invention it should be appreciated that the process is novel and thermochemically sound and also that pilot experiments have demonstrated it to be both simple and practicable. Most of the equipment has already been shown in large scale practice to be completely suitable for iron and steelmaking, and the one unit yet to be completely proved in practicethe iron carboniser-is obviously simple in construction and operation and capable of producing at a rate, in tons per hour, considerably faster than its associated oxygen steel furnace.
Heat balances taken out for various types of charges confirm that the process is thermochemically valid over a wide range of conditions and show also, if economics demand, that the ratio of carbon to silicon or other chemical fuels consumed, can be greatly varied. Representative thermobalance results are given in Table A for several sets of conditions.
TABLE A.HEAT BALANCE DATA BASED ON 100 TON CHARGES OF IRON CARBONISER METAL AND VARIOUS COOLANTS, PLUS FERROSILIGON OR SILICON CAR BID}? ADDED DIRECT TO OXYGEN CRUDE STEEL FU NA E 68% 94% 94% 94% Coolant type Red. Red. Red. Scrap Red. Scrap Ore Ore Ore Ore Coolant (tons) 51.0 74. 85. 0 100.0 48.0 64. 0 Hot metal carbon (t0ns). 4. 3 4. 3 4. 3 4. 3 4. 3 4. 3 Combined carbon plus free carbon in silicon carbide (tons) 2. 0 Carbon consumed (tons) 4. 0 3. 9 5. 9 4. 1 4. 0 4.1 Silicon as ferrosilicon added to crude steel furnace (tons) 4. 0 4. O 4. 0 2. 0 2. 0 Silicon, as silicon carbide added to crude steel furnace (tons) 4. 0
:Si ratio 1. 00 0.97 1. 57 1.00 2.00 2. 00 Steel from hot metal (tons)- 91. 7 91. 8 91. 8 91. 92. 8 92. 7 Steel from coolant (tons) 41.0 65.7 75. 3 99. 5 42. 5 63.0 Total steel (tons) 132. 7 157. 5 167. 1 191. 0 135. 3 155. 7 Steel gain per cycle, percent 44. 7 75. 8 82.0 109. 4 45.8 64. 9 Gaseous oxygen (tons) 5. 77 9. 83 12. 42 11.49 7. 8 8. 75 Oxygen from ore (tons). 1. 50 18. 1. 03 Total oxygen (tons) 11. 33 14. 38 11. 49 8.83 8. 75 Slag from silicon alloy (tons) 13. 7 13.7 13.7 13.7 6. 85 7.0 Slag from ore or scrap (tons)- 4. 1 5. 9 7. 0 0. 6 3. 83 0. 45 Total slag (tons) 17. 8 19. 6 20. 7 14. 3 10. 68 7. 45 Slag, percent. 13. 4 12. 8 12.2 7. 3 7. 8 4. 5
Assuming carbon and silicon to be the major fuels consumed in an acid oxygen crude steel furnace, for example, the cost of silicon per ton of product would primarily depend on: (a) the quantity of silicon used per ton of products; and (b) the cost of silicon per unit in the silicon source.
As the ratio of carbon to silicon consumed in the acid oxygen crude steel furnace may be increased from say, 1:1 to 4:1 or even more by decreasing the silicon and increasing the amount of recycling, silicon can be adjusted to the optimum ratio with carbon, depending on the economics.
The same would prevail in a furnace lined with neutral refractories as regards carbon and aluminium.
Because oxygen consumption for a given production depends on the total weight and type of fuel elements consumed and not specifically on the degree of recycling, and because of the simplicity and rapidity with which the liquid steel may be transferred to theiron carboniser, and then reconverted to iron, recycling would not necessarily appreciably increase overall cost.
Another economic point to consider is that the larger oxygen production units required for the process would be more efiicient and would appreciably decrease oxygen cost per ton of gas.
Regarding (b), the cost of silicon per unit in the silicon source would depend on several factors including the type of silicon bearing material used, its silicon content, method of manufacture, transport charges, and many others, and similar conditions would appertain to carbon-aluminium usage.
An interesting economic feature of the new process is that the gas cleaning plant would not necessarily have to be greatly increased in size compared with the conventional BOS plant because in the latter case, the cleaning plant is only working about one third of the heat time, and some advantage could be taken of this.
While all research costs money, proving the process by pilot plant investigation could be carried out comparatively cheaply because most of the operations are conventional and well established. Under these circumstances attention need mainly be concentrated on the iron carboniser and the cyclic crude steel furnace. Both of these units are very simple in design, and pilot equipment could be constructed at relatively low cost.
A metal capacity of five hundred pounds in each case should be sufiiciently large to prove the process beyond reasonable doubt before large scale development need be undertaken. In this regard it is relevant to recall that much of Henry Bessemers original pilot work was conducted in a forty pound capacity clay crucible using batches of only a few pounds of iron.
While a high frequency furnace of say one thousand pounds capacity Would be an ideal unit to use in combination with the experimental iron carboniser and crude steel furnace, a gas fired tilting crucible furnace, a cupola or other available unit for melting or otherwise providing small quantities of liquid iron or steel. would serve the purpose.
An adequate supply of gaseous oxygen would be desirable and ultimately necessary but oxygen gas would not be initially essential for proving the iron carboniser and its functioning as a cyclic unit because, although a regular supply of liquid steel to the carboniser is a primary requirement, it need not be oxygen steel, and consequently could be obtained by working the pilot crude steel furnace as a surface blown Bessemer, using air from a portable compressor or other available supply.
Large quantities of raw materials would not be necessary as all liquid steel could be converted. to iron and cast as such for use as coolant in subsequent cycles. This would simplify raw material and liquid metal handling, and the use of cast iron as coolant scrap would, like oxygen, have the desirable property of increasing coolant capacity.
What is claimed is:
1. A cyclic process for the production of pig iron and/ or steel comprising supplying a charge of liquid pig iron derived from an iron carboniser unit together with added supplements of iron bearing materials jointly with heat producing components, to an oxygen crude steel furnace and blowing the charge to crude steel of increased volume from the initial charge of pig iron, then returning the crude steel at temperatures of l500-l700 C. to the iron carboniser, where the heat gained in the oxygen crude steel furnace is utilised for solution of an additional amount of carbon, thus producing anincreased amount of pig iron of which a proportion is utilised as pig iron and the balance returned to the oxygen steel furance in a cyclic sequence.
2. A process according to claim 1 in which the added iron bearing materials are selected from. the group consisting of iron oxides, prereduced iron, ore, scrap steel, scrap iron and iron bearing alloys.
3. A process according to claim 1 in which the heat producing components are selected from the group consisting of metallurgically acceptable elements, compounds, alloys and mixtures yielding heat by oxidation in the oxygen crude steel furnace.
4. A process according to claim 3 in which the heat producing components are selected from the group consisting of the elements carbon, silicon, aluminium.
5. A process according to claim 3 in which the heat producing components are selected from the group con sisting of carbides of iron, manganese, titanium, zirconium, aluminium, silicon and calcium.
6. A process according to claim 3 in which the heat producing components are ferro alloys selected from the group consisting of ferrosilicon, ferroaluminum, ferrozidconium, ferromanganese, and ferrotitanium.
7. A process for the production of pig iron or of steel according to claim 1 in which the oxygen crude steel furnace employed is an acid oxygen steel furnace.
8. A process for the production of pig iron or of steel according to claim 1 in which the oxygen crude steel furnace is a neutral oxygen steel furnace.
9. A process for the production of pig iron or of steel according to claim 1 in which the oxygen crude steel furnace is a basic oxygen steel furnace.
10. A process according to claim 1 in which the source of oxygen employed in the oxygen crude steel furnace comprises tonnage oxygen of a lower degree of purity than that normally used in basic oxygen steel processes, thereby achieving economy in oxygen cost.
References Cited UNITED STATES PATENTS OTHER REFERENCES Comstock, George E, Titanium in Iron and Steel, 1955, p. 25.
L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 7548, 60