Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B1/00—Preliminary treatment of ores or scrap
  • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B11/00—Making pig-iron other than in blast furnaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B13/00—Making spongy iron or liquid steel, by direct processes
  • C21B13/0006—Making spongy iron or liquid steel, by direct processes obtaining iron or steel in a molten state
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/005—Manufacture of stainless steel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B1/00—Preliminary treatment of ores or scrap
  • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
  • C22B1/24—Binding; Briquetting ; Granulating
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B1/00—Preliminary treatment of ores or scrap
  • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
  • C22B1/24—Binding; Briquetting ; Granulating
  • C22B1/2406—Binding; Briquetting ; Granulating pelletizing

Definitions

• THIS INVENTION relates to the production of steel and, more particularly, but not exclusively, to the production of stainless steel wherein chrome oxide in the form of the particular ore, is added substantially directly to iron as opposed to conventional processes which usually involve the preparatory treatment of a chromium ore to produce, for example, high carbon ferrochromium prior to its addition to iron.
• Stainless steel is most commonly produced by alloying iron and high carbon ferrochromium to produce a high carbon alloy which is then subjected to argon-oxygen decarburisation in order to lower the carbon content to acceptable levels.
• the so-called Hamilton-Evans process involves the melting of iron and the provision of a large quantity of slag, known as the reception slag (a mixture of limestone, fluorspar, mill scale and chromite) on top of the molten steel.
• reception slag a mixture of limestone, fluorspar, mill scale and chromite
• the reception slag a mixture of limestone, fluorspar, mill scale and chromite
• the receptor slag also prevents segregation of the reactants which would otherwise occur on the surface of the melt which, in turn, would lead to chromite accretion and dissolution of reductant in the melt; and, the basicity of the receptor slag, being high, assists in the prevention of silica enrichment of the residual oxide and protects the furnace lining from excess wear.
• the disadvantages of this process are that the efficiency is low regarding silicon utilisation and chromium recovery; the tap-to-tap time is long, (of the order of 5 hours) because of the bulk mass of the molten slag; considerable quantities of Cr2O3 are contained therein; and excessive wear on the furnace lining results in consequence of these disadvantages. Furthermore, it is difficult to produce an alloy containing more than 13% chromium because of the high slag to metal ratio.
• Wild process is similar to the above except that the silicon content can be maintained below 0,5% by suitable alternate additions of chromite and ferro-silicon.
• the slag resulting from the reduction process is removed from time to time and the tap-to-tap time is believed to be about 8 hours which is also unduly excessive.
• a process for the production of steel in which at least one of the alloy metals of the steel chosen from the group comprising the required chromium, manganese, vanadium, nickel, cobalt and molybdenum is provided in the form of a finely divided oxide of the alloy metal intimately mixed with a finely divided reductant with the mixture of oxide and reductant being in an agglomerated form.
• the finely divided oxide to have a maximum particle size of one millimetre, and preferably less than seventy five microns in which case the maximum particle size of the reductant is about lmm, and preferably about less than seventy five microns; for the reductant to be present in an amount of at least about 0,4 times the stoichiometric amount required to reduce all the alloy metal and iron oxides in the oxide, and preferably about 1,3 times the stoichiometric amount; and for the oxide to be an ore.
• the agglomerated mixture of oxides of chromium together with the reductant may be in the form of briquettes, pellets or other agglomerated units and can either be added to an existing bath of molten iron or, alternatively, can be heated together with the solid iron with which the alloy metal is to be alloyed. Any usual or suitable binders can be used for the agglomerate.
• Tests indicate that solid state reduction commences at about 1200°C and progresses rapidly towards completion at between 1300°C and 1400°C whilst melting of the material takes place at 1500°C and 1550°C.
• the solid state reduction takes place extremely rapidly and a batch of stainless steel, for example, can be produced in as short a time as 45 minutes using the process of this invention.
• the alloy metal when it is chromium, it may be added in the form of chromite and the reductant may be ferrosilicon or ferro-alumino-silicon. In such an instance the reduction approaches substantial completion at 1400°C even in an oxidising environment such as air.
• Production of the steel may take place in any suitable furnace including electrical arc furnaces, plasma-arc furnaces, induction furnaces and the like in order to supply the marginal energy that is required for the progress of the reduction, and to compensate for the heat losses.
• Table 1 Composition of the raw materials used for crucible tests (in mass %) Chromite Ferrosilicon Iron Flakes Cr2O3 43,0 Si 79,4 C 0,015 FeO 21,6 Fe 18,8 Mn 0,35 Fe2O3 4,7 Al 1,4 P 0,02 SiO2 5,30 Ca 0,07 S 0,02 CaO 0,50 C 0,10 Oxygen 0,05 MgO 9,97 P 0,035 Fe balance Al2O3 14,20 S 0,005 TiO2 0,72 Mn 0,15 S 0,01 P 0,003
• tests were conducted in a 200kVA furnace manufactured by applicant.
• the furnace was of a known d.c. plasma-arc furnace type employing a single graphite electrode located centrally above the furnace bath.
• a direct current power supply was employed in which the molten bath formed, in use, the anode, while the graphite electrode formed the cathode.
• the furnace which had an outside diameter of 830mm and a refractory lining thickness of 140mm, was lined with a refractory material wherein the Al2O3 content was approximately 90 percent.
• the hearth was lined with a similar material to a thickness of 300mm and a number of mild steel rods were used to make the d.c. (anode) electrical connection from the molten bath through the hearth refractory to the anode cable.
• the feed materials consisted of steel scrap, nickel and composite pellets made from chromite fines and ferrosilicon with bentonite as a binder.
• the composition of the pellets is set out in Table 4.
• the analysis of the starting materials can be seen in Table 3.
• pellets Prior to the testwork the pellets were air dried for 24 hours and further indurated for approximately 4 hours at 800°C in order to develop pellet strength.
• Each heat or test consisted of two stages. The first stage being simply a melt-down of the steel scrap and nickel requirements over a half-hour period, and the second stage a smelt in which the chromite and ferrosilicon composite pellets were charged over a further one-hour period. In both stages the furnace power and energy requirements were adjusted in order to achieve and maintain a melt temperature of between 1550 and 1600°C.
• Table 3 Composition of the raw materials used for the d.c. plasma-arc tests (in mass %) Chromite Ferrosilicon Cr2O3 40,10 Si 80,7 C 0,015 FeO 15,0 Fe 18,5 Mn 0,35 Fe2O3 12,40 Al 0,39 P 0,02 SiO2 4,61 S 0,02 CaO 0,57 C 0,08 Oxygen 0,05 MgO 10,25 P 0,035 Fe balance Al2O3 16,10 S 0,005 TiO2 0,95 Mn 0,15 S 0,006 P 0,004 C 0,01
• tests were conducted in a 300kVA three phase a.c. open-arc furnace manufactured by the applicant.
• the furnace was of a known type employing 3 graphite electrodes with a P.C.D. of 500mm located centrally above the bath.
• a three phase alternating current power supply was applied to the molten bath by means of the electrodes.
• the furnace had an outside diameter of 965mm and a composite refractory lining thickness of 155mm made up from an outer lining of magnesia and an inner lining of high alumina refractory bricks (90 percent Al2O3).
• the hearth was lined with a high alumina castable, the base of which being located flush with the taphole to minimise the quantity of material retained in the furnace between tests and so eliminating any dilution effects.
• the feed materials consisted of steelscrap, nickel and composite pellets.
• the composite pellets were made from chromite fines and ferrosilicon utilising either bentonite or sodium-silicate as a binder.
• the bentonite pellets were air dried for 24 hours and further indurated for 4 hours at 800 °C, whilst the sodium-silicate pellets were air dried for 12 hours and then oven dried at 120 °C for a further 12 hours.
• the compositions of the chromite, ferrosilicon and scrap are given in Table 5 and the pellet composition for each test is given in Table 6.
• Table 5 Composition of raw materials used for the a.c.
• each heat or test consisted of two stages viz: a melt-down followed by the smelt-down. During the melt-down the steel scrap and nickel were hand fed over a half hour period following which the composite pellets were charged by way of an automatic feed system over a period of approximately 1 hour. The power input into the furnace was adjusted such that an operating temperature of 1550°C was maintained.
• results of the tests conducted in the 200kVA plasma arc furnace and the 300kVA electric arc furnace while serving to describe the process of the invention also indicate a lower recovery of chromium in the melt than was expected on the basis of the initial crucible tests. It is postulated that the answer for this lies in the difference in the operational conditions between the plasma arc and electric arc furnaces on the one hand and the tube furnace used for the crucible tests on the other.
• the benefits of the invention will be maximised using a two stage process in which the agglomerated material is initially treated at a lower temperature to promote solid phase reduction prior to being charged to a high temperature electric arc furnace for example.
• Rotary kilns or rotary hearth kilns would be suitable. In such a process a higher proportion of the alloying metal would find its way into the melt.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Life Sciences & Earth Sciences (AREA)
• Environmental & Geological Engineering (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Geochemistry & Mineralogy (AREA)
• Geology (AREA)
• Manufacture And Refinement Of Metals (AREA)

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Citations (12)

• 1993
  • 1993-08-10 ZA ZA935789A patent/ZA935789B/xx unknown
  • 1993-08-10 ZW ZW98/93A patent/ZW9893A1/xx unknown
  • 1993-08-11 KR KR1019930015520A patent/KR950006013A/ko not_active Withdrawn
  • 1993-08-11 FI FI933547A patent/FI933547A7/fi unknown
  • 1993-08-11 EP EP93306349A patent/EP0583164A1/en not_active Withdrawn
  • 1993-08-11 JP JP5199495A patent/JPH06172916A/ja active Pending
  • 1993-08-11 CA CA002103843A patent/CA2103843A1/en not_active Abandoned
  • 1993-08-11 BR BR9303358A patent/BR9303358A/pt not_active IP Right Cessation
  • 1993-08-11 NO NO932850A patent/NO932850L/no unknown

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