Co-Production of Steel, Titanium and High Grade Oxide

Abstract

Steel and pigment-grade TiO 2 with up to 99.99% (TiO 2 +ZrO 2 ) are produced continuously by direct smelting ilmenite or titaniferous magnetite concentrates at ultra-high temperature. Both oxidic and metal phases are independently force circulated in fully backmixed melt circulation reactors. Close proximity to thermodynamic equilibrium permits liquidus composition melts to be contained within reactors with protective linings of unmelted shells comprised of oxycarbide of the solidus composition. The unmelted shells are maintained above the critical temperature of the ductile to brittle fracture transition by supercritical steam generation, whilst the endothermic heat is supplied by electrical conductive heating. Smelting reduction with natural gas or carbon with low sulphur content, followed by in-line continuous vacuum refining makes available a continuous feed of refined melt for further in-line processing at somewhat lower temperatures using known emerging technologies for titanium metal production.

1 . A process plant for continuous smelting of minerals and related materials, in which at least one melt circulation loop is employed, comprising two immiscible bulk liquid phases, as opposed to a single liquid phase with a relatively thin liquid or solid phase floating thereon, and in which both liquid phases are forced-circulated in closed loop paths at rates very considerably greater than that associated with product phase additions or withdrawals to or from the melt circulation loop and typically with melt circulation ratios in excess of 1500/1 in these terms with a view to establishing fully back-mixed conditions in both liquid phases with temperature and compositional variations in both phases thus minimized so that with the provision of adequate retention time and inter-phase contacting area, thermodynamic equilibrium is closely approached for smelting in furnaces operating within the realms of ultra-high temperature pyrometallurgy at temperatures normally in excess of 1850° C. and approaching 2200° C., for which chemical reaction rates are in general very rapid and hence kinetically not rate-controlling and thus performance exclusively dependent on transport phenomena (heat, mass and momentum transfer, thereby making available a continuous refined melt feed for in-line downstream processing, employing present day more normal melt upper temperature limits of 1700-1800° C., perhaps extended upwards somewhat, as required to make high-grade products. 
     
     
         2 . A process plant, in which the mineral being smelted as claimed in  claim 1  is ilmenite or titaniferous magnetite and the two immiscible liquid phases are liquid un-refined steel as the denser phase in contact with an upper oxidic melt bulk phase composed principally of titanium oxides, titanium oxycarbide and various impurity oxides, all in solution in the liquid state and the refined products are pigment-grade titanium dioxide (up to 99.99% TiO 2 +ZrO 2 ) and or using known emerging technology production of commercially pure titanium metal. 
     
     
         3 . A process plant according to  claims 1  and  2 , in which the equivalent output of un-refined liquid steel as actually produced within the melt circulation loop or reverted thereto is withdrawn continuously or intermittently and subjected to refining in advance of continuously casting a steel product and at the same time reverting a specified amount within the range 1× to 4× of refined liquid steel product back to the liquid steel melt circulation loop to enhance extraction of undesirable impurities out of the force-circulated oxidic melt into the circulating liquid steel within the melt circulation loop. 
     
     
         4 . A process plant according to  claims 2  and  3  in which the impurities extracted by the circulating liquid steel from the oxidic melt are those associated with colouration or detraction of whiteness from titanium dioxide pigment materials subsequently manufactured from the oxidic melt or its derivatives, viz. the elements chromium, manganese, iron and vanadium. 
     
     
         5 . A process plant according to  claim 1 , in which a solid reducing agent selected from sources of elemental carbon, such as carbon black, calcined anthracite coal, petroleum coke or wood charcoal is used to effect the chemical reactions involved in carbothermic smelting, subject to the sulphur content of the particular solid reductant in its readily available form or after prior desulphurisation being low enough to avoid deleterious effects on product purity. 
     
     
         6 . A process plant according to  claim 1 , in which a gaseous reducing agent, such as natural gas or manufactured gas with high methane content, of the purity used in power generation or otherwise desulphurised so that the sulphur content is low enough to avoid deleterious effects on product quality, is used for carbothermic smelting and, as a result of carbon being preferentially consumed, a hydrogen enriched fuel gas is produced. 
     
     
         7 . A process plant according to  claims 1  and  6 , in which natural gas top blowing extends over the whole liquid surface areas available to reduce the chemical reaction intensity as methane, preheated no hotter than 350° C., discharges from radiation-shielded nozzles within the oxycarbide ceiling immediately above the melt surface, directly onto the melt under precisely controlled non-splash conditions, as established from published research, so that the jets are in what is termed the potential core region and thus un-decomposed methane contacts the melt surface before gas entrainment to effect greater than 99% efficiency of carbon transfer to the melt, relative to the 100% theoretical value, and dissolved carbide rapidly assimilated into the bulk melt without disruption of the chemical equilibria therein, so that the protective oxycarbide linings are not adversely affected, which is crucial to the success of the new technology, and without precipitation of oxycarbide or other solid phases in the bulk of the melt, whilst at the same time maintaining carryover of solid carbon particles into the bulk of the gas phase above the melt surface close to zero. 
     
     
         8 . A process plant according to  claim 1 , in which at least one of the bulk liquid phases is over-flown, withdrawn by an electric heated siphon or otherwise transferred in an amount equal to that actually produced in the melt circulation loop referred to in  claim 1 , to a forehearth, sump or tundish containing a barometric leg, such that the particular liquid phase is admitted continuously or intermittently to a melt circulation loop for continuous vacuum refining. 
     
     
         9 . A process plant according to  claim 1 , in which the vacuum refining is conducted at a reduced pressure in the vicinity of 1 mbar using steam jet vacuum pumping systems as used in vacuum degassing of steel. 
     
     
         10 . A process plant, in which the liquid phase as referred to in  claim 9  is an oxidic melt at a temperature typically in the range 1850-2200° C. composed principally of titanium II oxide (TiO) and titanium oxycarbide, along with other titanium oxides and residual quantities of impurity oxides all in liquid solution, is refined with the express purpose of reducing the concentration levels of the impurity oxides down to specification limits in the melt withdrawn from the continuous vacuum refining melt circulation loop via a barometric leg into a tundish or sump at substantially atmospheric pressure so that the melt can continue to further in-line processing stages. 
     
     
         11 . A process plant according to  claim 10 , in which the further in-line stages are known emerging technology for electrochemical de-oxygenation-based titanium metal production, carbochlorination to produce a titanium tetrachloride intermediate product, which itself can continue in-line to a reactor for electrowinning of titanium metal employing known emerging technology with a fused calcium fluoride-based electrolyte to which titanium tetrachloride is admitted either as a gas or liquid, again to produce liquid titanium metal initially in advance of titanium metal ingot production. 
     
     
         12 . A process plant according to  claim 11 , in which the melt from continuous vacuum refining is added to fully back-mixed melt circulation loop at substantially the same temperature as it was during continuous vacuum refining so that one arm of the melt circulation loop is used to add carbon to the circulating melt by top blowing or injection of natural gas and the other undergoing chlorination by carefully admitting chlorine so that reaction intensity is closely under control and the resulting gas mixture composed of various chlorides of titanium, a small excess of Cl or Cl 2  and principally CO and various other metallic chlorides present as impurities are passed onwards to the next processing stage. 
     
     
         13 . A process plant, in which the gases from the carbochlorination melt circulation loop of  claim 12  are passed to a so-called “condenser”, in which the gases are quenched with CO/CO 2  equilibria frozen at 1100° C. or above to prevent carbon deposition, employing fused sodium chloride close to its melting point, so that freeze lining principles can be utilised, as the solvent for absorption of gaseous thorium tetrachloride in particular, along with gaseous calcium chloride and minor amounts of other metallic chlorides with relatively low volatility. 
     
     
         14 . A process plant according to  claims 1 ,  2 ,  7  to  10 , in which fully back-mixed melt circulation loops operating close to thermodynamic equilibrium at temperatures normally above 1850° C. and upwards to 2200° C. are protected from the inevitable aggressive attack on conventional refractory linings normally expected, particularly at the aforementioned very high temperatures by establishing and maintaining stable linings of solid compounds composed of the solidus composition relative to melts which themselves are close to the liquidus temperature, whilst keeping such linings throughout above the critical temperature of the ductile to brittle fracture transition. 
     
     
         15 . A process plant according to  claim 14 , in which the un-melted shell lining is composed of titanium oxycarbide of the solidus composition associated with the melt being contained therein close to the liquidus temperature, and to preclude formation of cracks or fractures leading to lining failure, the lining throughout is kept above the ductile to brittle transition temperature by encasing the cooler face of the titanium oxycarbide lining at temperatures around 800° C. in a supported sheath of heat resistant alloy with controlled heat input and removal by electrical conductive heating and radiative heat transfer to steam boiler or steam re-heating/superheating tubes, respectively. 
     
     
         16 . A process plant according to  claim 1 , in which large reactors of swimming pool dimensions or “swimming pool” reactors are used to ensure that sufficient inter-phase area is provided for close proximity to thermodynamic equilibrium at operating temperatures normally above 185C° C. and upwards towards 2200° C. is attained, and in which special steps are taken to ameliorate the adverse effects of differential thermal expansion by the provision of adequate space within the furnace enclosure and skid mounting or more sophisticated means such as “bogey” tracking, supportative steel pontoons within launders or troughs containing a fusible alloy or liquid metal inside the launders extending the fill length of the furnace on both sides to carry the weight of roof structures, so that such structures are effectively floated during warm-up or cooling-down and thus are free to expand or contract laterally and longitudinally until the operating temperature is reached without further cyclic variation, at which time the fusible alloy or liquid metal is partially pumped out of the launders, permitting the structure to bear down to compress refractory fibre board or similar initially compressible material to establish a gas-tight seal.