AND METAL

BACKGROUND OF THE INVENTION

[0001] The present invention is in the technical field of pyrometallurgy and relates to a method of producing refined or nitrated ferromanganese alloys or metal in a DC arc furnace.

[0002] Manganese is used, principally, in steel and stainless steel production.

[0003] Traditionally, high grade manganese products (with Mn content in excess of 85% and less than 0.15% carbon) were produced in an aluminothermic process in reaction vessels. Refined manganese alloy grades (with Mn content between 75 - 95%) such as low (less than 0.5% carbon content) and medium carbon ferromanganese (from 1 -2% carbon content), hereinafter referred to as "LCFeMn" and "MCFeMn" respectively, and were produced via a siliconthermic process in metal ladles. This process has high cost implications.

[0004] The bulk of manganese product produced today consists of low or lower grade high carbon ferromanganese ("HCFeMn"), or ferrosilicomangenese ("FeMnSi"), with Mn content of between 60 - 80%, which principally is produced employing carbothermic reduction in an AC "submerged" arc furnace.

[0005] To improve upon the HCFeMn or FeMnSi grades various additional steps are required, requiring further capital expenditure into additional processing capacity to accommodate these steps. [0006] For example, in the reduction of the carbon content of HCFeMn, the alloy, tapped from the AC arc furnace, reports to a converter for oxygen refining in which carbon is burnt off. The disadvantage is that the process step results in a high (15- 20%) loss of manganese through its vaporized oxides. [0007] Another example of a multiple production step process employed in the state of the art, necessary to achieve a high grade product from metallurgical grade manganese ores, is an ore-lime melting step in an arc furnace followed by silicothermic reduction during a molten mixing step, employing the Perrin process.

[0008] The invention aims to at least partially address the aforementioned problems.

SUMMARY OF INVENTION

[0009] Hereinafter, "a high grade manganese product" means an alloy or metal product in which manganese is present in a concentration of at least 75% (m/m), and carbon is present in a concentration less than 2% (m/m). [0010] The invention provides for a pyrometallurgical method for producing a first high grade manganese product by reducing a manganese oxide reactant, in a first reaction zone within a closed vessel DC arc furnace, with a reducing agent, in an inert or nitrogen rich atmosphere.

[0011] The manganese oxide reactant may be input to the furnace as part of a charge. [0012] The reduction may be aluminothermic, wherein the reducing agent is aluminum, added to the furnace as part of the charge in a fine granulated form or as chips.

[00 3] The reduction may be silicothermic, wherein the reducing agent is silicon or ferrosilicon. Preferably the ferrosilicon is of a grade FeSi 75.

[0014] The first reaction zone may be a solid/liquid phase comprising of a slag layer and an underlying metal/alloy layer.

[0015] The temperature of the first reaction zone may be in a range 1600°C to 1900°C. Preferably, the temperature range of the reaction zone is 1700°C to 1800X.

[0016] The first high grade manganese product may be manganese or a ferro or ferrosilico alloy of manganese.

[0017] The inert or nitrogen rich atmosphere may be achieved by introducing, to an interior of the closed vessel DC arc furnace, an inert gas or nitrogen respectively. The introduction of the inert gas or nitrogen may purge the vessel of prior gas content. Preferably, nitrogen is introduced to make up, at least, 95% of the atmosphere.

[0018] A second high grade manganese product may be produced by oxidation of a vapour of the first high grade manganese product, for example vaporized manganese, in a second reaction zone, with nitrogen.

[0019] The second high grade manganese product may be a manganese nitride. [0020] The manganese nitride may be one or more of the following: MnN, Mn ₆ N ₂ , Mn ₃ N ₂ , Mn ₂ N, Mn ₅ N ₂ and Mn ₄ N. Preferably, the manganese nitride is Mn ₄ N.

[0021] The second reaction zone may occur within a gas phase. The second reaction zone may be located above the liquid/solid phase within the furnace and may extend to an off-gas located within an off-gas conduit which removes an off- gas from the furnace.

[0022] The temperature of the second reaction zone may be below 1 100°C, preferably in a range 600°C to 1 100°C.

[0023] The manganese nitride may be extracted from the second reaction zone and separated from other off-gas content by filtration.

[0024] The filtered manganese nitride may be refined or recycled to the furnace as part of the charge.

[0025] The first high grade manganese product and manganese nitride may be refined by passing a refining gas through a vessel (ladle), containing molten product or nitride, and allowing the refining gas to percolate through the product or nitride.

[0026] The refining gas may be oxygen or air.

[0027] The manganese oxide reactant may be an ore, preferably a calcined ore. The ore may be one or more of the following: pyrolusite, braunite, hausmannite, rhodonite, psilomelane, manganite, rhodochrosite and manganocalcite. [0028] Alternatively, the manganese oxide reactant may be manganese or manganese bearing waste stream, for example a dust or sludge waste stream or a high grade slag.

[0029] A fluxing agent may be introduced, with the charge, to the furnace to control one or more of the following parameters: the viscosity of the slag layer, the melting point of the slag layer, the capacity of the slag layer to absorb phosphorous and the capacity of the slag layer to absorb sulphite.

[0030] The fluxing agent may be CaO, MgO or Si0 ₂ based.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention is further described by way of example with reference to the accompanying drawings wherein:

Figure 1 is a process diagram, which includes the method of the invention, illustrating major equipment and process flows; and

Figure 2 is a process flow diagram focussing on the process steps of the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0032] Figure 1 of the accompanying drawings illustrates a process 10, which includes a method of the invention 12, illustrated in greater detail in Figure 2, in which a high grade manganese product 14 is produced from a manganese oxide input 15. [0033] The manganese oxide input 15 can be an ore, such as, for example pyrolusite, braunite, hausmannite, rhodonite, psilomelane, manganite, rhodochrosite and manganocalcite. The manganese oxide input can also be manganese bearing waste stream such as dust sludge or a high grade slag. Preferably, an ore such as pyrolusite (Mn0 ₂ ) is used in the process 10.

[0034] Ore material is temporarily stored in stockpiles 16 before reporting to a separator 18 which separates an oversize fraction from a sized fraction 20 of the ore material. This sized fraction is input to a calciner 22, with or without the addition of limestone (depending upon the ore type), and calcined to thermally decompose the oxides, such as pyrolusite, or carbonates, such as rhodonite, of the ore, removing volatile fractions and producing a manganese ore of a more stable oxidation state. In so doing, the possibility of eruptions when the ore is smelted (a step which will be described below) is substantially reduced.

[0035] Also, this is an important step in the fluxing process which will be described below in greater detail.

[0036] The calcined ore 24 reports to a batching system 26, which includes a plurality of hoppers, respectively designated 28A, 28B, 28C ...28N, each of which is dedicated to receive a furnace feed ingredient, one of which is the calcined ore 24. The other furnace feed ingredients include a fluxing agent 30 and a reducing agent 32, in the form of granules or sized scrap, which in this particular embodiment is aluminium. In other embodiments of the invention, employing silicothermic reduction, the reducing agent 32 can be silicon, metal or, preferably a ferrosilicon known as FeSi 75. [0037] The importance of fluxing, how fluxing alters important process parameters and the types of fluxing agents 30 used, will be more fully explained below.

[0038] Recycled product from the .smelting process, such as manganese nitrides, can be input to the furnace feed at this stage. This will be explained below in more detail.

[0039] In the batching system 26, each hopper releases its respective content, of ore 24, fluxing agent 30 or aluminium 32, at a predefined power to feed rate ratio, into a mixer 34, which mixes the feed ingredients into a feed mix 35 before release to a DC stationary or tilting DC arc furnace 36. [0040] The process of the invention 12 employs stationary or tilting DC arc furnace

36 which has a closed vessel body 38. The roof of the closed vessel body is pierced by a plurality of feed chutes 40, through which the feed mix 35 is introduced to an interior of the furnace 36, and an off-gas duct 42, through which gas and dust particles leave the furnace. An electrode 44 (see Figure 2), carrying a DC current, penetrates the vessel 38 to terminate, at an interior end 46, just above an open bath 48 of furnace smelt content. The vessel body is further penetrated, at respective sides, by a metal and slag taphole, respectively designated 50A and 50B, (or in the case of a tilting DC furnace, a single pouring spout 50), through which a metal or alloy product 51 and slag 52A is released from the furnace. [0041] The open bath 48 comprises an upper liquefied slag layer 52, of flux and aluminium oxides, at a temperature typically in the range 1700 - 1800°C, and a lower metal layer or alloy layer 54, at a temperature typically in the range 1600 - 1700X. [0042] In use of the furnace 36, nitrogen 37 is passed into the vessel through one of the feed chutes 40, alternatively, through a channel in the electrode 44, to purge the interior of the vessel 38 of any pre-existing gaseous content and replacing the interior of the vessel with an inert atmosphere made up of, at least, 95% nitrogen. With little or no oxygen, the possibility of manganese oxidising to undesirable manganese oxides is significantly reduced.

[0043] In the smelting process in the vessel 38, temperatures exceed 2000°C in the arc between electrode 44 and open bath 48; the fed mix liquefies, reacts and separates between the slag layer 52 and the metal/alloy layer 54.

[0044] The reactions which are taking place in an interface between these layers are:

Mn _x O _y + AI Mn + Al ₂ 0 ₃ + Mn(g) (equation 1 )

If the reducing reaction is silicothermic, the reaction equation is:

Mn _x 0 _y + FeSi Mn + SiO ₂ + Mn(g) (equation 2)

[0045] The molten manganese metal, or ferro-alloy of this metal, settles from the interface into the metal layer from where it can be tapped off at taphole 50A from where the metal or alloy product enters a refining stage and a final product sizing process 55 before dispatch as a final product 14.

[0046] With the open bath 48 of the DC furnace 36, manganese will fume off the bath, represented in the above equations as the manganese gas product, and pass directly into the nitrogen rich gas phase above the bath. Here, the temperature above the bath is too high for the manganese to react with the nitrogen. [0047] Reaction of the gaseous manganese occurs where the temperature is in a range 600°C to 1 100°C. A drop in temperature to within this range occurs in off-gas found in the off-gas duct 42, between a furnace off-gas outlet 57 and a dry gas cleaning facility 58 (hereinafter referred to as a "gas reaction zone"). To ensure a drop in temperature from about 1700°C, above the bath 48 of the furnace 36, to a temperature within the abovementioned range, the off-gas stream within the gas reaction zone is temperature quenched with a nitrogen stream 59 produced in the gas cleaning facility as will be more fully described below.

[0048] Within the gas reaction zone, the manganese will react with nitrogen as represented in equation 3 below.

Mn(g) + N ₂ -> M _x N _y (equation 3)

Mn + N ₂ -> M _x N _y (equation 4)

[0049] The nitride synthesis process, represented in equations 3 and 4, can produce one or more of many possible manganese nitride products, such as: MnN, Mn ₆ N ₅ , Mn ₃ N ₂ , Mn ₂ N and Mn ₄ N. The eventual product, or cocktail of nitride products, which results from this part of the process will depend upon the variables of temperature and the nitrogen to manganese concentration ratio. Therefore varying these parameters will vary the composition of the nitride product. These parameters can be controlled by controlling the rate and the temperature of the nitrogen stream 59 recycled from the facility 58. The preferable product is Mn ₄ N.

[0050] These manganese nitrides are solids, which in the form of dust particles, report to the fume abatement and dry gas cleaning facility 58 as part of the off-gas stream. In the facility, the manganese nitrides are separated from the gaseous components of the off-gas stream, on a series of high temperature filters (not shown) housed within the facility 58. The nitrogen stream 59 is thus produced by the separation, with this stream channelled back to the gas reaction zone in the off- gas duct 42, some of this stream being released to the atmosphere, the amount depending upon requirement. This is illustrated in Figure .

[0051] The filtered, separated manganese nitride particles 60 fall into collection funnels 62, beneath the facility 58, to be channelled along one of two optional processing streams: a recycle stream 64, or a refining stream 66. These two streams are illustrated in Figure 2.

[0052] The recycle stream 64, comprising the manganese nitride product 60, reports to the batching system 26 to form part of the fed mix 35 as required. The product 60 of the refining stream 66 reports to a ladle gas refining system 68.

[0053] The manganese nitride product 59 can be an important component of the fed mix 35 to regulate the process parameters of the smelting process within the furnace 36. As examples of this regulation: should higher concentrations of manganese be required in the fed mix 35, and ultimately in the end product 14; and should the temperature need to be reduced within the furnace; manganese nitride can be recycled into the fed mix. The manganese nitride has a temperature quenching effect, an important function when the temperature within the furnace 36 needs to be optimally regulated.

[0054] The manganese nitride product 60 is a high grade manganese product in itself. The product 60 can be mechanically agglomerated, aided by the use of binders, and sold as briquettes or pellets. [0055] The gas refining system 68 is interposed between the furnace 36 and the final sizing process 55. In the process of gas refining, the molten metal / alloy is fed from taphole 50A into a ladle 70 (or a series of ladles) which has a porous ceramic plug at its base. It is through this porous plug that a refining gas, from a gas stream 72, permeates the ladle to percolate through the molten metal / alloy content 74 of the ladle.

[0056] The refining gas, principally, comprises oxygen. This gas can also be air. Percolating oxygen through the ladle content 74 has the effect of oxidising the impurities or contaminants typically found in a manganese metal / alloy smelt product. Examples of these contaminants include Ca, Mg, Li, Al, Ti, Si and V. Oxygen will oxidise these contaminants to their respective oxides which will combine to form a discrete slag layer 76 atop the metal / alloy layer 78 that can be tapped off as a slag stream 80 and discarded. The now purified metal / alloy can be tapped off as a high grade manganese product 14, which reports to the sizing process 55 before dispatch and sale.

[0057] Sulphur and phosphorous are common non-metallic contaminants of a metal/alloy product. The reduction of these contaminants in the metal/alloy product is achieved by desulphurisation and dephosphorisation respectively whereby fluxing (described below) is used to increase the capacity of the slag layer 52 to absorb these contaminants resulting in the contaminants not being available to contaminate the product 14.

[0058] The refining gas can include inert gasses in a gas mix to reduce the partial pressure of O ₂ and CO and aid in stirring the contents 74 of the ladle 70. [0059] The composition of the fluxing agent 30, introduced to the furnace 36 as part of the feed mix 35, is an important variable to the smelting process and, consequently, to the efficiency of this process and ultimately the composition of the product 14 itself

[0060] Fluxing of a slag is done to reduce the melting point and viscosity of the slag layer 52. This has the effect of improving manganese metal or alloy recovery by reducing the manganese oxide content of the slag layer 52 and, with reduced viscosity of this layer, allowing the metal/alloy product 4 to separate from the slag interface and settle, by gravity and the additional forces created by the DC coupling or current flow (from cathode/electrode to anode), into the metal/alloy layer 54 for recovery.

[0061] The type of fluxing agent used is dependent upon the original ore mineralogy of the ore and elemental analysis. Mineralogy and elemental composition varies considerably between ore bodies.

[0062] Lime (calcined limestone (CaO)) or dolime (calcined dolomite (MgO)) are used as fluxing agents 30 of choice to flux (reduce melting point of) a SiO ₂ rich ore (acid ore) or when the slag layer 52 has a high alumina (Al ₂ 0 ₃ ) content.

[0063] Silicon oxide (Si0 ₂ ) is used to flux a basic CaO or MgO rich ore.

[0064] During the aluminothermic reaction explained above, alumina is produced. This component of the slag layer 52 has an extremely high melting point (liquefies at 2070 ^° C). Therefore to operate at 1750 ^° C in the slag layer 52, which is a preferable temperature, an appropriate mix of CaO/MgO + SiO ₂ , as a fluxing agent, needs to be added. [0065] The value of the sulphur partition coefficient usually rises with an increase in CaO content in slag. Lime additions will therefore increase the sulphur capacity of the slag, known as desulphurisation, and as the slag is discarded, this prevents or minimises sulphur adsorption by the metal/alloy product (CaO + S = CaS + O).

[0066] Dephosphorisation is the capacity of the slag to absorb phosphorous. The distribution ratio of phosphorous between the CaO-CaF2-SiO2 bearing slag and metal increases with increasing CaO content. Therefore both CaO and CaF2 will reduce phosphorous contamination of the metal/alloy product 14.