PRODUCTION OF HIGH CARBON FERROMANGANESE

BACKGROUND OF THE INVENTION

[0001] This invention relates to the smelting of manganese ore for the production of high carbon ferromanganese (HCFeMn).

[0002] Conventional smelting of manganese ore for the production of high carbon ferromanganese (HCFeMn) is carried out in AC submerged-arc furnaces and, to a lesser extent, in blast furnaces. These furnaces are typically operated in a full burden configuration with the charge consisting of lumpy ores and/or lumpy ore sinters to provide sufficient burden permeability. This configuration is able to minimize manganese losses to the off-gas. However the furnace operation is associated with unsafe and sporadic occurrences of violent burden eruptions as well as limitations in the furnace smelting capacity due to electrical dependence of the furnace operation on the slag properties and burden characteristics. This electrical constraint limits the scale-up of this type of furnace to a maximum power rating of 50 MW thus making it impossible to benefit from the advantage of economy of scale.

[0003] Due to shortages in electricity supply, an increased cost of electricity and a recent drop in commodity prices the electric smelting industry currently faces serious challenges in maintaining volume production and in containing unit production costs. These factors translate into an urgent need for improved smelting technologies and processes that offer increased furnace smelting production, to meet existing demands, through efficient utilization of the electrical energy.

[0004] An object of the invention is to provide a smelting process to address, at least partly, the aforementioned factors.

SUMMARY OF THE INVENTION

[0005] The invention provides a process for the production of high carbon ferromanganese which includes the step of smelting carbon-based manganese ore micro-pellets in a smelting furnace in a shallow burden configuration.

[0006] The micro-pellets may be pre-treated by means of at least one of the following: by being preheated or by being pre-reduced prior to smelting. This step leads to a reduction in furnace energy requirements.

[0007] The micro-pellets may have a size in the range of 2 to 5 mm.

[0008] The micro-pellets may be prepared from ore and reductant fines agglomerated with an organic binder. The fines may be less than 106μm.

[0009] Optionally a flux may be added to the micro-pellets to improve the kinetics of the reduction reactions.

[0010] The invention is primarily concerned with the processing of fully or partially reduced manganese ore micro-pellets as this enables a furnace to be operated at an increased power density which, in turn, leads to an increase in the smelting capacity of the furnace through the efficient utilization of electrical energy.

[0011] Preferably pre-reduction is carried out in a unit which is separate from the smelting furnace.

[0012] If untreated pellets are to be smelted the power-to-feed balance must be optimized to ensure that metallisation of the manganese occurs prior to melting, and to avoid localised overheating of the melt. This particular smelting step may also require optimisation of the furnace design and operation, as well as consistent control and optimisation of the products’ inventory in the furnace.

[0013] If pre-reduction is carried out prior to smelting then energy for the pre-reduction step may be provided by means of a CO/syngas medium which can act as a gaseous reductant, and as a fuel and energy carrier. In the case of a green energy supply configuration, the CO/syngas may be preheated in a thermal solar source to a temperature that provides an efficient pre-reduction of carbon-based ore manganese pellets.

[0014] Preheated CO/syngas may be blown through the burden of micro-pellets to provide heat energy and to bring about reduction of manganese in the micro-pellets. Unreacted CO/syngas together with product gasses may be combusted in a head space above the burden to provide additional process heat energy. Preferably the smelting is carried out in a DC open-arc furnace although it is possible to make use of a brushed-arc or an immersed-arc configuration.

BRIEF DESCRIPTION OF THE DRAWING

[0015] The invention is further described by way of example with reference to the accompanying drawing which is a flow chart representation of a process according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0016] The accompanying flow chart illustrates a smelting process 10 according to the invention.

[0017] Manganese ore 12, referred to herein as Gloria ore, is milled (14) arid then sieved (16) to produce fines i.e. milled ore 18 passing 106μm.

[0018] The milled ore 18 is mixed with anthracite 20 milled to 100% passing 106μm prior to mixing and with an organic binder 22 in a blending step 24, thereby creating a blended mixture 28 which is subjected to a pelletizing process 30 to produce agglomerated pellets 32. The agglomerated pellets 32 are screened (34) to collect micro-pellets 36 in the size class 2 to 5 mm.

[0019] The micro-pellets 36 are heated in a furnace 38 which is purged using Argon gas 40 to provide an inert atmosphere. After a reaction time of four hours at a reaction temperature the pellets are cooled to room temperature to produce a product of pre-reduced pellets 44. Energy for the pre-reduction step is preferably derived from a CO/syngas medium which optionally is preheated and which can act as a gaseous reductant, and as a fuel and energy carrier. For example preheated CO/syngas is blown through the burden of micro-pellets to provide heat energy and to bring about reduction of manganese in the micro-pellets, and unreacted CO/syngas together with product gasses are combusted in a head space above the burden to provide additional process heat energy.

[0020] The pellets 44 are batch-fed into a DC open-arc furnace 50 for smelting under controlled conditions. Outputs from the furnace 50 include off-gas i.e. fumes 52, slag 54 and a high carbon ferromanganese (HCFeMn) product 56.

[0021] Table 1 gives the chemical composition of the ore 12 and Table 2 give the chemical composition of the anthracite 20.

[0022] Table 3 shows the composition of the materials used to prepare the agglomerated pellets 32.

[0023] Six batches of pellets were submitted to the pre-reduction process. Table 4 gives the pre-reduction reaction temperatures used for the different batches of pellets. Table 5 shows, for each batch of pellets, the mass loss values after pre-reduction. The mass loss increased with an increase in temperature. It is unclear whether the mass loss was due to the reduction of metal oxides or the oxidation of carbon.

[0024] Table 6 reflects the overall mass balance of smelting tests conducted on the six batches.

[0025] Tables 7, 8, 9 and 10 respectively show the chemical compositions of the pre reduced pellets 44, the metals 56, the slags 54 and the fumes 52, for the six batches. No metals were produced in Runs 2 and 4. However the ferromanganese metals which were

produced in Runs 1 , 3, 5 and 6 had an Mn content greater than 50%. The slags 54 generated (Table 9) were rich in MnO except for the slag 54 in Run 3.

[0026] Elemental accountabilities on a mass percentage basis are shown for Runs 5 and 6 in Table 1 1 . Good accountability of Mn was obtained when untreated pellets were smelted in Run 5. This implies that Mn volatilization was significantly decreased in the process of the invention compared to the results obtained in Reference 1 refer to note hereinafter). The accountabilities of Al, Ca, Mg and Si from Run 5 are low. This indicates a loss of these elements through the process. Smelting of untreated pellets was characterized by the generation of large volumes of fumes 52. This resulted in intermittent blockages of the off-gas port and brief periods of poor extraction. It was during these periods that the fumes 52 escaped through the feed port resulting in loss and low accountability of the affected elements. The smelting of the pre-reduced pellets 44 generated lower volumes of fumes 52 and hence better accountabilities were obtained for Run 6. The lower accountabilities of Fe and Mn could be due to the loss of the metals via volatilization as higher reaction temperatures (>1600°C) were measured in Run 6.

[0027] Table 12 shows elemental recoveries on a mass percentage basis for metal 56, slag 54 and fumes 52, for Runs 5 and 6. The lower recovery of Mn to metal, when pre-reduced pellets 44 were smelted (Run 6), is believed to be due to the poor extent of reduction and metallization that occurred during the pre-reduction step. In addition the pre-reduced pellets 44 had insufficient carbon available for the further reduction of Mn oxides to Mn metal during the smelting step. It appears that a large amount of the carbon added to the pellets burnt off during the pre-reduction step.

[0028] A metal 56 produced in Run 6 was possibly because metallization was observed in pellets 44 pre-reduced at 1200°C and 1300°C, and because about 12,5% carbon still remained in pellets 44 pre-reduced at 1200°C. Thus the degree of Mn metallization and the amount of residual carbon contained in the pellets after the pre-reduction step are important parameters as they influence the metal yield, and the recovery of Mn to the metal phase during the smelting step.

[0029] The test work provides experimental evidence that the process of the invention is technically feasible for the treatment of manganese ore. This conclusion is based on the following:

(1 ) Mineralogical examination of pre-reduced pellets indicated that a low extent of reduction and poor manganese metallization were obtained during the pre-reduction step. It is suspected that the oxygen partial pressure inside the pre-reduction units was not low enough to enable reduction reactions to occur. Instead, it appears a large fraction of the carbon added to the pellets simply burnt off.

(2) Smelting tests that processed pre-reduced pellets did not produce any metals

(except for Run 6). All tests that smelted untreated pellets produced metal, but the metal yields obtained were very low with the highest metal yield achieved in Run 5 at 1 1.3% (Table 6).

(3) Good accountability of Mn was obtained when untreated pellets were smelted in Run 5. This implies that Mn volatilization was significantly decreased in this process compared to results obtained from previous tests on carbothermic smelting of manganese ore in the DC open-arc furnace (Reference 1 ). This could be the result of solid state reduction taking place prior to melting when the pellets were heated in the furnace. This is an important finding in favour of the process of the invention.

(4) However, about 23% of Mn could not be accounted for when pre-reduced pellets were smelted in Run 6. This could be attributed to loss via volatilization (due to poor Mn metallization in the pellets) since high operating temperatures (>1600°C) were measured in Run 6.

(5) A recovery of 81 % Mn to the metal was achieved when untreated pellets were smelted in Run 5 and only 18% of the Mn was recovered to the metal when pre-reduced pellets were smelted in Run 6 (Table 12). The lower recovery of Mn to the metal, when pre-reduced pellets were smelted, could have resulted from the poor extent of reduction and metallization that occurred during the pre-reduction step. In addition, the pre-reduced pellets had insufficient carbon available for further reduction of Mn oxides to Mn metal during the smelting step.

(6) The degree of Mn metallization and the amount of residual carbon obtained in the pellets after the pre-reduction step are important parameters as they influenced the metal yield and the recovery of Mn to the metal phase during the smelting step.

(7) No major technical and/or operational difficulties were encountered during the smelting tests and no unsafe events like violent burden eruptions were experienced during operation of the furnace. The furnace operation was independent of the slag properties and the furnace could be readily controlled to reach the necessary temperatures to melt the charge. This implies that larger furnaces with higher power ratings could be implemented with this process to increase furnace smelting capacity.

(8) A more stable power loading was maintained when pre-reduced pellets were smelted. Smelting of untreated pellets was associated with a release of large volumes of off-gas which interfered with the transferred plasma arc, thus causing a slightly unstable power loading during smelting.

(9) Smelting of pre-reduced pellets was associated with lower operating temperatures due to lower furnace energy requirements. The high volumes of off-gas generated when untreated pellets were smelted resulted in higher heat losses through the off-gas requiring more furnace power input.

(10) The process was able to obtain a molten and fluid slag phase without the addition of any flux, implying that this process could have lower energy requirements (than conventional smelting) and an increased furnace smelting capacity.

(1 1 ) The test work demonstrated that smelting of untreated carbon-based micro-pellets could significantly decrease manganese volatilization when compared to results obtained in tests on the carbothermic smelting of manganese ore in a DC open-arc furnace.

(12) The test work further illustrated that lower operating powers could possibly be used, and hence lower furnace energy requirements, when pre-reduced pellets were smelted.

Reference 1 : Schoukens, A., & Ford, M. (1984). The production of Ferromanganese and Silicomanganese in a 100 kVA DC transferred-arc plasma furnace. Rand burg: Mintek.

CLAIMS

1 . A process for the production of high carbon ferromanganese which includes the step of smelting carbon-based manganese ore micro-pellets in a smelting furnace in a shallow burden configuration.

2. A process according to claim 1 which includes the step, prior to the smelting step, of preheating or pre-reducing the micro-pellets.

3. A process according to claim 1 wherein the micro-pellets have a size in the range of 2 to 5 mm.

4. A process according to claim 3 which includes the step of forming the micro-pellets from ore and reductant fines agglomerated with an organic binder and, optionally, a flux.

5. A process according to claim 1 wherein, prior to the smelting step, the micro-pellets are pre-reduced in a unit which is separate from the smelting furnace.

6. A process according to claim 5 wherein energy for the pre-reduction step is provided by means of a CO/syngas medium which optionally is preheated and which can act as a gaseous reductant, and as a fuel and energy carrier.

7. A process according to claim 1 wherein preheated CO/syngas is blown through the burden of micro-pellets to provide heat energy and to bring about reduction of manganese in the micro-pellets, and unreacted CO/syngas together with product

gasses are combusted in a head space above the burden to provide additional process heat energy.

8. A process according to claim 1 wherein smelting is carried out in an inert atmosphere.