BACKGROUND TO THE INVENTION

Ferrochrome alloy is produced by smelting chrome ore, fluxing agents and carbon reductants in a DC arc or AC submerged arc furnace. Tapping of molten slag and metal from the furnace takes place intermittently. When enough smelted ferrochrome has accumulated in the hearth of the furnace, a tap hole is drilled open and a stream of molten metal and slag rushes down a trough into a chill or ladle. The metal phase (Ferrochrome alloy) solidifies in castings and is crushed for sale or further processed. The slag is a waste material and is usually discarded.

During the production of ferrochrome alloy, most of the chromium and iron together with some silicon and carbon report in the metal phase as ferrochrome alloy. However, the ferrochrome alloy and slag produced in a furnace do not reach thermodynamic equilibrium, due to kinetic constraints in the furnace. As a result a substantial amount of chromium, some 5-20% in the form of chrome oxide (Cr ₂ 0 ₃ ), reports in the slag phase and is not recovered to the alloy. Furthermore, the slag contains ferrochrome alloy in droplet form (about 1 to 8%), which is also lost to the discarded slag. Metal recovery plants are used in most ferrochrome plants to recover some of the metal in the slag at a cost.

It is an object of this invention to provide a means to recover chromium from slag in a process for the production of ferrochrome alloy. SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a process for recovering ferrochrome alloy and chrome from slag containing entrained ferrochrome alloy and Cr ₂ 0 ₃ , wherein:

the slag is mixed with a molten ferrochrome metal phase containing silicon (known as "charge chrome" or "high carbon ferrochrome") for silicon present in the metal phase to reduce Cr ₂ 0 ₃ in the slag phase in a silicothermic reduction reaction to chromium, which reports to the metal phase; and thereafter

separating an end slag phase from an end metal phase.

In this process, the slag becomes fluid due to the reduction of the Cr ₂ 0 ₃ content and the increase in Si0 ₂ content in the slag, this lowers the liquidus temperature of the slag, reduces the volume of solid constituents in the slag and allows good slag and metal separation and thus also results in the recovery to the metal phase of most of the ferrochrome alloy previously entrained in the slag.

The slag may contain 1-10%, typically 1-3% by mass entrained ferrochrome alloy metal droplets, and 5-20%, typically 10-16% by mass Cr ₂ 0 ₃ .

The end slag separated from the process typically contains 0.5 - 1% by mass ferrochrome alloy droplets and 0.5 - 4% by mass Cr ₂ 0 ₃ .

The ferrochrome metal phase may contain from 1-10% by mass, preferably 5-8% by mass, most preferably about 6% by mass silicon.

The end metal phase separated from the process typically contains from 0.1-3% by mass silicon and can be controlled according to client needs.

The slag may be an acidic slag (by "acidic slag" is meant (%CaO + %MgO)/%Si0 ₂ <1 on a weight basis, or in other words more acidic components than basic components), and an advantage of the process of the present invention is that no or little flux/es need/s be added during the reduction of Cr ₂ 0 ₃ to chromium. Acceptable refractory lining life can be achieved if the process temperature is controlled below the liquidus temperature of the slag. The "liquidus temperature" is the lowest temperature where some of the slag will start to solidify. A further advantage of not needing to add fluxes is that the slag volume reduces significantly, leading to a much more favourable slag to alloy ratio, which improves the mixing efficiency which is important to allow the silicothermic reduction to be completed in an acceptable time.

Some fluxes may however still be added to ensure slag saturation and to mitigate damage and wear to the refractory material.

The process temperature of the molten metal and molten slag is preferably controlled below the liquidus temperature of the slag (typically 1570°C for a charge chrome slag), to avoid super heating of the slag which will lead to a fast rate of consumption of the refractory material, and ideally above 1475°C, at a temperature adequately above the solidus temperature of the molten metal phase, to prevent solidification of a significant portion of the ferrochrome alloy. The "solidus temperature" is the temperature where all of the ferrochrome alloy will solidify or becomes solid.

The ideal process temperatures will depend on the slag and ferrochrome alloy composition supplied. A typical process temperature range may be from 1475 to 1570°C, ideally 1500 to 1550°C for the processing of a typical acidic charge chrome slag produced in a submerged arc furnace, which will allow from a fluid molten slag and alloy, required for fast reaction kinetics, good slag and alloy separation and good refractory life.

The process temperature may be increased by adding oxygen to the process and/or adding less coolant materials, including cold ferrochrome alloy, cold ferrochrome slag and fluxes. The process temperature may be decreased by the addition of one or more of the following alone or in combination: cold ferrochrome, chrome ore, cold ferrochrome slag, and/or flux/es.

The silicothermic reduction reaction may take from 2 to 60 minutes, typically 5 to 20 minutes in order to achieve reduction of Cr ₂ 0 ₃ in the slag to chromium.

In accordance with a further aspect of the invention, the silicon content of the molten metal phase added to the process is controlled during the smelting of the chromium ore, to provide the desired silicon content of from 1-8% by mass, preferably 5-8% by mass, most preferably about 6% by mass.

The required amount of silicon in the metal phase can be calculated by determining the amount of Cr ₂ 0 ₃ to be reduced, as a function of the slag to ferrochrome ratio and the % Cr ₂ 0 ₃ in the slag from the furnaces and the final % Cr ₂ 0 ₃ in the slag after processing and the final silicon content required in the ferrochrome alloy. An equilibrium curve of the particular ferrochrome slag and ferrochrome alloy produced from furnaces needs to be established from thermodynamic data that is in the public domain. This will enable a producer to select the final silicon in the end metal and the end slag after silicothermic reduction to be achieved. Low Cr ₂ 0 ₃ content in the end slag of below 2% can be achieved, while at the same time achieving a relatively low silicon content in the end metal of typically 1% to 3%, which is in general desired by ferrochrome clients. The % Cr ₂ 0 ₃ in the end slag and silicon in the end metal after the silicothermic reduction reaction can be planned by controlling the mass ratios of silicon and Cr ₂ 0 ₃ , taking into account the equilibrium curve. The % Cr ₂ 0 ₃ in the furnace slag, silicon in the furnace ferrochrome, mass of furnace slag and ferrochrome needs to be determined to calculate if there is an excess or shortfall of silicon in the furnace ferrochrome supplied. If there is an excess silicon in the ferrochrome supplied in terms of the desired final silicon in the metal or slag % Cr ₂ 0 ₃ , the volume of Cr ₂ 0 ₃ can be increased to the required calculated level by increasing the molten ferrochrome slag added from other furnaces or the addition of cold raw materials including ferrochrome slag or chrome ore. A higher silicon content in the metal and lower content of Cr ₂ 0 ₃ in the slag can alternatively be accepted. If there is a short-fall of silicon in the ferrochrome supplied in terms of the desired final silicon in the metal or slag % Cr ₂ 0 _3i the volume of Cr ₂ 0 ₃ can be reduced by adding less molten ferrochrome slag, or accepting that a lower silicon in the final metal and higher Cr ₂ 0 ₃ in the slag will be produced.

In the process of the present invention, there is also the opportunity to remove silicon from the metal phase to levels below 0,5%, if required. This can be achieved by adding an excess of Cr ₂ 0 ₃ to the reduction reaction to ensure a high Cr ₂ 0 ₃ in the end slag or alternatively utilising an additional reduction step with adequate Cr ₂ 0 ₃ addition, which would ensure that the Cr ₂ 0 ₃ in the end slag would be typically 4% and above.

The silicothermic reaction takes place in a reaction vessel, for example a converter or ladle or other custom made vessel in which slag and metal can be mixed.

Typically, the vessel is a converter and the slag and metal are mixed by the blowing of stirrer gas/ses through the metal and slag. Alternatively, the metal and slag ^, may be mixed with a magnetic stirrer.

The stirrer gas/ses may be an inert gas or gasses or a combination of inert gas or gasses and oxygen. The ideal inert gas is argon, but other inert gasses such as nitrogen and carbon dioxide may also be used. Steam or water spray, which will break-up into H ₂ and 0 ₂ , may also be used as the gas.

A lower carbon content of typically 4% to 6% by mass may be produced when oxygen is used as one of the stirring gasses. Lower carbon contents can be produced by increasing the volume of oxygen used for stirring or alternatively ensuring that the silicon in the metal has been effectively removed during the reduction, the slag tapped from the converter, oxygen can be blown to reduce the carbon to the desired level, following which the final metal can be tapped, the Cr ₂ 0 ₃ generated in the converter can be recovered when the next batch of ferrochrome metal and slag is loaded into the converter, during the silicothermic reduction.

BRIEF DESCRIPTION OF THE DRAWING

The Drawing is a cross-sectional view of a converter.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

This invention allows chrome recovery in the production of ferrochrome (known as "charge chrome" or "high carbon ferrochrome") from smelting of chromium ore to be increased from a typical amount of about 70% to 84%, to more than 90%, typically about 95%, by the reduction of Cr ₂ 0 ₃ in the slag with silicon from the metal phase of the smelting process and the recovery of most of the metal droplets in the slag.

In the Drawing, a reaction vessel in the form of a converter is indicated generally by the numeral 10. The converter 10 comprises a steel shell 12 which is lined with a refractory material 14. Tuyeres or porous plugs 16 are provided in the base of the converter 10 for introducing stirring gas(ses) 18 into the converter 10.

Molten metal and slag from the smelting of chromium ore is tapped from a furnace into ladles from the conventional production of ferrochrome and then poured into the converter 10, so that the converter 10 contains molten metal 20 and molten slag 22. The molten metal 20 is ferrochrome alloy which contains silicon from the smelting process (known as "charge chrome" or "high carbon ferrochrome"), and the molten slag 22 may contain about 1-10%, typically 1.5-3% by mass ferrochrome alloy metal droplets and 7-20%, typically 10-16% by mass Cr ₂ 0 ₃ . The silicon is a strong reductant which, due to fast reaction kinetics in the converter 10, reduces most of the Cr ₂ 0 ₃ in the slag 22 (silicothermic reaction: 3Si + 2Cr ₂ 0 ₃ → 3Si0 ₂ + 4Cr), and chromium is recovered to the metal 20. The mixing of the molten metal 20 and molten slag 22 at a controlled temperature is able to achieve reduction of Cr ₂ 0 ₃ in the slag 16 by silicon in the metal 14 within 2 to 60 minutes, typically 5 to 20 minutes and achieve up to 95% recovery of chrome from the overall ferrochrome production process. Furthermore, the slag 22 becomes fluid, due to the reduction of the Cr ₂ 0 ₃ content and the increase in Si0 ₂ content in the slag which lowers the liquidus temperature of the slag and allows good slag and metal separation. Most of the ferrochrome alloy previously trapped/entrained in the furnace slag is recovered to the metal 20.

A typical start and end composition for charge chrome metal in a process of the present invention is as follows:

A typical start and end composition for charge chrome slag is as follows:

Element Start End

Cr ₂ 0 ₃ 10% 0.5 - 4%

FeO 5% 0.06%

Si0 ₂ 32% 41%

MgO 22% 23%

CaO 5% 6%

Al ₂ 0 ₃ 27% 29% The percentage silicon in the metal 20 is preferably controlled during the smelting of the chromium ore to provide a desired silicon content of 1-8% by mass, preferably 5-8% by mass, typically about 6% by mass, to allow for adequate silicon to reduce the Cr ₂ 0 ₃ in the molten slag 22 to low levels. The required amount of silicon in the metal 20 can be calculated by determining the amount of Cr ₂ 0 ₃ to be reduced, as a function of the slag to ferrochrome ratio and the % Cr ₂ 0 ₃ in the slag from the furnaces and the final % Cr ₂ 0 ₃ in the slag after processing and the final silicon content required in the ferrochrome alloy.

An equilibrium curve of the particular ferrochrome slag and ferrochrome alloy produced from furnaces needs to be established from thermodynamic data that is in the public domain. An example of such an equilibrium curve is provided below:

0% 1% 2% 3% 4% 5% 6%

&2θ ₃ in Slag

This will enable a producer to select the final silicon in the end metal 20 and the end slag 22 after silicothermic reduction to be achieved. Low Cr ₂ 0 ₃ content in the end slag of below 2% can be achieved, while at the same time achieving a relatively low silicon content in the end metal of typically 1% to 3%, which is in general desired by ferrochrome clients. The % Cr ₂ 0 ₃ in the end slag and silicon in the end metal after processing in the converter can be planned by controlling the mass ratios of silicon and Cr ₂ 0 ₃ into the converter taking into account the above equilibrium curve. The % Cr ₂ 0 ₃ in the furnace slag, silicon in the furnace ferrochrome, mass of furnace slag and ferrochrome needs to be determined to calculate if there is an excess or shortfall of silicon in the furnace ferrochrome supplied. If there is an excess silicon in the ferrochrome supplied in terms of the desired final silicon in the metal or slag % Cr ₂ 0 ₃ , the volume of Cr ₂ 0 ₃ can be increased to the required calculated level by increasing the molten ferrochrome slag added from other furnaces or the addition of cold raw materials including ferrochrome slag or chrome ore. A higher silicon content in the metal and lower content of Cr ₂ 0 ₃ in the slag can alternatively be accepted. If there is a short-fall of silicon in the ferrochrome supplied in terms of the desired final silicon in the metal or slag % Cr ₂ 0 ₃ , the volume of Cr ₂ 0 ₃ can be reduced by loading less molten ferrochrome slag to the converter or accepting that a lower silicon in the final metal and higher Cr ₂ 0 ₃ in the slag will be produced.

Low silicon levels in ferrochrome, typically high in demand by clients, are difficult to produce in submerged arc furnaces. This invention allows for alloy with higher silicon content to be produced in furnaces, which allows for more stable furnace conditions. The percentage silicon in the metal can be controlled to specific levels as required by clients in the process vessel.

There is also the opportunity to remove most of the silicon in the metal, to levels below 0,5% by mass, if required. This can be achieved by adding an excess of Cr ₂ 0 ₃ to ensure a high Cr ₂ 0 ₃ in the end slag or ideally utilising an additional reduction step with adequate Cr ₂ 0 ₃ addition, which would ensure that the Cr ₂ 0 ₃ in the end slag would be typically 4% and above.

The vessel should be lined internally with a refractory material 14. The appropriate refractory can be selected by taking into account the composition of the ferrochrome slag produced, which will also be influenced by the composition of the ores smelted.

In accordance with this invention, the molten metal 20 and molten slag 22 are mixed in the converter 10 using a stirrer gas 18 (typically an inert gas such as argon), while controlling the temperature to maintain the molten metal 20 and molten slag 22 below the liquidus temperature of the slag, which must be determined for a particular slag produced by a given producer and avoid super heating of the slag at temperatures high above the liquidus temperature of the slag which will lead to a fast rate of consumption of the refractory material 14. The temperature of the molten metal 20 and the molten slag 22 should be controlled adequately above the solidus temperature of the molten metal 20 ensure that most of the alloy stays liquid. For example the temperature may be from 1475 to 1570X, ideally 1500 to 1550°C for the processing of a typical acidic charge chrome slag produced in a submerged arc furnace.

As mentioned above, the temperature in the converter 10 must be controlled. The silicothermic reduction of the Cr ₂ 0 ₃ in the slag is an exothermic reaction which will allow the process temperature to rise. Oxygen may be added with the stirrer gas 18 and less cold raw materials like ferrochrome fines should be loaded to increase the process temperature if required. The process temperature can be lowered by the addition of one of the following or a combination, which will add to the economic benefits of this invention, as the material is smelted with available energy generated by the exothermic reactions from the reduction of the Cr ₂ 0 ₃ by Si and the oxidation of Si and C by the 0 ₂ gas:

• Ferrochrome fines, which value will be upgraded as it normally sells at a discount,

• Alloy that sells at a discount such as ferrochrome from a metal recovery plant,

• Chrome ore,

β Cold ferrochrome slag,

• Fluxes.

Although this invention has been described with reference to the use of a converter, an appropriate reaction vessel such as ladles or custom made vessels can be designed for the application, taking into account factors such as the ferrochrome tapped per tap from the furnace and the slag volume to be handled.

An advantage of the process of this invention is that no fluxes need to be added to the process vessel, as an acidic slag would allow acceptable refractory lining life, if the temperature is controlled below the liquidus temperature of the end slag and a refractory compatible with the slag is chosen for the converter. Maintaining a basic slag with the addition of fluxes such as burned lime and burned dolomite would not be essential to ensure good refractory life. Slow wear can be expected if the slag temperature is marginally above the liquidus temperature of the slag. Fluxes including burned lime and burned dolomite or chrome ore may however still be added to ensure slag saturation at higher process temperatures. The advantages of not having to add fluxes include the following:

• The slag volume in the converter can be reduced significantly, which will allow for much superior mixing of molten slag and molten metal, which in turn will improve the reaction efficiency,

• The addition of a large volume of fluxes to maintain a basic slag, require a significant amount of process heat to bring the cold fluxes up to the process temperature. The available process heat can be used more productively to smelt low value ferrochrome such as fines and upgrade it to lumpy product,

• A significant cost saving is realised by not having to add fluxes. The mixing efficiency can be further improved by loading of the molten ferrochrome slag in more than one batch, which will improve the molten metal to slag ratio in the processing vessel.

A niche ferrochrome product with higher chromium content, low sulphur, silicon according to client specifications and lower carbon content can be produced:

• The additional recovery of chromium to the alloy results in the production of an alloy with about 3.5% higher chromium content, compared to the ferrochrome supplied from the furnace. ^β Desulphurisation of the ferrochrome takes place during the processing in the converter due to the capacity of the slag to desulphurise.

A lower carbon content of typically 4% to 6% by mass may be produced when oxygen is used as one of the stirring gasses. Lower carbon contents can be produced by increasing the volume of oxygen used for stirring or alternatively ensuring that the silicon in the metal has been effectively removed during the reduction, the slag tapped from the converter, oxygen can be blown to reduce the carbon to the desired level, following which the final metal can be tapped, the Cr ₂ 0 ₃ generated in the converter can be recovered when the next batch of ferrochrome metal and slag is loaded into the converter, during the silicothermic reduction.

Further benefits of this invention include:

• Desulphurisation will allow for cheaper reductants high in sulphur to be used in the furnaces, which can be a considerable cost saving,

• The processed slag will be low in entrained alloy and the cost of processing the slag through a metal recovery plant can be saved.

• Very little additional raw materials and energy are consumed to produce the additional chromium units, which is predominantly refractories and process gasses. Carbon credits can be earned due to this.

• The logistics costs from smelter to clients and other sales costs of the additional chromium produced will be less that of conventional ferrochrome production, as the chromium units per ton ferrochrome sold will be higher,

© The addition chrome units produced should be less than 25% of the cost of the conventional production of ferrochrome in a submerged arc furnace.