This is a 371 of PCT/CA2012/000387, which claims the benefit of U.S. provisional application 61/457,638, which is incorporated herein by reference in its entirety.
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This invention relates to metallization of metal oxides in ores and subsequent refining to commercial products such as steels and ferroalloys. This invention particularly relates to carbon levels in solid products of reduction and the subsequent de-carburization of liquid iron and iron alloys in converters.
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There are two commercially important reactors in current commercial ironmaking: (a) the blast furnace (BF), and (b) the submerged arc furnace (SAF). For kinetic advantages and for permeability of fluid flows, the smelting or melting compartments of these two reactors are packed with metallurgical coke (which is 90% carbon balance ash). In general, the resultant hot metal is very close to carbon saturation (more than 4% from the BF and more than 6% from the SAF) and contains other impurities such as Mn, Si, P, etc. due to carbon reduction of gangue oxides in the raw materials. Carbon and these impurities have to be removed in the subsequent refining processes to produce end products for market.
Different metal oxides may be reduced to their elemental metallic states by reducing agents of different reducing power. A simple example is given here to illustrate the thermodynamic nature of the stability of metal oxides and carbon reduction. When a mixture of metal oxides and carbon (as the reducing agent) is gradually heated up to a high temperature to initiate the reduction reactions, the degree of metallization of each oxide in Direct Reduction Iron (“DRI”) depends on the maximum temperature reached and the amount of carbon available. Based on the thermodynamics of chemical reactions under equilibrium conditions, for example, when a mixture consisting of lead oxide, nickel oxide, iron oxide, manganese oxide, chromium oxide, silica, titanium oxide, alumina is reduced by carbon; these reactions would take place sequentially, following the order as listed of the oxides shown above. After the reduction of all lead and nickel oxides present in the system, the remaining carbon in the mixture will start to reduce iron oxide and so on. At the exhaustion of carbon, all reactions stop. For the mixture of oxides mentioned above, if substantial recovery of chromium is sought, then, the recovery of iron, nickel, lead would be complete and some dissolved Si and Ti would be in the metallic phase at a high enough temperature.
DRI made according to existing commercial “Direct Reduction” processes of ironmaking may be divided into two groups: (1) DRI made in a shaft furnace and rotary kiln which is limited in carbon by chemical reactions, not by supplies, and varies with operating conditions. (2) DRI made in Rotary Hearth Furnaces which has both residual carbon as well as iron oxides with uncertain amounts due to re-oxidation of sponge iron and incomplete reduction.
Ironmaking technology based on the reduction of chemically self-sufficient ore-coal composite pellets in a Paired Straight Hearth (PSH) Furnace can produce DRI with a high degree of metallization and well controlled carbon levels (see for example U.S. Pat. Nos. 6,257,879 and 6,592,648, the entire contents of which are incorporated by reference herein). This is because re-oxidation of sponge iron can be avoided or effectively limited to very low values. Liquid product made by melting DRI of a PSH Furnace has two advantages: (1) selective reduction (SR) to produce low carbon liquid iron through the design of green balls and the use of the PSH Furnace, (2) simpler processing in the refining of low carbon hot metal to end products.
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OF THE INVENTION
The theoretical basis of “Selective Reduction” is based on the thermodynamic nature of the stability of metal oxides and carbon reduction. When a mixture of metal oxides and carbon is gradually heated up to a high temperature to initiate the reduction reactions, the degree of metallization of each oxide in DRI depends on the maximum temperature reached and the amount of carbon available. Based on the thermodynamics of chemical reactions under equilibrium conditions, for example, when a mixture consisting of lead oxide, nickel oxide, iron oxide, manganese oxide, chromium oxide, silica, titania, alumina is reduced by carbon; these reactions would take place sequentially, following the order as listed of oxides shown above. After the reduction of all lead and nickel oxides present in the system, the remaining carbon in the mixture will start to reduce iron oxide and so on. At the exhaustion of carbon, all reactions stop. If, for the mixture of oxides mentioned above, substantial recovery of chromium is desired, then, the recovery of iron, nickel, lead would be complete and there would be some dissolved Si and Ti in the metallic phase at a high enough temperature.
One could apply this concept of “effectively controlled carbon availability” to control the recovery of iron in iron ore below 100% as illustrated in following two examples: One is the smelting of low grade nickel ore in blast furnace, with a typical hot metal containing less than 5% wt Ni and balance mainly iron and carbon. It would be advantageous to increase nickel content through limiting the recovery of iron (but not of nickel) by controlling carbon available in green balls. A higher Ni to Fe ratio in DRI, subsequently in hot metal, will lead to more valuable products. The second example concerns high phosphorous iron ores which are abundant in many parts of world. These have very limited usage in blast furnace ironmaking today because of essentially 100% recovery of P in hot metal and excessive cost of P removal in subsequent steelmaking. In my process ore-coal composite pellets may be designed such that in the melting of DRI there would be a slag of controlled iron oxide level and basicity to keep a substantial portion of the P in the slag.
In summary, my invention is the design of ore-coal composite pellets and the use of a PSH Furnace, so as to take advantage of (1) carbon as the most powerful reducing agent and (2) the introduction of a mechanism to cut-off all reduction reactions at the exhaustion of carbon.
DESCRIPTION OF DRAWINGS
FIG. 1 is a binary phase diagram for the system AL2O3—B2O3;
FIG. 2 is a binary phase diagram for the system B2O3—MgO;
FIG. 3 is a binary phase diagram for the system SiO2—B2O3;
FIG. 4 is a binary phase diagram for the system CaO—B2O3;
FIG. 5 is a photomicrograph of direct reduction iron from green balls without flux additions;
FIG. 6 is a photomicrograph of direct reduction iron from green balls with a flux addition of 4 g silica per 100 g of ore; and,
FIG. 7 is a photomicrograph of direct reduction iron from green balls with a flux addition of 5 g of silica and 1.1. g boron oxide in borax.
DESCRIPTION OF PREFERRED EMBODIMENTS
De-carburization and Refining of Hot Metal
Currently, liquid metal from ironmaking furnaces is further processed to commercial products often in the same plant. Hot metal from a BF containing 4.5% C and “carbon ferrochrome” from a SAF containing over 6% C are raw materials for making of steels of much less than 1% C and medium carbon ferrochrome of around 2% C, respectively. Therefore, de-carburization is a necessary refining step in the current industrial setting.
In the case of steelmaking from blast furnace hot metal, there are two necessary and sequential refining steps: (1) de-carburization and removal of dissolved nitrogen and silicon in liquid iron in the Basic Oxygen Furnace (BOF), then, (2) final control of temperature and composition of liquid steel in a Ladle Furnace (LF) before casting. There are two slags produced in traditional flow sheets, one in the BF for the removal of gangue in ore and ash in coal and coke, and the other in a converter for the de-carburization and removal of impurities in hot metal, such as Si, Mn, S and P.
The present invention may be exemplified by the experimental results reported below. Composite pellets (also referred to as “green pellets”) made of typical iron ore and medium or high volatile coal with a C/O ratio (where C stands for total carbon in carbonaceous reductants and O stands for combined oxygen in reducible oxides in ore, both in atomic-grams) in the range of 0.95 to 1.05 may be reduced to DRI of 95% metallization in a laboratory under PSH Furnace operating conditions. Subsequently, the melting of the resultant DRI under a controlled atmosphere, without any flux additions to control S & P, may be carried out in an induction furnace to achieve the separation of liquid metal from slag. The following typical compositions in wt% of liquid metal are given below:
For C/O=0.95: C 0.02, Si 0.09, O 0.02, N 0.003