Claims:We claim:
01. An economical process (100) of producing synthetic slag (20) from aluminum dross (30) in an arc furnace (50), characterized in that
the arc furnace (50) has a plurality of graphite electrodes (51) placed in a symmetrical formation, a first taphole (59) at a lower level and a second taphole (60) relatively there above and circumferentially at a different location, and
a charge mixture (10) comprising the aluminum dross having alumina and aluminum, an oxidizing agent, and calcium oxide, a ratio of the calcium oxide and an “aluminum-free” alumina being 650-680:680-720;
wherein,
the charge mixture (10) is maintained at a temperature between 1350°C to 1450°C,
a first reaction 2Al + OA2O3 = Al2O3 + 2OA, and
a second reaction 12CaO + 7Al2O3 = 12CaO.7Al2O3 (or C12A7)
commence simultaneously, and complete in 20 to 50 minutes, producing the synthetic slag (20) .
02. The economical process (100) of producing the synthetic slag (20) from the aluminum dross (30) as claimed in claim 01, wherein the arc furnace acts as a submerged arc furnace with the plurality of electrodes dipped into the liquid and arcing takes place inside a melt of the charge furnace.
03. The economical process (100) of producing the synthetic slag (20) from the aluminum dross (30) as claimed in claim 01, wherein the aluminum-free alumina comprises 51 grams of alumina produced for every 27 grams of aluminum during the first reaction.
04. The economical process (100) of producing the synthetic slag (20) from the aluminum dross (30) as claimed in claim 01, wherein the oxidizing agent is ferric oxide Fe2O3.
05. The economical process (100) of producing the synthetic slag (20) from the aluminum dross (30) as claimed in claim 01, wherein the Al2O3 is producible at 32 to 62% cost with respect to prior art, and whereby the synthetic slag (20) is correspondingly produced. , Description:FIELD OF THE INVENTION
The present invention relates to aluminum dross, particularly to a process to use aluminum dross waste into a useful product. More particularly the present invention relates to an economical process to produce synthetic slag for steel making.

BACKGROUND OF THE INVENTION
Aluminum industry worldwide generates a huge quantity of waste known as aluminum dross. Aluminum dross is primarily ash skimmed from the surface of molten aluminum during the process of producing/smelting and also casting. Aluminum dross is made up mainly of Al2O3 or aluminum oxide, commonly known as alumina, along with a generous amount of fluxes like fluorides, sodium etc. Aluminum dross also invariably contains unwanted metallic aluminum in the form of small beads or larger pieces. There are known attempts to convert this waste into useful products, the initial procedure being crushing and pulverizing, whereby alumina part becomes fine powder while the metallic aluminum remains as beads/nuggets etc. which is screened out, which is then melted in crucibles and converted to ingots, bars or notch bars etc. as finished products. But this process still leaves behind large quantities of aluminum dross powder, which also comes under the category of hazardous material.
US 2012/0297926 relates to a method for producing an agglomerated product containing alumina which is useful as a component to be incorporated into a synthetic slag as used in Steel making. The product is formed from powdered aluminum dross which is formed with water into pellets or briquettes and wherein the components of the pellets or briquettes are allowed to react at elevated pressure to release ammonia. The pellets or briquettes may then be calcined.
201921010688 also relates to the utilization of aluminum dross based briquetted synthetic slag during tapping of low carbon aluminum killed (LCAK) steel grades.
Thus, one of the ways is to pelletize the powder, dry the pellets, and then calcine it to convert the free aluminum metal to alumina, as described in the above cited prior art. Also, in this pelletizing process, a lot of ammonia and other gasses are generated, creating pollution problems and requiring elaborate scrubbing systems. This would require pelletizing machine, a drier, a gas scrubber and a furnace or kiln for calcination.
KR1019970043110 relates to a method for reforming slag generated as a byproduct in a steelmaking process.
For above process, a known source for CaO is calcined lime. But for Al2O3 there could be a number of sources. The main source for many years had been calcined bauxite, which is getting costlier day by day. Also, bauxite has some undesirable impurities. Silica (SiO2) is high which is not much preferred by many steel makers. Also, it has high TiO2, which is unwanted for certain grades of steel, such as ball bearing steel.
Prior art seems to miss a known serious cost issue as now discussed.
A typical chemistry of aluminum dross powder being about 40-60% alumina with 10-20% aluminum in metallic powder or various compound forms, while a good source of alumina for manufacture of synthetic slag, however, has a serious problem due to presence of metallic aluminum. The free aluminum present reacts with carbon, from lining and electrodes, to form aluminum carbide Al4C3, which has a melting point of 2500o C. and accumulates at the bottom of arc furnace, making an unmeltable hard chunk. As a result, the arc furnace has to be shut down every 3-4 days and this hard material has to be dug out. This usually involves several days’ work and adds unwanted cost.
The present invention effectively addresses this aspect of economic significance and technical advance.

SUMMARY OF INVENTION
The present invention has technical advance as well as economic significance, and stems from the industrial need wrt management of a hazardous waste. The present invention aims at producing synthetic slag for producing quality steel while at the same time attempting to inventively use a hazardous industrial waste and reduce adverse impact on environment. It is clearly understood that aluminum dross does have a large amount of aluminum along with alumina; and that alumina and aluminum have very different characteristic, making them unusable in a mixture form.
The present invention is a process of producing synthetic slag in an arc furnace with graphite electrodes placed in a symmetrical formation for homogeneous reaction parameters in the arc furnace, using aluminum dross powder having a fair percentage of both – alumina along with aluminum. An arc furnace is most economical in terms of quick start and stop and consumes minimum energy, however, graphite electrodes and carbon linings in presence of free and unwanted aluminum, present in the aluminum dross, result in formation of an undesirable aluminum carbide Al4C3. Aluminum carbide has a melting point of 2500o C and accumulates at the bottom making an unmeltable hard chunk. As a result, the furnace has to be shut down every 3-4 days and this hard material has to be dug out. This involves several days of unproductive work, and therefore cost. A preferred embodiment of the present invention pre-empts this situation by adding Ferric Oxide (Fe2O3) to the aluminum dross along with calcium oxide commonly known as lime, and a charge mixture is obtained. This charge mixture is added to the arc furnace. This step is significantly different than prior arts, wherein different process and or reactions are carried out sequentially.
When the furnace temperature is maintained between 1350°C to 1450°C,
a first reaction 2Al + Fe2O3 = Al2O3 + 2Fe, and
a second reaction 12 CaO + 7 Al2O3 = 12CaO.7Al2O3 (or C12A7)
commence simultaneously.
Particularly, ferric oxide or Fe2O3 is selected for this inventive process since
- Iron (Fe) and Aluminum (Al) are situated closely in electrochemical series and can therefore participate in oxidation and reduction reaction at same temperature,
- Al has a higher affinity for Oxygen than for Carbon at the prescribed temperature; and
- Fe as a byproduct is a useful outcome.
As a variation any oxidizing agent (abbreviated as OA) or oxide which exhibits similar chemical behavior wrt Aluminum and whose byproduct is of economic significance can substitute ferric oxide. Thus, a first reaction is broadly represented as 2Al + OA2O3 = Al2O3 + 2OA, wherein OA implies a cation of the oxidizing agent, preferably a trivalent metallic ion. Calcium oxide and aluminum oxide, represented commonly as CA, can combine in a number of ways to different compounds of varying eutectics. Illustratively, it can form CA, where C stands for Calcium Oxide (CaO) and A stands for alumina (Al2O3). When one molecular weight of CaO combines with one molecular weight of Al2O3, then it is commonly represented as CA. CA2, CA3, in general CN1AN2 (wherein N1 represents number of molecules of Ca and N2 represents number of molecules of Al2O3) etc. are high eutectics complexes, i.e., with high melting point, and are mostly used as high alumina cements or refractory cements. The main deciding factor for the end product is the ratio of CaO and Al2O3 in the mixture. As per the present invention, CaO and Al2O3 are mixed in the ratio of 650-680:680-720 to obtain a combination of C12A7. This combination has the lowest eutectic for CaO-Al2O3 combination, with a melting point of 1380-1400 deg C. This is the desired synthetic slag as per present invention. Its easy meltability and fluidity of the melt makes it most economical and most profitably useful in steel making process for removal of sulfur and other (mainly Al2O3) inclusions from steel - During the process of steel making, the molten metal in general contains high amounts of sulfur, (anywhere between 0.1% to 0.8% or higher). The desirable level is 0.05%. and the synthetic slag produced as per present invention helps reduce sulfur by forming calcium sulfate and which then goes into waste slag. Also, the steel may contain some micro particles of alumina (Al2O3) picked up from Refractories, which the synthetic slag removes, resulting in cleaner higher grade steel. Importantly, to arrive at a correct weight of the ingredients of the charge mixture, 102 gm of alumina is considered for every 54 grams of Aluminum present in the aluminum dross. In other words, when 54 grams of Aluminum is present in the charge mixture, it is considered equivalent to 102 grams of alumina, which shall get produced in the course of the rection. Hence aluminum-free alumina is considered for calculation of correct percentage. The charge mixture is maintained at the prescribed temperature for typically 20-50 minutes when the second reaction resulting in the formation of C12A7 is completed. In our inventive process, this conversion of unwanted free aluminum to alumina is achieved in the melting furnace itself, by adding a calculated amount of mill scale (iron oxide) to the charge mixture, which on melting reacts with the aluminum, converting it to alumina, and in the process the iron oxide is reduced to iron metal, which settles at the bottom, giving a byproduct of cast iron. Thus, aluminum is no longer a nuisance and output is valuable. There is no generation of obnoxious gases.
The arc furnace used for this process is more particularly a submerged arc furnace, which is essentially a regular arc furnace, however since the charge mixture is a low conducting material, the electrodes are dipped into the liquid and arcing takes place inside the melt, which is why it is termed submerged arc furnace. The submerged arc furnace has two tapholes, a first taphole at a lower level and a second taphole relatively there above and circumferentially at a different location. The first taphole is used to periodically tap out an accumulated by product (58) which is cast iron when the oxidizing agent is ferric oxide, and the second taphole (60) is for withdrawing the end product which is the synthetic slag 12CaO.7Al2O3 (or C12A7).
The economic significance of the present invention is easily understandable from following comparative product costings, based on a plant infrastructure for 20 TPD (Tonnes Per Day) production, and with comparable fixed investment; and with a boundary condition that to achieve C12A7 phase of calcium oxide and aluminum oxide, the two oxides are mixed in almost equal proportions. The Al2O3 extracted and producible as per present invention is at 32 to 62% cost with respect to prior art, and whereby the synthetic slag is correspondingly produced. The inventive process is based significantly on use of hazardous waste and is consequently further environment friendly.

BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a process flow diagram of the present invention.
Figure 2 is a sectional front view of an arc furnace deployed for the present invention.
Figure 3 is a sectional top view of the arc furnace deployed for the present invention.
Figure 4 is an electrochemical series of elements participating in chemical reaction of the present invention.
Figure 5 shows characteristics of various eutectics of Calcium Oxide and Alumina combination (CA).

DETAILED DESCRIPTION OF INVENTION
The present invention, which has technical advance as well as economic significance, shall now be described in detail with the help of drawings and flow diagrams. It is to be understood that several variations are possible around the inventive concept of the present invention and the detailed description of the preferred embodiment should not be construed to limit this invention in any way.
The present invention stems from the industrial need to have a process invention of economic significance and technical advance wrt management of a hazardous waste. The present invention aims at producing synthetic slag for producing quality steel while at the same time attempting to inventively use a hazardous industrial waste and reduce adverse impact on environment.
Synthetic slag during secondary steelmaking maximizes the efficiency of the steel refining process by (i) improving steel quality, (ii) improving productivity, (iii) reducing costs, and (iv) protecting the environment (since it removes the need of addition of fluorspar also called fluorite). The other objectives of use of the synthetic slag during secondary steelmaking are as follows:
• To cover the liquid steel with an insulating layer to reduce heat losses which otherwise takes place due to heat radiation.
• To remove the possibility of reoxidation of steel from atmospheric oxygen.
• To absorb inclusions, present in the liquid steel.
• To desulphurize liquid steel. Using synthetic slag of desired basicity and sulphide capacity, deoxidized steel can be desulphurized to as low as 0.005 % of sulphur.
• The use of the synthetic slag practice is to be attractive especially when there is low capital cost of equipment associated with the application of this practice.
Synthetic slag is used during the secondary steelmaking because of the following:
• Synthetic slag greatly influences deoxidation and desulphurization of the liquid steel.
• It greatly shortens the steelmaking time, efficiently remove impurities and improve the steel quality.
• It is a required additive for production of special steels such as bearing quality steel.
• It reduces dust float pollution effectively.
• It lowers melting temperature of slag.
• It helps in increasing the slag fluidity.
• It helps in absorbing inclusions and impurities, thus producing cleaner steel.
• Synthetic slag is stable and nearly dust-free and hence easy to handle and there is no pollution during its handling.
It is clearly understood that aluminum dross does have a large amount of aluminum along with alumina; and that alumina and aluminum have very different characteristic, making them unusable in a mixture form.
Figure 1, the present invention is a process of producing synthetic slag (20) in an arc furnace (50) with graphite electrodes (51) placed in a symmetrical formation for homogeneous reaction parameters in the arc furnace (50), using aluminum dross powder (30) having a fair percentage of both – alumina along with aluminum.
Figure 2,3, an arc furnace (50) is most economical in terms of quick start and stop and consumes minimum energy, however, graphite electrodes (51) and carbon linings (55) in presence of free and unwanted aluminum, present in the aluminum dross (30), result in formation of an undesirable aluminum carbide Al4C3. Aluminum carbide has a melting point of 2500o C and accumulates at the bottom making an unmeltable hard chunk. As a result, the furnace has to be shut down every 3-4 days and this hard material has to be dug out. This involves several days of unproductive work, and therefore cost.
A preferred embodiment of the present invention pre-empts this situation by adding Ferric Oxide (Fe2O3) to the aluminum dross (30) along with calcium oxide commonly known as lime, and a charge mixture (10) is obtained. This charge mixture (10) is added to the arc furnace (50). This step is significantly different than prior arts, wherein different process and or reactions are carried out sequentially.
When the furnace temperature is maintained between 1350°C to 1450°C,
a first reaction 2Al + Fe2O3 = Al2O3 + 2Fe, and
a second reaction 12 CaO + 7 Al2O3 = 12CaO.7Al2O3 (or C12A7)
commence simultaneously.
Particularly, referring to Figure 4, ferric oxide or Fe2O3 is selected for this inventive process since
- Iron (Fe) and Aluminum (Al) are situated closely in electrochemical series and can therefore participate in oxidation and reduction reaction at same temperature,
- Al has a higher affinity for Oxygen than for Carbon at the prescribed temperature; and
- Fe as a byproduct (58) is a useful outcome.
As a variation any oxidizing agent (abbreviated as OA) or oxide which exhibits similar chemical behavior wrt Aluminum and whose byproduct (58) is of economic significance can substitute ferric oxide. Thus, a first reaction is broadly represented as 2Al + OA2O3 = Al2O3 + 2OA, wherein OA implies a cation of the oxidizing agent, preferably a trivalent metallic ion.
Figure 5, calcium oxide and aluminum oxide, represented commonly as CA, can combine in a number of ways to different compounds of varying eutectics. Illustratively, it can form CA, where C stands for Calcium Oxide (CaO) and A stands for alumina (Al2O3). When one molecular weight of CaO combines with one molecular weight of Al2O3, then it is commonly represented as CA. CA2, CA3, in general CN1AN2 (wherein N1 represents number of molecules of Ca and N2 represents number of molecules of Al2O3) etc. are high eutectics complexes, i.e., with high melting point, and are mostly used as high alumina cements or refractory cements. The main deciding factor for the end product is the ratio of CaO and Al2O3 in the mixture. As per the present invention, CaO and Al2O3 are mixed in the ratio of 650-680:680-720 to obtain a combination of C12A7. This combination has the lowest eutectic for CaO-Al2O3 combination, with a melting point of 1380-1400 deg C. This is the desired synthetic slag as per present invention. Its easy meltability and fluidity of the melt makes it most economical and most profitably useful in steel making process for removal of sulfur and other (mainly Al2O3) inclusions from steel - During the process of steel making, the molten metal in general contains high amounts of sulfur, (anywhere between 0.1% to 0.8% or higher). The desirable level is 0.05%. and the synthetic slag produced as per present invention helps reduce sulfur by forming calcium sulfate and which then goes into waste slag. Also, the steel may contain some micro particles of alumina (Al2O3) picked up from Refractories, which the synthetic slag removes, resulting in cleaner higher grade steel.
Importantly, to arrive at a correct weight of the ingredients of the charge mixture, 102 gm of alumina is considered for every 54 grams of Aluminum present in the aluminum dross. In other words, when 54 grams of Aluminum is present in the charge mixture, it is considered equivalent to 102 grams of alumina, which shall get produced in the course of the rection. Hence aluminum-free alumina is considered for calculation of correct percentage. The charge mixture is maintained at the prescribed temperature for typically 20-50 minutes when the second reaction resulting in the formation of C12A7 is completed.
In our inventive process, this conversion of unwanted free aluminum to alumina is achieved in the melting furnace itself, by adding a calculated amount of mill scale (iron oxide) to the charge mixture (10), which on melting reacts with the aluminum, converting it to alumina, and in the process the iron oxide is reduced to iron metal, which settles at the bottom, giving a byproduct of cast iron. Thus, aluminum is no longer a nuisance and output is valuable. There is no generation of obnoxious gases.
The arc furnace (50) used for this process is more particularly a submerged arc furnace, which is essentially a regular arc furnace, however since the charge mixture (10) is a low conducting material, the electrodes (51) are dipped into the liquid and arcing takes place inside the melt, which is why it is termed submerged arc furnace. The submerged arc furnace has two tapholes, a first taphole (59) at a lower level and a second taphole (60) relatively there above and circumferentially at a different location The first taphole (59) is used to periodically tap out an accumulated by product (58) which is cast iron when the oxidizing agent is ferric oxide, and the second taphole (60) is for withdrawing the end product which is the synthetic slag 12CaO.7Al2O3 (or C12A7).
The economic significance of the present invention is easily understandable from following comparative product costings, based on a plant infrastructure for 20 TPD (Tonnes Per Day) production, and with comparable fixed investment; and with a boundary condition that to achieve C12A7 phase of calcium oxide and aluminum oxide, the two oxides are mixed in almost equal proportions.
Illustrative Prior Art ONE - Calcined bauxite contains average 75-78% Al2O3. Therefore 100 kg of bauxite will give us only 77 kg Al2O3. Cost of calcined bauxite at present is Rs11/- per kg. (for 77kg Al2O3), i.e., Rs.14.30/- per kg of Al2O3.
Illustrative Prior Art TWO - In the 2nd process, dross powder is pelletised, dried, then calcined. This involves a number of processes and equipment and labour. Present price of dross powder is Rs.3 to 3.50/- per kg. to convert it requires pelletisers, driers, and calcination kilns and Rs.3/- to 3.50 is added. After all the processing the cost of final product comes to about Rs.6.50 to 7/- per kg of with average 88% Al2O3 content, i.e., Rs.7.38/- per kg of Al2O3.
In our process as per invention described above, the dross powder cost is same as above, that is, INR 3.00 to 3.50/- per kg. Extra addition is approx. 3-4% mill scale (iron oxide). Cost of mill scale is about Rs.8/- per kg. Final product we get 88% Al2O3 content and cost is Rs.4/- per kg, therefore cost of Al2O3 works out to Rs.4.55/- per kg Al2O3.
The Al2O3 extracted and producible as per present invention is at 32 to 62% cost with respect to prior art, and whereby the synthetic slag (20) is correspondingly produced. The inventive process is based significantly on use of hazardous waste and is consequently further environment friendly.