CA3037916A1 - Method to prepare a steel and slag aluminum deoxidizer from aluminum dross and dross residue characterized by low nitride contents and low hydro-reactivity - Google Patents

Abstract

This invention relates to a method for the preparation of a steel and slag aluminum deoxidizer characterized by low aluminum nitride contents and low water reactivity, using aluminum dross residue or aluminum dross fines, with reduced costs and environmental benefits.

Description

METHOD TO PREPARE A STEEL AND SLAG ALUMINUM DEOXIDIZER FROM
ALUMINUM DROSS AND DROSS RESIDUE CHARACTERIZED BY LOW
NITRIDE CONTENTS AND LOW HYDRO-REACTIVITY
FIELD OF THE INVENTION
This invention relates to a method for the preparation of a steel and slag aluminum deoxidizer characterized by low aluminum nitride contents and low water reactivity, using aluminum dross residue or aluminum dross fines, with reduced costs and environmental benefits.
BACKGROUND OF THE INVENTION
New product application and superior steel grades require new cleaner, desulfurized and alloyed steels.
This requires more active and reducing slag, with lowest possible oxygen activity.
The utilization of aluminum dross to decrease the quantity of reducible metal oxides, namely iron and manganese oxides in the carry-over slag during steel transfer from the basic oxygen furnace (BOF) or electric arc furnace to the transport ladle, is recognized as an effective, cost-saving practice.
In Reference 1, Aydemir compared the efficiencies of aluminum dross and solid aluminum in lowering reducible oxide contents in steel slag. He concluded that aluminum dross with 30-35% Al and 55-60% A1203 produces a more lasting effect than pure aluminum, which was attributed to the formation of low melting point calcium aluminate due to the presence of alumina in the dross. In another industrial trial, it was concluded that Al dross (containing 46.7 % Al, with 86 % of particles size in the 5-30 mm range) is more efficient for ladle slag deoxidation, resulting in final FeO + MnO contents which are only half of those by using Al granules, on the same Al metal basis content (Reference 2).
For a similar reason, the use of aluminum dross has been reported to enhance sulfur removal and steel cleanliness when treating hot metal with lime, a phenomenon attributed to a decrease in slag viscosity (Reference 3).
More recently, in 2014, dross residue with low metallic aluminum and high impurity levels, such as aluminum nitride and alkali salts, has been tested as ladle slag reducer. In Reference 4, both the FeO and S contents of BOF slag were successfully reduced, but the nitrogen content of the steel significantly increased from 66 to 129 ppm N.
This is attributed to the fact that the reaction of aluminum nitride with reducible oxides generates gaseous nitrogen, which dissolves in the steel, according to the following reaction between AIN and iron oxide:

2 AIN + 3 FeO = 3 Fe + Al2O3 + N2 Dross fines and dross residues being available in large quantities, and at relatively low cost on a metal comparative basis, there is a need to find ways to upgrade at least a significant proportion of these materials for their utilization as slag conditioners and deoxidizers while minimizing the detrimental effect on steel caused by the presence of aluminum nitride. This would further help to minimize the environmental impact caused by the disposal of dross residues.
Dross is the material which forms on the surface of molten non-ferrous metals during remelting and metal handling operations, when the molten metal is in contact with a reactive atmosphere such as air. Aluminum dross normally consists of metal oxides entraining a considerable quantity of un-reacted metal and, for cost-efficiency reasons, recovering the metal content of dross is considered sound practice.
Aluminum recovery from dross is usually done in rotary salt furnace (RSF), which produces salt cake residue. Disposal of this waste in landfill has become a major issue and is now prohibited in many countries. This has led to the development of various salt-free dross methoding technologies that facilitate oxide waste recycling.
Some of these methods are based on different mechanical pre-treatments (milling, classification, screening) to separate a coarse-aluminum-rich fraction, which can be directly remelted, thus limiting waste, as described in Reference 5.
Another approach uses electric-powered rotary furnaces, rather than fossil-fuel-powered RSF, which do not require salt to separate metal from the oxides. For example, plasma furnaces (Reference 6) and electric-arc furnaces (Reference 7) generate non-metallic residues called NMP (Non-Metallic Product), which essentially consist of a mixture of alumina and magnesium oxides, other metal-oxide impurities, aluminum nitrides, chlorinated and fluorinated salts, and variable amounts of residual metallic aluminum.

A typical NMP primary dross residue composition, generated by the salt-free plasma dross technology called NOVALTM, is described in Reference 8.
Due to its high equivalent alumina content (corresponding to the total alumina derived from aluminum metal and aluminum nitride oxidation), the NOVALTM technology is successfully utilized as a base material for the production of calcium aluminate, or as an alumina additive for the production of Portland cement.
However, as stated above, NMP requires high-temperature oxidative pre-treatment before it can be used to formulate steel and slag conditioners, due to the presence of aluminum nitride in concentrations equal to and often higher than metallic aluminum concentrations, as illustrated in Ref 8, where the AIN/AI ratio is greater than 1.
In addition to the metallurgical problems caused by the presence of AIN, many other factors, such as highly variable compositions and hydro-reactivity, impede large-scale use of NMP for high-quality product fabrication, compared with raw materials from natural sources.
Therefore, there is a genuine need to develop new approaches allowing to upgrade significant proportions of this type of salt-free residue by making the best use possible of their metallic aluminum contents while minimizing aluminum-nitride-related problems.
SUMMARY OF THE INVENTION
The present invention includes a method to prepare an aluminum reducing material of substantially homogenous composition from different mixtures of NMPs residues from salt-free recycled aluminum dross or fine dross material characterized by low AIN
content and high Al to AIN ratio.
According to an alternate aspect method of the invention, there is provided the method as defined above, further comprising the following steps:
a') separation of NMP dross residues or aluminum dross to generate fine particle size material smaller than about 10 mm;
b) classification of the smaller particles into multiple size fractions according to respective AIN and Al concentrations;
c) preparation of different blends of MNP dross-residue and/or aluminum dross granulometric fractions selected according to AIN and Al contents of the multiple fractions

- 3 -so as to derive a product of substantially homogeneous composition with the criteria required for its anticipated end-use.
According to an alternate aspect method of the invention, there is provided the method comprises the following steps:
a') comminution of NMP dross residues or aluminum dross to generate fine particle size material smaller than about 10 mm;
b') characterization of the AIN and Al contents of different size fractions of NMPs residues or fine materials from aluminum dross according to the particle-size distribution of the granular material, from between about 10 to 0.1 mm;
c') preparation of different blends of MNP dross-residue and/or aluminum dross granulometric fractions selected according to AIN and Al contents so as to derive a product of substantially homogeneous composition with the criteria required for its anticipated end-use.
Optionally, the methods of the present invention may further comprise the step of:
d) adding a predetermined quantity of aluminum to the blended product to reach the final product composition to maximize Al content while minimizing AIN to Al % w/w ratio of less than one.
It has been shown that the material prepared by selectively using the coarse end of the particle size distribution of NMP dross residue or dross material is characterized by a much lower hydro reactivity, by a factor of about 10, compared with the entire size distribution of the original materials.
The blended material derived from the method of the present invention can be used as is, without further pre-treatment; for example, in steel foundries as a steel and slag deoxidizer, or for the preparation of blended mixtures or briquetted products for use as slag conditioner and deoxidizer by mixing with other compounds such as lime, magnesium oxide, alumina or fluoride salts.
DESCRIPTION OF THE INVENTION
Description of the drawings FIGURE 1 is a flow chart of a particular embodiment of the method of the present invention.

- 4 -Abbreviations and Definitions The term "about" as used herein refers to a margin of + or ¨ 10% of the number indicated.
For sake of precision, the term about when used in conjunction with, for example: 90% means 90% +1- 9% i.e. from 81% to 99%. More precisely, the term about refer to + or -

5% of the number indicated, where for example: 90% means 90% +1- 4.5% i.e. from 86.5% to 94.5%.
As used herein the singular forms "a", "and", and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a cell"
includes a plurality of such cells and reference to "the culture" includes reference to one or more cultures and equivalents thereof known to those skilled in the art, and so forth. All technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs unless clearly indicated otherwise.
As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes"
and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, un-recited elements or method steps.
Detailed description of particular embodiments The innovative method of the present invention comprises the characterization and utilization of one or several size fractions of NMP or fine dross material, selected on the basis of their respective AIN and Al concentrations, for the preparation of a substantially homogenous aluminum deoxidizer or conditioner suitable for steel foundry applications, among others.
The NMP dross residue derived from the salt-free aluminum dross treatment method is a particularly suitable material for the purposes of this invention because it is available in suitable grain-size distributions or can be easily milled to suitable granulometry. Also, the fines that are invariably found in aluminum dross are also usable either as is or in combination with NMP dross residue.
The residue from plasma dross treatment, as described in ref 8, characteristically has aluminum nitride (AIN) concentrations higher than or equal to those of metallic aluminium.
For the aforementioned reasons, this greatly limits the potential use of NOVALTM product in steel metallurgy without high-temperature pre-treatment to oxidize the nitrides, at the risk of losing their aluminum content. Moreover, NOVALTM is highly hydro-reactive, which generates toxic, flammable gases on contact with water, and further complicates its transportation and use.
That is why the large majority of NOVALTM applications rely on high-temperature pre-methoding to eliminate its Al and AIN contents, transformed into alumina through oxidation, while recovering the energy value of these oxidizing reactions.
However, the distribution of AIN concentrations in NOVALTM particles has been found to vary greatly and unexpectedly according to particle size, a characteristic that would allow upgrading a significant proportion of the NOVALTM particles while minimizing the problems associated with high AIN concentrations. As opposed to other constituents found in NOVALTM particles, such as iron and silicon oxides, and/or carbon, AIN
concentrations increase considerably as particle size decreases. Moreover, AIN concentration increases in the non-metallic phase according to the number and intensity of mechanical milling operations, for a comparable particle-size distribution.
Also, nitrogen concentrations have been found to vary inversely to that of metallic aluminum, which is surprising initially, assuming that the all of the nitrogen in NOVALTM is in the form of aluminium nitride (AIN).
Table 1 shows the varying concentrations of several NOVALTM constituents, according to particle size of the material. The AIN/AI concentration ratio rapidly increases as particle size decreases. Therefore, it would appear possible to derive aluminum-rich concentrate by fractioning a NMP blend to about 30-60 mesh, thus yielding a considerable proportion of the initial mass, as characterized by an AIN/AI ratio below one.
Segregation effects in the concentrations of various minor phases according to particle size are well known in the mining industry. What is surprising and unexpected here is that aluminum nitride contents increase inversely to those of other constituents such as SiO2, Fe2O3, as well as that of aluminum.
Without wishing to be bound by theories or claiming to fully understand the overall physico-chemical mechanisms involved, it is possible to formulate a hypothesis to explain the behaviour of aluminum nitride by considering it as an unstable intermediate species involved in a progressive aluminum to aluminum oxide conversion reaction in contact with

- 6 -air. Indeed, X-ray diffraction analyses of NOVALTM samples frequently reveal the presence of aluminum oxynitride diluted in excess amounts of alumina. In these conditions, nitrogen cannot be exclusively associated with the presence of aluminum, in AIN form, but shared with that of alumina as A1803N6 in a progressive oxidation reaction. This could explain the increase in nitrogen contents in the finest NOVALTM particles, the alumina content of which increases with a decrease in metallic Al content, as shown in Table 1. This is also compatible with the observation that the nitride content increases with the mechanical intensity of NMP particles comminution, as illustrated in TABLE 1.

.................................................................
TYPICAL NMP CHEMICAL COMPOSITION vs PARTICLE SIZE
Size Mesh %AIN %Al AIN/AI %A1203* %Si02 %Fe203 Size mm 10-20 6 45 0.13 33 2.5 0.41 2 -0.85 20-30 14 32 0.44 38 2.4 0.40 0.85-0.60 30-60 18 15 1.2 52 2.2 0.33 0.60-0.25 60-120 19 10 1.9 56 2.2 0.38 0.25-0.125 120-325 20 6 3.3 58 2.0 0.35 0.125-0.045 <325 22 3 7.3 60 1.9 0.33 <0.045 -- AVERAGE 18 14 1.3 56 2.2 0.36 * MgO = 13 %.
In a particular embodiment, the deoxidizer is prepared from the coarse end of the particle-size distribution, with typical particle sizes varying between about 3 and 0.25 mm, which results in a product of much lower hydro-reactivity compared to the entire particle-size distribution of the original NMP or fine dross material. This significantly facilitates their handling and transportation.
For example, the water reactivity of two NMP samples of the same chemical composition, but of different particle size was compared, as shown in TABLE 2:

- 7 -Water reactivity in L of gas/kg/hr measured on two size fractions of the same NMP, according to Note 1.
Size range coo Volume of gas.
2 mm X 0.50 mm 1.1 mm 0.3 - 0.5 L/kg/hr 2 mm X 0.075 mm 0.180 mm 5 - 6 L/kg/hr Note 1: Recommendations on the transport of dangerous goods. Manual of tests and criteria. Sixth revised edition. United Nations, NY and Geneva, 2015. ISBN 978-139155-8.
In accordance with a particular embodiment of the method of the invention, there is provided a method to produce a low-cost aluminum deoxidizer from aluminum dross fines and/or NMP dross residue, for use as a flux in the manufacture of steel, comprising the steps of:
a) separating on a size basis, a primary non-metallic product (NMP) dross residue and/or aluminum dross fines into particles (S) smaller than or equal to about 10 mm and particles larger (L) than about 10 mm;
b) classifying said smaller particles (S) into multiple size fractions (Fs) according to respective AIN and Al concentrations;
c) choosing and blending said multiple size fractions (Fs) to form a blended product having an AIN to Al % w/w ratio smaller than about 1;
d) optionally, adding a predetermined quantity of aluminum to the blended product to reach the final product composition to maximize Al content while minimizing AIN to Al %
w/w ratio of less than one.
According to an alternative embodiment, the method as defined herein further comprises the steps of:
a') comminuting said larger particles (L) to form a secondary smaller particles (SS) of NMP residue and/or fine dross powdery material and an aluminum-enriched coarser fraction;

- 8 ¨

a") separating said secondary fractions (SS) from said aluminum enriched coarser fraction;
b') classifying said secondary smaller particles (SS) into multiples fractions (Fss) according to respective AIN and Al concentrations;
c') mixing a quantity of Fs and a quantity of Fss, as per their respective AIN
and Al concentrations to achieve a desired final AIN to Al % w/w ratio lower than about one, thus forming a blended product; and d') optionally, adding a predetermined quantity of Al to said blended product to reach a final product composition of having a AIN to Al ratio lower than about 0.75.
Particle size According to a particular embodiment of the method, the smaller particles (S) of step a) or a') are sized from about 10 mm to about 0.1 mm; more particularly from about 5 mm to about 0.1 mm; most particularly from about 3 mm to about 0.25 mm.
Size fractions According to a particular embodiment of the method, particularly in step b) or b'), the smaller particles (S) are classified into fractions corresponding size distribution of: about 3 to about 0.85 mm (Fs1); from about 0.85 to about 0.60 mm (Fs2); and from about 0-.60 mm to about 0.25 mm (Fs3).
AIN/AI ratios According to a particular embodiment of the process, particularly in step c) or c'), the fractions are combined to achieve a % weight ratio of AIN / Al smaller than about 0.75;
more particularly the AIN/AI ratio is smaller than about 0.5; most particularly the AIN/AI ratio is smaller than about 0.25; still most particularly the AIN/AI ratio is smaller than about 0.1.
Method According to a particular embodiment, the method is carried out exclusively with non-metallic particles (NMP) dross residue.
According to a particular embodiment, the method is carried out on a continuous basis or batch-wise. In a preferred embodiment, the described method is conducted continuously.

- 9 -Example Figure 1 shows a particular embodiment of the present invention using the NMP
dross residue obtained from a salt-free dross recycling furnace. The NMP material (2), similar to the NOVALTM product described in REF 8, is removed from the salt-free dross treatment furnace (1) and sent by conveyer, after cooling, to be separated into two fractions using a sieve (3) or any other suitable classification system known to experts in the field. The fine fraction (4), in the 2-3 mm range, is called Fp, while the coarser fraction (5) is usually milled in a semi-autogeneous mill (6), or any other similar communition system such as described in US Patent No. 6,199,779 B1, in order to concentrate the aluminum into an enriched fraction (7) for further methoding while a secondary stream (8), of particle size distribution similar to that of the Fp stream, is derived and called the secondary stream Fs.
The primary (4) and secondary (8) NOVALTM material sample streams are then divided into two separate granulometric fractions, as previously determined according to the aluminum nitride content variation versus the particle-size distribution of each stream. This separation is done using either mechanical screening (9,10) or other suitable splitting devices on a size separation basis.
Then, a suitable quantity of the coarser fraction of each stream (11,12), determined according to the final composition of the desired product in terms of Al and AIN, is blended in a rotary mixer (13), or other mixing apparatus known to experts in the field, until a product of homogeneous composition has been achieved.
Optionally, and if necessary, any other product of defined composition (15) such as aluminum material can be added to the final mixture in order to obtain the desired formulation, or if quantities in the NMP and/or dross material are not sufficient.
The finer, AIN-rich residual fractions (16,17) are subsequently used, individually or as a blend, as base material for high-temperature applications where their high energy contents provide a cost-effective benefit, as illustrated in Ref 9.
The present invention has been described in terms of particular embodiments found or proposed by the present inventor to comprise preferred modes for the practice of the invention. It will be appreciated by those of skill in the art that, in light of the present disclosure, numerous modifications and changes can be made in the particular

- 10 -embodiments exemplified without departing from the intended scope of the invention. All such modifications are intended to be included within the scope of the appended claims.
References 1. Use of aluminum dross for slag treatment in secondary steel making to decrease the amount of reducible oxides in ladle furnace. A thesis submitted to the graduate school of natural and applied science of Middle East Technical University, by Onur Aydemir, January 2007.
2. R. Reddy et al. Advances in Molten Slags, Fluxes and Salts. Proceedings of the 10th International Conference on Molten Slags, Fluxes and Salts, 2016. Springer, 10 January 2017.
3. U.S. Patent No. 5,873,924, Kinsman et al. Desulfurizing mix and method for desulfurizing molten iron, 1999.
4. Li, Y.-L., et al. Development of ladle slag reducer using aluminum dross.
Iron and Steel, Volume 49, Issue 3, March 2014, pp 17-23.
5. U.S. Patent No. 6,199,779 B1, March 13, 2001. J. Mosher.
6. U.S. Patent No. 5,421,850, June 6, 1995. G. Dube et al.
7. N. Unlu et al. "Comparison of salt-free aluminum dross treatment methodes".
Resource, Conservation and Recycling, Elsevier, 36, 2002, pp. 61-72.
8. R. Breault et al. "Aluminum plasma dross treatment method and calcium aluminate production:
Closing the loop with no residue" Fourth International Symposium on Recycling of Metals and Engineered Materials, October 22-25, 2000, pp. 1,183-1,194.
9. R. Breault et al. "Market Opportunities for the Alcan Plasma Dross Residues." Light Metals, 1995, J. Evans Ed., The Minerals, Metals and Materials Society. 1995.

- 11 -

Claims (12)

1. A method to produce a low-cost aluminum deoxidizer from aluminum dross fines and/or NMP dross residue, for use as a flux in the manufacture of steel, comprising the steps of:
a) separating on a size basis, a primary non-metallic product (NMP) dross residue and/or aluminum dross fines into particles (S) smaller than or equal to about 10 mm and particles larger (L) than about 10 mm;
b) classifying said smaller particles (S) into multiple size fractions (Fs) according to respective AIN and Al concentrations;
c) choosing and blending said multiple size fractions (Fs) to form a blended product having an AIN to Al % w/w ratio smaller than about 1;
d) optionally, adding a predetermined quantity of aluminum to the blended product to reach the final product composition to maximize Al content while minimizing AIN to Al %
w/w ratio of less than one.

2. The method of claim 1, further comprising the steps of:
a') comminuting said larger particles (L) to form a secondary smaller particles (SS) of NMP residue and/or fine dross powdery material and an aluminum-enriched coarser fraction;
a") separating said secondary fractions (SS) from said aluminum enriched coarser fraction;
b') classifying said secondary smaller particles (SS) into multiples fractions (Fss) according to respective AIN and Al concentrations;
c') mixing a quantity of Fs and a quantity of Fss, as per their respective AIN
and ASI
concentrations to achieve a desired final AIN to Al % w/w ratio lower than about one, thus forming a blended product; and d') optionally, adding a predetermined quantity of Al to said blended product to reach a final product composition of having a AIN to Al ratio lower than about 0.75.

3. The method of claim 1 or 2, wherein in step a) or a'), said smaller particles (S) are sized from 5 mm to about 0.1 mm.

4. The method of claim 1 or 2, wherein in step a) or a'), said smaller particles (S) are sized from 3 mm to about 0.25 mm.
- 12 ¨

5. The method of claim 4, wherein in step b) or b'), said smaller particles (S) are classified into fractions corresponding size distribution of: about 3 to about 0.85 mm (Fs1);
from about 0.85 to about 0.60 mm (Fs2); and from about 0-.60 mm to about 0.25 mm (Fs3).

6. The method of claim 5, wherein in step c) or c'), said fractions are combined to achieve a % weight ratio of AIN / Al smaller than about 0.75.

7. The method of claim 5, wherein in step c) or c'), said fractions are combined to achieve a % weight ratio of AIN / Al smaller than about 0.5.

8. The method of claim 5, wherein in step c) or c'), said fractions are combined to achieve a % weight ratio of AIN / Al smaller than about 0.25.

9. The method of claim 5, wherein in step c) or c'), said fractions are combined to achieve a % weight ratio of AIN / Al smaller than about 0.1.

10. The method of any one of claims 1 to 9, carried out exclusively with non-metallic particles (NMP) dross residue.

11. The method of any one of claims 1 to 10, wherein said method is carried out on a continuous basis or batch-wise.

12. The method of claim 11, carried out continuously.
- 13 ¨