WO2012049696A1

WO2012049696A1 - Process and reactor for the plasma transformation of powdery by-products of bauxite processing into a solid, inert and compact product

Info

Publication number: WO2012049696A1
Application number: PCT/IT2011/000343
Authority: WO
Inventors: Aldo Imerito
Original assignee: Ecotec Gestione Impianti S.R.L.
Priority date: 2010-10-08
Filing date: 2011-10-06
Publication date: 2012-04-19
Also published as: ITRM20100523A1; IT1402152B1; EP2625141A1

Abstract

Subject matter of the present invention are a process and a plasma reactor for transformation of red muds, which is a bauxite processing by-product, starting from powdery matrix into a solid, inert and compact residual product, with a remarkably reduced volume, permitting increasing life cycle of dumps using available volumes and from which industrially interesting products can be extracted, e.g. an iron-containing fraction, that can be used in a production cycle of the steel-making sector. A block diagram of the process according to the invention, in one specific embodiment within a transferred arc plasma reactor, is shown in figure 1.

Description

PROCESS AND REACTOR FOR THE PLASMA TRANSFORMATION OF POWDERY BY-PRODUCTS OF BAUXITE PROCESSING INTO A SOLID, INERT AND COMPACT PRODUCT

The present invention relates to the technical field of transformation of by-products of Bayer process for extraction of bauxite for economical and/or ecological exploitation.

As it is well known, most used industrial process to obtain alumina from bauxite is the so-called Bayer process. Steps characterizing the process are:

- grinding bauxite:

- solubilization at a temperature of some tens of Celsius degrees and following separation of not solute products;

- precipitation of AI(OH)3 by lowering temperature;

- calcination.

Different compositions and percentages of Al, Ti, Fe and Si oxides are present in bauxite. Extraction efficiency if very low and this causes presence of large amounts of metallic elements into treatment byproducts, generically known as red muds.

During the treatment, different iron hydroxides present in bauxite are dehydrated thus making a solid solution of Fe₂03 (containing about 10% of AI2O3), said solution then crystallizing and creating main constituent of red muds, the typical composition of which is the following:

pH 12.5

Fe₂0₃ 30 + 60%

AI2O3 11 + 20%

Si0₂ 2.5 + 14%

Ti0₂ traces + 11%

CaO traces ÷ 11%

Na₂0 1.5 + 8%

Red muds are diluted and sent to a pressure filter, wherein some components are recovered: then as sewages, they are pumped away from the system to be discharged within artificial ponds. This technique has a relevant environmental impact since these secondary products are not efficiently disposed and presently have no industrial application. Thus, red muds are really dangerous wastes, the treatment of which is still a serious problem. Although red muds are presently handled in such a way to reduce their impact at the minimum level, they still represent a great hazard for human health; furthermore, some sites still are influenced by inheritance of past wrong managing. Surface deposits wherein red muds are stocked must be built and managed with particular attention to prevent contamination of stratum below and of surrounding grounds, and to prevent that powder material is dispersed into air, thus causing harmful effect for health; in fact, said dusts have a strongly alkaline composition and cause irritant effects for skin, eyes and respiratory apparatus.

Only in the last years studies and experiments have been promoted for a suitable treatment of said wastes. In some cases, high content of aluminum within material has brought to modify production cycle in order to reduce their alkaline charge, so as to obtain an inert mud that can be used to fill in exhausted mines, as substrate for re-implantation of original plants or for other agricultural objects, or as landfill material for coastal zones. Finally, it has been tested the use of red muds for producing building materials.

Usually, pyro-metallurgical treatments, employed for transformation of dangerous wastes, fusion and/or concentration of metallic minerals, or of solid matrixes containing metals, exploits high temperatures and complex industrial systems in order to obtain metal at a set purity grade or metallic oxides with a lower oxidation state.

In order to overcome the transition regime, which is necessary to bring treatment system at a regime state and maintaining regime temperature, very large amount of power is necessary, and often, amount of recovered material is low, thus making the process economically not convenient.

Not reduced fraction of post treatment mineral is a cinder containing inside remarkable amounts of metals, such as iron, titanium and aluminum, as well as magnesium and silicium.

Red muds are considered a very dangerous waste, the treatment and/or disposal of which is a huge cost for modern society.

Thus, the needing exist in this specific technical field of transforming red muds by a more advantageous process both under the economical and ecological point of view. The above needing is satisfied by the process according to the present invention, further giving advantages that will be well evident in the following.

It is therefore specific subject of the present invention a process wherein powdery by-products from processing of bauxite (red muds) are transformed, subjecting them at a plasma generated by an inert gas, into an inert, compact and solid product, considerably reduced in volume, with fractions of industrial interest, said process comprising the following operations::

- optional drying of red mud at a temperature in the range between 60 and 250°C;

- optional adding of an agent for increasing the electrical conductivity;

- optional adding of a fluidizing agent for modifying thermodynamic features of the charge to be subjected to treatment, e.g. calcium fluoride;

- feeding into an arc plasma reactor;

- use of an inert gas for generation of plasma, said inert gas being chosen from the group comprising nitrogen and/or argon;

- optional creating of reducing conditions within the reactor; environment;

- treating with the following operational conditions into plasma arc reactor;

- potential difference between the electrodes comprised within the range 10 - 10.000 volt;

- temperature comprised within the range 600 - 3500°C within the reaction chamber;

- obtaining the above solid, inert and compact product, considerably reduced in volume, having an aspect similar to a volcanic slag comprising a glass matrix having inside crystalline micro regions consisting of metals contained in the product.

Particularly, according to the invention, drying is carried out at a temperature comprised in the range 100-1 15°C.

Always according to the invention, agent for increasing the electrical conductivity is selected from the group comprising water and saline aqueous solutions. Furthermore, according to the invention, creation of reducing conditions within the reactor can be obtained introducing at least a reducing gas chosen from the group comprising methane, propane, hydrogen and mixtures thereof.

Technology adopted for generation of plasma exploits transferred arc configuration, wherein anode and/or cathode can consist of a bar, of a plate, of a ring or of a plaque within the reactor.

The invention further concerns a reactor with the crucible in refractory, metallic or ceramic material, or in any case resistant to elevated temperatures, suitable for the plasma transformation of powdery byproducts coming from bauxite processing (red mud) in a solid, inert and compact residue as per the process described in the above, comprising the following parts:

- means for drying the material to be treated;

- means for feeding the material to be treated and the optional additives into the reactor;

- anode and cathode;

- means for inducing the discharge between anode and cathode, through direct or alternating current;

- optional means for giving the anode and/or the cathode a vertical, horizontal or rotatory motion within the reactor;

- means for creating reducing conditions within the reactor.

Particularly, according to the invention, in the reactor with the crucible in refractory metallic material, the anode and/or the cathode consists at least in part of the reactor bottom or of the reactor itself.

Still according to the invention, in the reactor with the crucible in ceramic refractory material, the anode or the cathode consists in a conductive material in the form of a bar, of a plaque or of a ring.

Furthermore, according to the invention, the cathode and/or the anode are made up of a conductive material, preferably stainless steel or graphite, in form of bar optionally perforated for allowing the flowing of at least a gas inside of it.

The invention further concerns to a solid, inert and compact product, obtainable by the above process, having an aspect similar to a volcanic slag comprising a glass matrix having inside crystalline micro regions consisting of metals contained in the product. It is further subject of the present invention the use in iron metallurgy, in aluminum metallurgy and in titanium metallurgy of iron-, aluminum- and titanium- containing fractions extracted from the solid, inert and compact product, considerably reduced in volume, obtainable from the above process; for example iron-containing fraction extracted from the above product can be advantageously used in the steel-making sector.

Adoption of the solution according to the present invention gives the following advantages:

- treatment by plasma reactor permits transforming what has been until now a very hazardous waste and the disposal or treatment of which was a very high cost for society, into a solid, inert and compact, not leached slag, with reduced volume and weight, from which it is possible extracting from treatment a series of products having an industrial interest, among which iron;

- plasma treatment starts acting immediately with respect to the other pyro-metallurgical systems, thus reducing transition regime time and permitting a remarkable energy saving;

red mud extraction from ferrous fraction and contemporaneous transformation of the same from diamagnetic fraction into paramagnetic fraction; paramagnetic phase can be easily individuated by subjecting the obtained product, finely ground, to a magnetic field.

Said iron paramagnetic product is a valuable commercial matter for metallurgic industry. Extraction of said iron compounds can be carried out by every one of the known mining and metallurgic techniques:

- red mud matrix is transformed into a compact, not powdery, matrix;

- a weight and volume reduction with respect to the material not subjected to treatment is obtained, mainly due to separation of water during the process; particularly, process permits reducing water contents of more than 40% in weight; transformation of matrix and its volume and weight reduction permit more rational use of dump basin, thus permitting longer cultivation time and a more easy handling of diffused powder emissions.

Plasma arc directly acts since from first ignition and reaction zone almost instantaneously reaches operative temperature. Plasma arc directly acts on material to be subjected to treatment, thus giving a product resembling volcanic cinder, comprising a glass matrix having inside crystalline micro regions, consisting of metals presents within the product, creating aggregates similar to a metallic alloy. Matrix to be subjected to the treatment is subjected to the process that, by temperature, electrical field applied and reducing environment, transforms diamagnetic iron compounds into paramagnetic iron compounds, red mud matrix into a compact matrix, which is no more powdery, into a basaltic-type residual, resembling a volcanic slag with reduced volumetric dimensions.

In the above, a general description of the invention has been given. A more detailed description will be provided by the help of figures and examples, aimed to better understand its objects, features and advantages.

Figure 1 represents a block diagram of an embodiment of the process for treatment of red mud according to the invention, within a transferred arc plasma reactor.

Figure 2 shows an image of red mud before treatment obtained by FEG-SEM (Field Emission Gun - Scanning Electron Microscopy) microscopy.

Figure 3 shows image of the obtained product, by FEG-SEM microscopy.

In order to analyze effects of treatment in the following examples, different analyses techniques have been used, such as X ray diffractometry and FEG-SEM microscopy, using a device permitting obtaining a tridimensional image of the sample employing electrons instead of light and permitting obtaining magnifications up to 1 ,000,000x. FEG-SEM (Field Emission Gun - Scanning Electron Microscopy) microscopy is a microscopy technique employing an electron beam carrying out a scanning of the sample to be examined and exploiting field emission to identify some peculiar features such as qualitative and quantitative composition, crystallization state, conductivity and so on.

Image of a sample before and after the treatment by the reactor according to the invention is shown in figures 2 and 3, wherein some operative and measurements parameters are put into evidence: "HV" represents tension of the field applied between sample-holder current and beam emitting electrode: "curr" represents electronic current of the scanning beam; "mag" is magnification with respect to real dimensions; "det" is the detector employed; "WD" represents system visual field width or extension; "HFW" represents treatment field extension. Example 1

Red muds have been employed in the test, the composition of which is the following:

Al₂0₃ 20%

Fe₂0₃ 28%

Ti0₂ 7%

Si0₂ 12%

Hydration H₂0 (%) 25%

CaO 2%

Na₂0 6%

400 g of sample have been dried at a temperature of 110°C; then, they have been inserted within refractory metal reactor. Reactor employed is made of AISI 316 stainless steel. In this configuration, cathode consists of a metallic bar at about 1 centimeter from anode, while the latter is positioned at the bottom of the reactor body and consists of a steel disc acting as electrode. Cathode is moved at a uniform rotator speed of rotation at about 1 centimeter from wall reactor; thus, besides acting as electrode, it also mixes sample to be subjected to treatment.

Nitrogen has been used as plasmogenic gas. After having filled in the reactor chamber with nitrogen, its flow has been adjusted for generation of plasma until reaching about 15 slm (standard liters by minute). A potential difference of 1000 volts under alternate current about has been applied between electrodes for 10 minutes. Potential difference application causes discharges, main effect of which is bringing iron compounds present in sample to reduction, and to obtain a volcanic slag aspect.

Example 2

Red muds have been employed in this test, the composition of which is the following:

AI2O3 20%

Fe₂0₃ 28%

Ti0₂ 7%

Si0₂ 12%

Hydration H₂0 (%) 25%

CaO 2%

Na₂O 6%

Sample before treatment is shown in figure 2. Sample is mixed with another substance and/or mixture of substances suitable to give a mix having very high conductivity features; particularly, in this example, water with a saline content of about 3% in weight has been employed.

400 g of sample have been dried at a temperature of 110°C;

400 ml of water have been added to the sample, to which 13 grams of sodium chloride have been added. Mix obtained has been introduced within reactor. Reactor consists of a ceramics crucible, having inside two tungsten bars constituting of cathode and anode. During the treatment, cathode is kept fixed at a distance of about 1 centimeter from anode. System can be supplied either by direct or alternate current, with a variability of tension between 10 volts and 10000 volts, and it can provide or not a sample mixing system. Direct current has been used in this example, and a potential difference of 300 volts for 10 minutes has been applied. Argon has been employed as plasmogenic gas. After having filled in the reactor chamber with argon, its flow has been adjusted for generation of plasma until reaching about 30 slm (standard liters by minute). Treatment time is reduced in presence of reducing gas, such methane, introduced within argon atmosphere.

Stable spheroid-shaped discharges are created within the reactor, main effect of which is bringing metallic compounds at reduction, particularly iron, aluminum and titanium, which are present within the sample, giving to the matrix the aspect of a volcanic cinder, figure 3.

Energy developed by plasma arc permits reaching such temperature at which sample is melt, with consequent stratification of components of the same sample; particularly, fraction containing iron concentrates in the lower layers of melt material, and, rising within the reactor, stratification of fraction containing titanium, of fraction containing aluminum and fraction containing silicium. Transformation of matrix is evidenced from FEG-SEM analysis. By comparing figures 2 and 3, it is well evident transformation of sample.

Summarizing, results of tele-microscopy analyses indicates that process has given, in both tests, a product, from which iron, aluminum and titanium can be then extracted by known methods, and the average composition of which is the following:

O 15.35%

Na 11.13% Mg 3.60%

Al 25.87%

Si 12.51 %

S 3.92%

CI 1.76%

Ca 8.04%

Ti 7.10%

Fe 10.71 %

Present invention has been described for illustrative, but not limitative, purposes according to its preferred embodiments, but it is to be understood that variations and/or modifications can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.

Claims

1. A process for the transformation of powdery by-products from the bauxite processing (red mud), by a plasma generated by an inert gas into a solid, inert and compact product considerably reduced in volume, with fractions of industrial interest, comprising the following operations:

- optional adding of an agent for increasing the electrical conductivity;

- optional adding of a fluidizing agent for modifying thermodynamic features of the charge to be subjected to treatment;

- feeding into an arc plasma reactor;

- optional creating of reducing conditions;

- treating with the following operational conditions into plasma arc reactor;

- obtaining the solid, inert and compact product, considerably reduced in volume, having an aspect similar to a volcanic slag comprising a glass matrix having inside crystalline micro regions consisting of metals contained in the product.

2. The process according to claim 1 , wherein drying is carried out at a temperature comprised in the range 100-115°C.

3. The process according to claim 1 or 2, wherein the agent for increasing the electrical conductivity is selected from the group comprising water and saline aqueous solutions.

4. The process according to any of the preceding claims, wherein the creation of reducing conditions within the reactor is obtained introducing at least a reducing gas chosen from the group comprising methane, propane, hydrogen and mixtures thereof.

5. The process according to any of the preceding claims, wherein the plasma has the transferred arc configuration, and anode and/or cathode consist of a bar, of a plate, of a ring or of a plaque within the reactor.

6. A reactor with the crucible in refractory, metallic or ceramic material, or in any case resistant to elevated temperatures, suitable for the plasma transformation of powdery by-products coming from bauxite processing (red mud) into a solid, inert and compact residue as per the process of claims 1 - 5, comprising the following parts:

- means for drying the material to be treated;

- anode and cathode;

- means for creating reducing conditions within the reactor.

7. The reactor with the crucible in refractory metallic material according to claim 6, wherein the anode or the cathode consists at least in part of the reactor bottom or of the reactor itself.

8. The reactor with the crucible in refractory ceramic material, or in any case resistant to high temperatures, according to claim 6, wherein the anode or the cathode consists in a conductive material in the form of a bar, of a plaque or of a ring.

9. The reactor according to any of preceding claims 6 - 8, wherein the cathode and/or the anode are made up of a conductive material, preferably stainless steel or graphite, in form of bar optionally perforated for allowing the flowing of at least a gas inside of it.

10. Solid, inert and compact product, characterized in that it is obtainable by the process according to any of the claims 1 - 5 and in that it has an aspect similar to a volcanic slag comprising a glass matrix having inside crystalline micro regions consisting of metals contained in the product.

11. Use of iron-containing fraction extracted from the solid, inert and compact product, considerably reduced in volume, obtainable from the process according to any of the claims 1 - 5, in the iron-metallurgy field.

12. Use of fractions of industrial interest extracted from solid, inert and compact product, considerably reduced in volume obtainable from process according to any of claims 1 - 5, in aluminum metallurgy field or in titanium metallurgy field.