SALTS

Technical field of the invention

The invention relates to a process for processing red mud, and producing rare-earth metal salts.

Background of the invention

The current global aluminum production is based on the Bayer process patented 1892, in which the alumina content is extracted from the sedimentary bauxite-mineral, named bauxite.

The principle of the method is that the grounded bauxite is digested by caustic (NaOH) alkaline cooking, and the alumina (aluminum oxide), aluminum oxide hydrates, gallium is recovered from the resulting slurry.

The residual alkaline slurry is called red mud. This slurry is washed in order to reduce the alkalinity, and to recover the sodium hydroxide content, sometimes dried and collected in red mud ponds.

These red mud ponds are hazardous waste sites, because of the corrosive effect of red mud due to its residual alkalinity, on the other hand, because the residue of the very fine-grained (clay) bauxite is also fine, and very dusty. This dusting out is typically very harmful, belonging to the category of PM 10 particles causing silicosis, asthma, lung cancer, and to the category of PM 2.5 particles causing cardiovascular diseases, and accumulating in the body. The airborne particles are the air-distributed fine-grained dust of less than 10 μιη (particulate matter, PM), the classification of which is performed based on the particle size, where categories PM 10 and PM 2.5, respectively, correspond to particle sizes less than 10 μπι, and 2.5 μπι.

Furthermore, the metal (hydroxide) and caustic soda residual alkalinity content of the red mud slurry, together with the leachate, infiltrate into the soil and the drinking water/natural water bases, and contaminates them.

For these reasons, it would be necessary to fully eliminate the red mud ponds. However, to date no profitable solution has been found that would make this, and in such a large volume possible, therefore billions of tons of red mud is deposited in the world, occasionally having spilled out, thereby causing industrial disasters.

This is despite the fact that the red mud contains a lot of important industrial raw material metals. Nearly half of it is iron oxide, it has high content of titanium dioxide, but it contains very important rare-earth metals, scandium, yttrium and gallium as well. The composition of an average, in terms of its useful material content, rather poor quality red mud waste [after the extraction of gallium (Ga)], based on the common measurements of MAL Hungarian Aluminum Production and Trade Company and MTA/VE Hungarian Academy of Sciences/University of Veszprem is as follows:

The composition of red mud (MAL and MTA/VE):

- Fe ₂ 0 ₃ (iron oxide) 40-45 % by weight, this gives the colour of the red mud; MTA/VE 37 % by weight;

- A1 ₂ 0 ₃ (aluminum oxide) 10-15 % by weight MTA/VE 14.3 % by weight;

- Si0 ₂ (silicon dioxide) 10-15 % by weight; present as sodium- or calcium-alumina-silicate, MTA VE 20 % by weight

- CaO (calcium oxide) 6-10 % by weight MTA/VE 7.7 % by weight

- Ti0 ₂ (titanium dioxide) 4-5 % by weight MTA/VE 3.8 % by weight

- Na ₂ 0 (bound soda) 5-6 % by weight MTA/VE 4.8 % by weight

- MgO MTA/VE 0.53 % by weight

- other materials, such as rare-earth metals: 1 ton of red mud contains 1 to 3 kg of rare-earth metals, and scandium Sc, yttrium Y, and if it is not obtained during the production of oxide hydrate, about 0.1 kg/ton of Ga gallium as well.

The amount of rare-earth metals and other metals in red mud are usually as follows:

20 to 30 ppm of gallium, 55 ppm of scandium, 120 ppm of yttrium, 240 ppm of lanthanum, 450 ppm of cerium, 10 ppm of praseodymium, 190 ppm of neodymium, 23 ppm of samarium, 160 ppm of gadolinium, 40 ppm of molybdenum, 410 ppm of zinc, 500-550 ppm of chromium, 55 ppm of cobalt, 42 ppm of uranium, 50 ppm of thorium, 1950 ppm of manganese.

The problem is that the concentration of each type of useful material (for all types of the components) is low as compared to the concentration of ores directly mined for the production of this material types. Therefore, the processing of red mud uneconomical.

According to the state of the art, a number of attempts have been made for the utilization of red mud, and for the recovery of the useful metals of red mud.

Typical red mud elimination method was the use of the red mud as soil conditioner material, for example by improving alkaline soils, but this method has not spread.

Taking advantage of the clay mineral nature of red mud, experimentation is going on today for the use as the construction material, both in the cement industry, and in the utilization as hous- ing block (DY Liu et al.: Materials 5, 1232-1246 (2012). H. Gu et al.: Waste Manag Res 30 (9), 961-965 (2012); W. Liu et al.: J. Hazardous Materials, 161 (1), 474-478 (2009)). There were initial success, but for example, the Australian laws prohibited the housing-purpose use of the relatively high radioactivity housing blocks. It does not seem to spread out a bit far-fetched either option, the red mud application for road repair. In order to utilize the metal materials of the red mud, the researchers mostly concentrated, and recently still focus on obtaining the iron (oxide) (MS Rukhlyadeva et al.: Inorganic Synthesis and Industrial Inorganic Chemistry (Russian J. of Applied Chemistry) 88 (3), 377-381 (2015). Jayassankar K. et al.: Int J. Minerals, Metallurgy and Materials, 19 (8), 679-684 (2012)).

It is a rather hard approach, because after obtaining the iron oxide content of the red mud, it should be competitive in the market, both in terms of price, and of the metallurgical quality with the iron ores, which fact already questions the economics of the idea, and thus of viability on industrial scale.

Apart from the market competitiveness with iron ores, successful attempts have been made (for example, in the Alumina Industry and Steel Research Institute of the Hungarian Academy of Sciences) for recovering the iron (oxide) content of the red mud, such that the red mud slurry was heated with charcoal, and the Fe ₂ 0 ₃ hematite ferric oxide content of the slurry, by using the resulting carbon monoxide, was reduced to Fe ₃ 0 magnetite ferrous ferric oxide. Then, the magnetite was taken out from the system using a magnetic separator, and after being pelleted, it was recy- cled in the metallurgy (US 9, 199,856 B2, X, Li et al.: Trans Nonferrous Met Soc China 25, 3467- 3474 (2015)).

Others modified the same procedure such that originally a carbon monoxide reducing gas was led onto the red mud, still others hydrogen led onto the red mud (W. Liu et al.: J. Hazardous Materials, 161 (1), 474-478 (2009) again, F. Kauss et al.: Chemie Ingenieur Technik 87 (11), 1535-1542 (2015)).

The Russian Rosintech Res. Inst, solved this problem such that they led the synthesis gas from an aqueous breakdown of natural gas (methane) to the mud (CH ₄ + H ₂ 0 = CO + 3H ₂ ) (Chemosphere 78 (9), 1116-1120 (2010)).

CN 101463420 A publication document discloses a method for comprehensive utilization of red mud. In its first step, the red mud is mixed with charcoal and calcium chloride, and heated for 3 hours at 1100 °C. The GaCl ₃ , TiCl ₄ , and ScCl ₃ components are separated by aqueous washing, and then the residual slurry is separated to magnetic and non-magnetic slurry by magnetic separator. The non-magnetic portion is used for the production of building materials. To this the rare-earth metals are recovered after the addition of sodium carbonate or an aqueous solution of crystalline oxalate by filtration and drying. To obtain the soluble ScCl ₃ GaCl ₃ components, the material is washed, then soaked for 2 hours, filtered, and the scandium is precipitated by the addition of oxalic acid crystals. After filtration, the Ga(OH) ₃ and Ti(OH) ₄ components are precipitated with aqueous ammonia. Then, hydrochloric acid is added, and the material is extracted with P ₂ 0 ₄ reagent. After re-extraction of the acidic solution, it is heated for 3 hours at 800 °C. US 6,248,302 Bl patent document discloses the treatment of red mud for recovering the metal content. According to the method, the iron, aluminum, silicon and titanium components are precipitated by acidic dissolution, heating and washing with water.

In these procedures the expensive reduction gas is an important element of the costs, but besides this, the red mud reduction gas system should also be heated to a temperature of about 500 °C, which requires large amount of natural gas, which, considering the gas prices as compared to the iron ore, makes these pellets uncompetitive in price in the market. (Neither the energy need of pelletization, nor the profitability of the processability of the thus obtained iron oxide in fluidized bed are even considered.)

Another disadvantage of the known processes is that the recovery of the metal content is difficult, and in particular not complete. Primarily, recovering the iron content in the form of iron oxides is problematic, where the residual iron content makes it difficult to recover the other metal components.

The preparation of the rare-earth metals is carried out from mineral ores, wherein the rare- earth metal generally exist as oxides or sulfates. The ores are enriched with gravity, magnetic and electrostatic separation and ore preparation methods, because the concentrations are rarely sufficient for the direct chemical digesting. The chemical digestion is carried out in aqueous alkali or acid. Recently ion exchange and the liquid-liquid extraction processes have widely spread (E. Bour-bos et al.: 1st European Rare-earth Resources Conference, Milos, September 4-7, 2014).

It is also known that in the daily life wastes containing organic materials are in large quantities formed, which, in some cases, can be quite dangerous. The processing of them is generally performed by thermal decomposition carried out at high temperature (pyrolysis), in which synthesis gas with H ₂ +CO composition is produced (EP 2638130 Al, WO 2009/151180 A, US 2011039956 Al).

The problem to be solved

There is need to develop a technique that enables both efficient processing of large amounts of red mud, thus the elimination of the red mud ponds hazardous to the environment, on the other hand, allows the recovering of metals and rare-earth metals in the red mud.

More particularly, there is need to develop a method, which allows the full recovery of iron oxides present in the red mud.

The solution for the problem

It has been found that recovery of iron oxides present in the red mud, is hampered by the fact that the iron oxide particles present in red mud are coated with soluble glass enclosure. This soluble glass (sodium silicate) is formed from the alkali (NaOH) content, and the Si0 ₂ content of the red mud, and surrounds the iron oxide particles in the form of a gel. The resulting soluble glass enclosure inhibits the separation of iron oxide, and thus makes it difficult to complete the processing of the red mud, and the subsequent recovery of the metal content of it.

We have found that the above problem is advantageously solved, if prior to the removal of the iron oxides, the soluble glass enclosure is decomposed by the transformation of soluble glass to sodium carbonate and silicic acid.

In addition, the economics of the process can be substantially improved, if the processing of red mud and simultaneously recovering metals and rare-earth metals present in the red mud, is coupled with waste management process of the wastes with organic content.

Brief description of the invention

An advantage of the process according to the invention is that it allows the complete removal of iron oxides by the breaking down the soluble glass enclosure surrounding the iron oxide particles, and thus the economical utilization of the iron content.

A further advantage of the process according to the invention is the concentrating of the extremely popular and expensive rare-earth metal content of the red mud in such a salt concentrate, which is then marketable and searched raw material for the rare-earth-metal vendor and user companies.

A further advantage of the process according to the invention is that is makes the processing of the red mud economical, so that it synergistically combines the processing of wastes containing organic materials, said materials generated in daily life, and sometimes being very dangerous, with the utilization of red mud waste. As a result of the process according to the invention, all variables of the red mud in ponds can be eliminated by this process, said technology also producing an outstanding high profit simultaneously.

Accordingly, the subject matter of the present invention is a process for recovering the iron oxide content of red mud, said process comprising the step of converting the sodium oxide present in the red mud in the form of soluble glass to sodium carbonate with carbonic acid.

Furthermore, the invention relates to the utilization of red mud, said process comprising the the following steps:

(a) providing red mud waste of alumina production (in the following: red mud);

(b) further, providing waste containing organic material, and converting to synthesis gas by high temperature pyrolysis;

(c) converting the sodium oxide present in the red mud in the form of soluble glass to sodi- um carbonate with carbonic acid; (dl) magnetizing the Fe ₂ 0 ₃ hematite ferric oxide present in the red mud and separating the anti-ferromagnetic Fe ₂ 0 ₃ hematite ferric oxide from the remained slurry by magnetic separator;

(d2) converting the Fe ₂ 0 ₃ hematite ferric oxide present in the red mud to Fe ₃ 0 ₄ magnetite ferrous ferric oxide with synthesis gas, and separating the Fe ₃ 0 ₄ magnetite ferrous ferric oxide from the remained slurry by magnetic separator;

(e) treating the remained slurry obtained in steps (dl) or (d2) with strong acid, thus obtaining metal sulphate solution and Si0 ₂ and Ti0 ₂ suspension.

In one embodiment of the process according to the present invention, the process further comprises the the following steps:

(d3) treating the Fe ₂ 0 ₃ hematite ferric oxide obtained in step (dl) and/or the Fe ₃ 0 ₄ magnetite ferrous ferric oxide obtained in step (d2) with the synthesis gas obtained in step (b), in which pure iron (Fe) is obtained.

In a further embodiment of the process according to the present invention, said process further comprises the steps of:

(d4) iron (Fe) obtained in step (d3) is treated with carbon monoxide gas (CO), in which the

Fe(CO) ₅ iron pentacarbonyl is obtained.

In a further embodiment, the process according to the present invention further comprises the steps of:

(el) the metals and rare-earth metals are recovered from the metal sulphates and rare-earth sulphates in the metal sulphate solution obtained in step (e) in a conventional manner.

In a further embodiment of the process according to the present invention, said process further comprises the steps of:

(e2) the Si0 ₂ and Ti0 ₂ slurry obtained in step (e) is separated by conventional techniques, obtaining pure Si0 ₂ quartz sand, and pure Ti0 ₂ titanium dioxide.

Description of the drawings

Figure 1 shows the various steps of the process and the material flows according to the invention as a block diagram.

Figure 2 shows the material balance, which repeats the above indicating the specific vol- umes on the basis of 1000 tons of red mud input.

Figure 3 shows a schematic arrangement of an apparatus for performing the process according to the invention. Detailed description of the invention

As mentioned above, this invention relates to a process for recovering the iron oxide content of red mud, comprising the step of converting the sodium oxide the red mud in the form of soluble glass (Na ₂ Si0 ₃ ), to sodium carbonate with carbonic acid.

The red mud used in the process according to the present invention is the alkaline slurry resulting from the slurry produced by the alkaline cooking of bauxite, which remains after obtaining the alumina. Preferably the thick red mud collected and deposited in the ponds may be used.

The soluble glass (sodium silicate) is the reaction product of the NaOH sodium hydroxide remaining from the Bayer process and Si0 ₂ silicon dioxide (sand), which usually occurs in the forms of orthosilicate (Na ₄ Si0 ₄ ), metasilicate (Na ₂ Si0 ₃ ), polysilicate ((Na ₂ Si0 ₃ ) _n ) and pyrosilicate (Na ₆ Si ₂ 0 ₇ ). The soluble glass well dissolves in water and alkali, and forms a gelatinous, gel-like material. This gel adheres to the iron oxide particles of the red mud, and highly impairs their gas- accessibility. Moreover, it agglutinates the iron oxide particles with other materials, for which the CaO calcium oxide, MgO magnesium oxide, and the like may be mentioned as examples. To ob- tain the iron oxide, this soluble glass enclosure should be broken down first. The soluble glass immediately decomposes in acidic media. To achieve this, any inorganic or organic acid such as sulfuric acid, phosphoric acid, carbonic acid and the like can be used. Due to its easy applicability, and in order to avoid the introduction of any foreign material into the system, breaking down of the soluble glass coverage surrounding the iron oxide particles present in the red mud, may pref- erably be achieved by using carbonic acid.

In the course of the carbonic acid treatment, the soluble glass surrounding the iron oxide particles glass is converted with H ₂ C0 ₃ carbonic acid to Na ₂ C0 ₃ sodium carbonate and H ₂ Si0 ₃ metasilicic acid (which is easily decomposed into water and Si0 ₂ silica sand). The reaction of sodium silicate and carbonic acid is shown in the following reaction scheme.

Na ₂ Si0 ₃ + H ₂ C0 ₃ = Na ₂ C0 ₃ + H ₂ Si0 ₃

The resulting products are no longer bound to the iron oxide particles, and particles of other materials are not adhered thereto, but are rather a separate part of the remaining slurry.

For the conversion the red mud is heated in a drum-type furnace usually at 200-400 °C, and typically at about 300 °C with the addition of C0 ₂ carbon dioxide, wherein the added C0 ₂ carbon dioxide, as a result the moisture content of the red mud, converts to carbonic acid and effects the breaking down of the soluble glass coating.

Before the carbonic acid treatment, the red mud deposited in the ponds is prepared. In doing so, it is excavated from the ponds using excavators, or other appropriate means, and with a closed- off conveyor belt supplied with dust-free technology, it is forwarded to a production schedules warehouse. Here it is stored out of reach of rain and wind for approximately one week. From this location it is fed into the drum-type furnace in such a manner that the added red mud is screened (in order to weed out any foreign bodies), then it is pre-powdered, in order to facilitate the subsequent operations for the gas permeability by shreding the eventually adhered blocks, lumps off. (In case of an about 200000 t/year production, about 90-150 t/h feedable red mud should be pre- stored, if we take into account the operational limits of the outlined procedure.)

The prepared red mud is fed in the drum -type furnace. The rotating drum -type furnace filled with the prepared red mud is then closed, and at room temperature, while rotation it is washed with an inert gas (preferably nitrogen). The displacement of air is checked using an oxygen analyzer connected to the gas outlet. After the displacement of oxygen (air) (the loss of out- flow at the outlet), the nitrogen is turned off, and the slurry in the drum-type furnace, while rotating the drum -type furnace, is run at a temperature of 100-150 °C in order to remove the remaining water content from the ponds, and to convert the red mud slurry to a dry powder for the additional operations. The drying is controlled by measuring the amount of steam by a device equipped to the gas outlet tube of the furnace, while the temperature is shown by a built-in thermometer and it is checked by the controlled heating of the furnace.

The amount of carbon dioxide to be fed in the drum-type furnace can be estimated on the basis of the Si0 ₂ and Na ₂ 0 content determined by a representative chemical analysis prior to the processing. In practice, the feeding of carbon dioxide in the drum-type furnace is continued until the outflow of water vapor content through the gas composition analyzer of the drum-type furnace is ceased.

Then, the powder-slurry of the red mud so modified is cooled to room temperature, preferably making use of the cooling effect of the carbon dioxide and/or nitrogen flushing.

For the task use of a rotating drum -type furnace is suitable, which is heated by 380V 50Hz industrial electricity, whereas the heating power (the current) is controlled by setting of the de- sired temperature.

After breaking down the soluble glass coverage surrounding the iron oxide particles present in the red mud, the iron oxide content may be practically fully recovered.

The present invention further relates to a process for the utilization of red mud, said process comprising the above-specified steps (a) to (e).

The red mud used in step (a) according to the invention is the residual alkaline slurry resulting from the slurry produced by cooking of the bauxite after the extraction of alumina. As preferred example, the thick red mud collected and deposited in the ponds a may be used.

In step (b) according to the present invention the waste containing organic material used in the production of synthesis gas can be for example waste paper, rice husking residues, straw, hemp, flax, the products of there, and the like. The waste containing cellulose is dry or wet waste, especially waste of about 10 % by weight moisture content.

The waste of the organic content can also be waste containing dioxin and furan, such as waste oil, transformer oil, agricultural chemicals residue, and the like, or other industrial hazard- ous waste, such as bated leather waste, oil sludge, contaminated gas black, tarry waste, plastic and the like.

The composition of the synthesis gas used in the invention is CO+H ₂ , and is generally in 1 : 1 mole ratio CO+H ₂ . The production of synthesis gas is carried out in a conventional manner. In doing so, it is preferred that the waste containing organic matter is heated in furnace, forge or plasma furnace at temperatures above 1000 °C isolated from air, and the resulting synthesis gas is purified in the usual manner and then transferred from tank or for direct use.

In one embodiment according to the present invention, the pyrolysis of the organic material containing waste is performed in a plasma energy pyrolysis system (e.g. PEPS Plasma Energy Pyrolysis System), briefly plasma forge.

The feature of plasma forge is that it is the most suitable for the disposal of hazardous waste (BAT technology). Accordingly, in this case the synthesis gas can be produced from wastes for example containing dioxins and furans (waste oils, transformer oils, agricultural chemicals residues, and the like), or other industrial hazardous wastes (bated leather waste, oil sludges, contaminated gas black, tarry waste, plastics and the like).

If the resulting synthesis gas differs from the 1 : 1 CO+H ₂ composition, the preferred gas composition is adjusted by addition of the missing specific gas. For example, in case of excess carbon monoxide, hydrogen is added from a bottle, a tank or by direct water decomposition, until the 1 : 1 molar ratio is reached, while in case of excess hydrogen, carbon monoxide is added from a bottle, a tank until the 1 : 1 molar ratio is reached.

In step (c) of the process according to the present invention the sodium oxide present in the red mud in the form of soluble glass is converted to sodium carbonate using carbonic acid.

Step (c) of the process according to the invention is preferably carried out as described above.

Step (dl) of the process according to the present invention consists of two parts. In the first part the Fe ₂ 0 ₃ hematite ferric oxide present in the red mud is magnetized, and in the second part the antiferromagnetic Fe ₂ 0 ₃ hematite ferric oxide is separated from the residual slurry using a magnetic separator.

The magnetization is carried out in a conventional manner. For example, the preparation of red mud may be passed through two magnetic separators connected in series, wherein in the first magnetic separator the strong magnetic field will magnetize the Fe ₂ 0 ₃ hematite ferric oxide crys- tallites, and then, the Fe ₂ 0 ₃ hematite ferric oxide crystallites exhibiting in this manner (anti- ferromagnetic properties are separated in the second magnetic separator from the red mud. As a result of this, pure Fe ₂ 0 ₃ hematite ferric oxide and residual powder mud is obtained.

Alternatively, the red mud prepared is magnetized by heating in an induction furnace mag- netizing. Induction furnaces also induce an alternating frequency magnetic field, which is suitable for magnetizing the paramagnetic hematite in the iron oxide Fe ₂ 0 ₃ to (anti-)ferromagnetic state.

Preferred is the use of the induction-heated drum-type furnace, because it next to changing the structure of the red mud, pre-magnetizes the originally paramagnetic Fe ₂ 0 ₃ hematite particles by its induced magnetic field, and this is an important step to bring the hematite particles to (anti- ferromagnetic state .

The pre -magnetized hematite particles may also be magnetized in a magnetic field formed by standard 380V 50 Hz, but ideal is using around nearly 1 kHz alternating current and a magnetic separator having about 1 Tesla magnetic field power.

After the magnetization the powder mud can be led to a magnetic separator, which is also preferably a device operating according to the principle of magnetic induction, established for the separation of ferromagnetic and (anti-)ferromagnetic materials, or in the same device design as that of the device with "magnetizing" function. They are sufficiently high-productivity machines to be able to separate the red mud powder appearing on the output of the drum-type furnaces.

In this magnetization process, wherein the hematite→ (anti-)ferromagnetic hematite con- version is carried out, the iron oxide content is present typically in the form of Fe ₂ 0 ₃ hematite ferric oxide, but in smaller amounts Fe ₃ 0 ₄ in the form of magnetite ferrous ferric oxide can also be present. This does not affect adversely the goodness, efficiency of the magnetic separation process.

In another embodiment of the process according to the present invention the Fe ₂ 0 ₃ hematite ferric oxide crystallites are first pre -magnetized. This can be accomplished for example, by such a manner that the drum-type furnace applied for converting the sodium oxide present in the form of soluble glass in the red mud to sodium carbonate is adjusted to induction heating, wherein the induction field will pre-magnetize the Fe ₂ 0 ₃ hematite ferric oxide crystallites. The magnetization of the pre-magnetized crystallites is then carried out e.g. as described above.

Step (d2) of the process according to the invention also consists of two parts. In the first part the Fe ₂ 0 ₃ hematite ferric oxide present in the red mud is converted to Fe ₃ 0 ₄ magnetite ferrous ferric oxide, which can be added to magnetic separator, and in the second part the magnetic Fe ₃ 0 ₄ ferrous ferric oxide is separated from the remaining slurry by magnetic separator.

The red mud is fed into a specially designed rotary drum-type furnace. Said drum-type fur- nace should be able to ensure the explosion-free use of synthesis gas. The rotating drum-type furnace filled with red mud is closed and flushed with an inert gas (preferably nitrogen) at room temperature with rotation. The displacement of air is checked by an oxygen analyzer connected to the gas outlet. After the displacement of oxygen (air) (the loss of outflow at the outlet) the nitrogen is turned off, and the slurry in the drum-type furnace, while rotating the drum-type furnace is run at a temperature of 100-150 °C, to remove the remaining water content from the ponds, and a dry powder of the red mud slurry be furthered for the next operations. The latter two operations can also be combined. The drying is controlled by measuring the steam content using a device equipped to the gas outlet tube of the furnace, while the temperature is shown by a built-in thermometer, and it is checked by the controlled heating of the furnace.

After sufficient drying, the temperature of the rotating drum-type furnace is raised to a temperature of 450-550 °C necessary for the reduction to magnetite with synthesis gas (because the reaction temperature of the reduction is about 500 °C, however, the hematite loses its anti- ferromagnetism above 600 °C).

Then, the 1 : 1 mole ratio synthesis gas CO+H ₂ is allowed onto the red mud powder system of the rotating and heated drum-type furnace. The synthesis gas is fed in a predefined volume in the system to prevent the over-reduction of iron oxides to FeO/Fe ₂ 0 ₃ in 1 : 1 mole ratio (because the relationship between magnetite and hematite is: Fe ₃ 0 ₄ → FeO x Fe ₂ 0 ₃ ).

The amount to be added is determined based on the prior and representative analytics of the hematite content in the red mud to be processed, and weight ratios of the 2Fe ₂ 0 ₃ + (CO+H ₂ )→ 4FeO + H ₂ C0 ₃ chemical reaction equation. (See our attached block scheme showing also the mass balance).

This reduction heating in the synthesis gas is continued monitoring of the hydrogen analyzer equipped to the gas exit, until the hydrogen is consumed. Then the red mud is cooled back to room temperature (ambient temperature), preferably with rotation during nitrogen purge.

The total processing time of the drum-type furnace operation is about 2 to 3 hours (therefore the calculated production capacity of the drum-type furnace operation for a 200000 t/y red mud production lines is around 90-150 t/h).

For the above process an induction-heated rotating drum-type furnace is preferably used. On one hand, such heaters and the temperatures achieved with them are well controllable on the other hand, the induction furnaces also induce an alternating frequency magnetic field, which is suitable for the magnetization of the paramagnetic Fe ₂ 0 ₃ hematite ferric oxide to anti- ferromagnetic.

Therefore, if the chemical transformation as described above (the preparation of magnetite from the hematite) is not perfect, and remaining Fe ₂ 0 ₃ would be in the system, it is preferably magnetized by the magnetic field, and it allows the separation by the suitable magnetic separator with induced magnetic field (together with the magnetite). By the repeated addition of the powder already having passed through the separator and having magnetically not selected to the separator, the magnetized hematite remains may completely separated from the slurry.

For the implementation of the the task a rotating drum-type furnace is suitable, which is heated by 380V 50Hz industrial electricity, whereas the heating power (the current) is controlled by setting of the desired temperature.

In one embodiment of the invention, the red mud is treated with the synthesis gas produced in step (b) in which the sodium oxide in red mud in the form of soluble glass is is converted to sodium carbonate [step (c)], and simultaneously the Fe ₂ 0 ₃ hematite ferric oxide is converted to Fe ₃ 0 ₄ magnetite ferrous ferric oxide, which can then be fed to magnetic reduction separator [step (d2)] . One principle of the process according to the the invention is that at least the reduction gas, and optionally the gas providing for the energy need of heating up to the reaction temperature is covered by the production of synthesis gas derived from waste.

The separation of Fe ₂ 0 ₃ hematite ferric oxide by the magnetization and then by magnetic separator implemented according to the step (dl) is not always sufficient for the complete removal of Fe ₂ 0 ₃ hematite ferric oxide present in the red mud. In order to remove the residual Fe ₂ 0 ₃ hematite ferric oxide the red mud is is treated according to step (d2) with the synthesis gas obtained in step (b).

In order to implement the treatment with synthesis gas, synthesis gas is passed into the red mud, and it is reduced at elevated temperatures. The reduction is preferably carried out in the manner described above.

The magnetized Fe ₂ 0 ₃ hematite ferric oxide obtained in step (dl) and/or the Fe ₃ 0 ₄ magnetite ferrous ferric oxide obtained in step (d2) is separated from remained slurry, in which pure Fe ₂ 0 ₃ hematite ferric oxide or Fe ₃ 0 ₄ magnetite ferrous ferric oxide is obtained. The separation is preferably performed, for example in a magnetic separator.

In one embodiment of the process of the present invention the resulting Fe ₂ 0 ₃ hematite ferric oxide, or Fe ₃ 0 ₄ magnetite ferrous ferric oxide is treated in the synthesis gas produced in step (b), during which pure, high surface activity, chemically highly active iron Fe is obtained [Step (d3)]. To this drum-type furnace is preferably used, which is heated to 800 to 1000 °C under a stream of synthesis gas.

In a further embodiment of the process according to the present invention, the Fe obtained in the form of high surface activity powder is treated with carbon monoxide gas, (CO), in which the Fe(CO) ₅ iron pentacarbonyl [step (d4)] is obtained.

The method comprises reacting iron Fe with CO carbon monoxide usually at 180 to 200 °C, and at a pressure of 100 to 200 atm. In the process according to the present invention this can be achieved at a temperature of 110 to 140 °C and at a pressure of 10 to 25 atm, since iron powder reduced with hydrogen is used in place of the usual sponge iron, said powder iron exhibiting unique surface and hence chemical activity. In doing so, as CO source the CO component of the synthesis gas can be used.

The residual slurry obtained in step (dl) or (d2) of the process according to the present invention, is treated with a strong acid such as hydrochloric acid or sulfuric acid, in particular with 96% industry strenght aqueous sulfuric acid [step (e)] . In doing so, the metal oxides and rare-earth metal oxides present in the residual slurry are converted to metal sulphates and rare-earth metal sulphates, and the metals and rare-earth metals are recovered from the resulting metal sulphates and rare-earth metal sulfates in a conventional manner [step (el)] .

In one embodiment of the process according to the present invention, the remaining slurry used for the acid treatment is first diluted with water in the ratio by weight from 1 to 2, preferably 1.2 to 1.8, then it is stirred at 20 to 100 °C, preferably 30 to 80 °C for 0.5 for 3 hours, preferably for 1-2 hours. At this point the metal oxides form an alkaline suspension, in which neither of tita- nium dioxide, nor silicon dioxide is dissolved. To the thus treated slurry, portions of a strong acid, particularly 96% industrial strenght sulfuric acid are added under stirring, during which the mixture is brought to a neutral state around of about pH=7. The neutralized slurry is aged, in which the rare-earth metal sulphates remain in solution, and the titanium dioxide, and silicon oxide settles.

The solution containing the rare-earth metal sulphates are separated by e.g. decantation, suction or centrifugation, and the solution is wholly or partly evaporated at a temperature up to 100 °C. Thus, a concentrated salt solution or dry metal salts are obtained, which may be separated to individual metal salts in a conventional manner.

The Si0 ₂ and Ti0 ₂ suspension obtained in step (e) of the process according to the present invention is separated by conventional techniques, obtaining pure Si0 ₂ quartz sand and pure Ti0 ₂ titanium dioxide [Step (e2)]. This is preferably achieved according to EP Al HU P 1200075 patent publication document.

The iron oxide produced according to the process of the present invention may be used in any suitable manner. Examples of the fields of application are as follows: a catalyst, a pigment, a water treatment flocculent with dissolution with hydrochloric or sulfuric acid, or metallurgical recovery.

Likewise, pure iron prepared by the process according to the present invention, iron pentacarbonyl, and rare-earth oxides may be used in any suitable manner.

Similarly, the titanium dioxide may be used in any suitable manner. The quartz sand can be used as washed sand for building construction, may be a starting material for refractory lining bodies, glass production component, high-purity casting sand, but it is suitable for ceramic purposes and for pressing building blocks as well.

Figure 3 shows schematically the main units of a 100 apparatus for carrying out the process according to the invention, and their arrangement.

The 100 apparatus comprises a 110 drum-type furnace, a 120 C0 ₂ tank, a 130 magnetic separator and a 140 acid-treatment tank, a 145 acid tank, a 150 synthesis gas tank, a 160 pyrolysis unit 170 iron settling unit a 180 centrifuge and a 190 magnetizing unit.

The 110 drum-type furnace has a 111 red mud inlet (not shown) receiving raw red mud from production unit or red mud ponds, a 112 C0 ₂ inlet receiving C0 ₂ from the 120 C0 ₂ tank through the L4 line, a 114 synthesis gas inlet receiving CO + H ₂ synthesis gas from the 150, a synthesis gas tank through the lockable LI pipeline, furthermore a 113 treated sludge outlet providing for sludge treated thermally or by induction heating (pre -magnetization). The 110 drum-type furnace may preferably be inductively heated, allowing the magnetization of Fe ₂ 0 ₃ hematite ferric oxide present in the red mud.

The 130 magnetic separator receives the treated red mud at the 131 treated sludge inlet said treated red mud being transmitted from the 110 drum-type furnace through the L3 line and the hematite or magnetite obtained as a result of magnetic separation is discarded at a 132 hematite/magnetite outlet, and the residual slurry remaining from the red mud is discarded through a 132 slurry outlet.

If no reduction is performed in the 110 drum-type furnace, the heat-treated slurry is transferred through a lockable L10 pipeline to the 190 magnetizing unit, which receives the red mud pre-magnetized by the 110 drum-type furnace through a 191 magnetizing inlet, and the magnetized iron-oxide -containing slurry is transferred through a 192 magnetizing outlet terminal, in one also lockable Ll l pipeline to the 130 magnetic separator, to its second 13 Γ treated mud inlet. In order to ensure that the stream of material can be fed to the 130 magnetic separator through the 190 magnetizing unit, the section of the L3 pipeline after its branching is also designed for being suitable to be turned off.

The 190 magnetizing unit may be a second magnetic separator, but also may be formed as an integrated part of the 130 magnetic separator. In the latter case obviously there is no need for the LlO, Ll l lines.

The slurry formed in 130 magnetic separator is transferred to to the 141 slurry-input of the 140 acid-treatment tank through pipeline L5. The 140 acid-treatment tank receives the sulfuric acid needed for the acid treatment at a 142 acid-inlet from the 145 acid tank through the pipeline L7. The metal sulphate salts resulting from the acid treatment are discarded through the 143 metal sulphate -output, while the remaining slurry is discarded through a 144 residual slurry outlet from the 140 acid treatment tank.

The 150 synthesis gas tank stores the CO+H ₂ synthesis gas produced in the 160 pyrolysis unit by the high temperature gasification (pyrolysis) of the waste with organic matter content. The synthesis gas is transferred from the 160 pyrolysis unit to the 150 synthesis gas tank through pipeline L9.

The 100 apparatus also comprises a 170 iron depositing unit, which receives hematite or magnetite iron oxide from the 130 magnetic separator at a 171 hematite/magnetite inlet, said hematite or magnetite iron oxide being transferred through the lockable L6 pipeline, furthermore, it receives the synthesis gas needed for the reduction at the 172 synthesis gas inlet, said synthesis gas is transferred from the 150 synthesis gas tank, through the L2 pipeline. The pure iron powder obtained as a result of the reduction may be discarded through the 170 iron depositing unit through the 173 Fe outlet.

The 100 apparatus according to the invention also comprises a 180 centrifuge, which sepa- rates the titanium oxide and sand discarded from the 140 acid-treatment tank from the slurry discharged through lockable pipeline L8. Accordingly, the 180 centrifuge has a 181 residual slurry inlet and outlets 182 and 183 for the Ti0 ₂ - and Si0 ₂ -outputs.

It should be noted in connection with the pipelines in the 100 apparatus according to the present invention, said pipelines connecting the respective components to each other, that is it obvious for the persons of ordinary skill in the art that the respective units how, and with what kind of pipelines must be connected to each other in order to ensure the appropriate material flow and the required operating parameters (such as pressure, flow rate, sectioning, and the like).

It is further noted that by closing one or more amongst the lockable pipelines (especially LI, L2, L3, L6, L8, L10, LI 1 pipelines) a given unit or even more selected units of the apparatus are detachable from the apparatus, thereby enabling the formation of a variety of operating modes corresponding to the above-described process, or configuration corresponding to various embodiments of the invention. The simplest structure, i.e. the configuration of minimum according to the equipment is surrounded by dotted lines in Figure 3.

Finally, it is noted that the processing units in the apparatus according to the invention are well known and widely used in the chemical industry, the functional design, material selection, sizing of said units therefore belong to the routine works of the person with ordinary skill in the art, and therefore they will not be discussed in detail in the description of the present specification.

The invention is further illustrated by the following examples without limiting the scope of claims to these examples. Examples

Example 1 :

Processing of red sludge from Almasfiizito recovering the hematite content

The composition of red mud deposited in the ponds at Almasfiizito (VEAB Monography):

A1 ₂ 0 ₃ 15-19%

Fe ₂ 0 ₃ 30-40%

Na ₂ 0 6-14%

MgO 0.3-1%

CaO 3-9%

V ₂ 0 0.2-0.4%

p ₂ o ₅ 0.5-1.0%

co ₂ 2-3%

S0 ₃ 0.8-2%

F 0.1-0.4%

C 0.15-0.2%

loss due to glowing 15-18%

Trace elements:

Ga 20-30 ppm

Se 55 ppm

Y 120 ppm

La 240 ppm

Ce 450 ppm

Pr 10 ppm

Nd 190 ppm

Sm 23 ppm

Gd 160 ppm

Mo 4 ppm

Zn 410 ppm

Cr 500-550 ppm

U 42 ppm

Th 50 ppm In the example, the calculation is made with the average value, and the processing of the red mud is shown for 1000 kg (1 1) of starting material.

The red mud with about 10-30% by weight moisture content is added to a heated, rotating drum-type furnace wherein operation is done at atmospheric pressure of 1 bar. The drum-type furnace is inductively heated, and 100 kg of C0 ₂ carbon dioxide is fed therein. By this manner the soluble glass is converted to 171 kg Na ₂ C0 ₃ sodium carbonate and H ₂ Si0 ₃ meta-silicic acid, while also carrying out the pre -magnetization of the Fe ₂ 0 ₃ hematite crystallite particles. The red mud slurry made digestable, and pre-magnetized in induction furnace, dried to powder is run through the strong magnetic field of a magnetic separator. The hematite particles thus made ferromagnetic are magnetically separated from the slurry and residue by a megnetic separator. Thus, 350 kg of Fe ₂ 0 ₃ hematite ferric oxide and 721 kg of residue slurry is obtained.

By treating the resulting Fe ₂ 0 ₃ hematite ferric oxide powder with 99 kg of CO+H ₂ synthesis gas 244 kg of iron metal powder is obtained.

To the 721 kg residual slurry water is added, and it is treated with 1625 kg of 96 % concentration industrial strenght sulphuric acid in acid-resistant and alkali-resistant tanks, under normal pressure, with occasional stirring at a temperature of 20-100 °C for several hours. The material is chemically neutralized, and the liquid phase containing the metal salts dissolved, is separated by centrifugation and concentrated. Thus, 1588 kg of metal sulfate salt, within this about 3.5 kg of rare-earth metal sulphate salt is obtained in the form of solution, which is, as desired, concentrated by evaporation.

From the resulting suspension after removing the metal sulphate salts by centrifugation, 125 kg of silica sand Si0 ₂ and 45 kg of Ti0 ₂ titanium dioxide is obtained.

Example 2

Processing of red sludge from Almasfuzito by reducing the hematite content

1000 kg (1 t) quantity of the red mud with the composition as described in Example 1 is processed. To this, the hazardous waste containing organic matter (waste oil, sludge, leather waste and the like) collected in this environment is gasified in a mobile PEPS type (Plasma Energy Py- rolysis Systems) hazardous waste disposal apparatus, and the reduction gas and energy need of processing is thus covered.

The Fe ₂ 0 ₃ hematite ferric oxide content of the red mud is reduced to Fe ₃ 0 ₄ magnetite ferrous ferric oxide. To achieve this, the red mud is fed in a rotating drum -type furnace, and using 72 kg of synthesis gas composed of 1 : 1 mole ratio CO+H ₂ , prepared from wastes it is heated to about 500°C. At this temperature another 11 kg of CO+H ₂ synthesis gas is fed on the red mud for the reduction. Thus, 338 kg of Fe ₃ 0 ₄ magnetite ferrous ferric oxide and 23 kg of carbonic acid gas are obtained.

To supplement the carbonic acid formed by the reduction, 55 kg of carbon dioxide is added to the drum-type furnace, by which, together with carbonic acid gas formed during the reduction, the soluble glass content of the red mud is converted to 171 kg of Na ₂ C0 ₃ sodium carbonate and H ₂ Si0 ₃ metasilicic acid.

The resultant Fe ₃ 0 ₄ magnetite ferrous ferric oxide is separated from the remaining slurry using a magnetic separator. Thus, besides 338 kg of pure Fe ₃ 0 ₄ magnetite ferrous ferric oxide, 721 kg of residual slurry is obtained.

By treating the resulting pure Fe ₃ 0 ₄ magnetite ferrous ferric oxide with 60 kg of CO+H ₂ synthesis gas, 244 kg of iron metal powder is obtained.

The resulting 721 kg of residual slurry is treated in acid-resistant, alkali-resistant tubes with 1625 kg of 96% industrial strenght sulfuric acid, under atmospheric pressure, with occasional stirring at 200 to 400 °C for several hours. Once dissolved, the slurry is chemically neutralized with sodium hydroxide, and the liquid phase containing the metal salts dissolved therein, is separated by centrifugation and concentrated. Thus, 1588 kg of metal sulphate salt, within this about 3.5 kg of rare-earth metal sulphate salt in particular is obtained.

After removing the metal sulphate salts from resulting suspension by centrifugation, 125 kg of silica sand and 45 kg of Si0 ₂ Ti0 ₂ titanium dioxide is obtained.

Example 3

Processing of the red mud form the Vietnamese Tanren Aluminum Factory by the reduction of the hematite content

The composition of the red mud from the Vietnamese Aluminum Factory is as follows:

Fe ₂ 0 ₃ 30.8 0 //o

MnO 0.02 0 //o

CaO 3.51 0 //o

K ₂ 0 0.11 0 //o

p ₂ o ₅ 0.22 0 //o

A1 ₂ 0 ₃ 15.6 0 //o

MgO 0.27 0 //o

Na ₂ 0 3.14 0 //o Rare-earth metals (especially Ce, La, and Nd) in total 28.59 ppm, and 0.81 ppm of Sc, and 0.99 ppm of Y.

The processing of the red mud is shown for 1000 kg (1 1) of starting material.

To this as waste containing organic material 160 kg of cellulose (about 10% moisture con- taining rice straw and rice husking residue) is gasified in drum-type furnace over 1.5 hours at 1000 °C, and thus the reduction gas and energy source need of the processing is covered.

The Fe ₂ 0 ₃ type hematite ferric oxide content of the red mud is reduced to Fe ₃ 0 magnetite ferrous ferric oxide. To achieve this, the red mud is fed into a rotating drum -type furnace, and heated to 500 °C with CO+H ₂ synthesis gas produced from 72 kg of cellulose waste. At this tem- perature it is treated with an additional 10 kg of 1 : 1 mole ratio CO+H ₂ synthesis gas. Thus, 298 kg of Fe ₃ 0 ₄ magnetite ferrous ferric oxide and 15 kg of carbonic acid gas is obtained.

Another 12 kg of C0 ₂ carbon dioxide is fed into the reduction block. As a result of both the reduction of magnetite hematite content of the red mud, and the carbonic acid formed from the introduced carbon dioxide, the soluble glass content of the red mud is converted Na ₂ C0 ₃ sodium carbonate and H ₂ Si0 ₃ metasilicic acid.

The resultant Fe ₃ 0 ₄ magnetite ferrous ferric oxide is separated from the remaining slurry by a magnetic separator. Thus, 723.4 kg of residual slurry obtained next to the 298 kg of pure magnetite Fe ₃ 0 ₄ ferrous ferric oxide.

By treating the resulting pure Fe ₃ 0 magnetite ferrous ferric oxide is with 58 kg of CO+H ₂ synthesis gas, 162 kg of iron metal powder was obtained.

The 723.4 kg of obtained residual slurry is treated in Door tubes with 960 kg of 96% industrial strenght sulfuric acid, under atmospheric pressure, with occasional stirring at 200 to 400 °C for several hours. Once dissolved, the slurry was chemically neutralized with sodium hydroxide and the liquid phase containing the metal salts dissolved therein are separated by centrifugation and concentrated. Thus, 922 kg of metal sulphate salt, including about 62.7 g of rare-earth metal sulphate salt is yielded.

After removing the metal sulphate salts from the resulting suspension by centrifugation, 317 kg of Si0 ₂ quartz sand and 25.8 kg of Ti0 ₂ titanium dioxide is obtained.

Example 4

The energy consumption of the process according to the invention is demonstrated for 1 ton of red mud, said energy may be shared into two parts, one part for heating to the reaction temperature, another part to produce the reducing: a. )

Taking a look at the main components of the red mud, it can be seen that

the specific heat of iron oxide is 0.7 kJ/kgK

the specific heat of alumina is 0.7 to 1.1 kJ/kgK

the specific heat of silica sand is 0.8 kJ/kgK

the specific heat of titanium dioxide is 0.7 kJ/kgK;

and there is substantial water content, wherein the specific heat of water is 4.2 kJ/kgK, and the specific heat of the steam is 2 kJ/kgK.

Others have measured the specific heat of red mud to 1.31 kJ/kgK.

Considering these facts the specific heat of the red mud slurry is accepted to approach the

1.5 kJ/kgK value.

When heated to 500 °C, the slurry (now taking the energy need of the heating of the drum- type furnace or other technical means and technological losses as zero ) and based on the Q=c.m.dT equation for 1 ton of red mud this value is 672 MJ (where dT=480 °C).

Taking the heating value of synthesis gas 10 MJ/kg, this also needs 67.2 kg of synthesis gas. b. )

At the same time one ton of red mud, according to the tests of MTA/VE, contains 370 kg of hematite ferric oxide. In order to reduce this amount of hematite to Fe ₃ 0 magnetite ferrous ferric oxide, we need 17.4 kg of (CO+H ₂ ) synthesis gas. Of course, in practice, we work substantial excess of gas, and therefore a+b) need is estimated by 85 kg of synthesis gas, that is, for the reduction of the hematite ferric oxide content of one ton of red mud to magnetite ferrous ferric oxide, 85 kg of synthesis gas composed of (CO+H ₂ ) is needed.

If this amount of CO+H ₂ synthesis gas is produced from a cellulose waste having about 10% moisture content (paper waste, rice husking residues, straw, hemp, flax, their products, and the like), this is possible with smaller amounts with drum-type furnaces as well. Then for 1 ton of fed cellulose, approximately 1 ton of produced synthesis gas will be obtained. Since the energy need of the conversion from 1 ton of cellulose to CO+H ₂ synthesis gas is 430.5 kWh/ton, we see that the energy need of the conversion of 1 ton of red mud hematite content to magnetite Fe ₃ 0 ferrous ferric oxide is about 36.5 kWh, where 304 kg magnetite ferrous ferric oxide is obtained from 1 ton of red mud.

It is a further advantage of the nearly 1: 1 molar ratio CO+H ₂ synthesis gas made from e.g. cellulose, is that after the reduction, C0 ₂ carbon dioxide and H ₂ 0 water is formed from the oxygen of Fe ₂ 0 ₃ iron oxide, which is converted to C0 ₂ +H ₂ 0=H ₂ C0 ₃ carbonic acid. If the the sodium silicate (soluble glass) [orthosilicate (Na ₄ Si0 ₄ ), metasilicate (Na ₂ Si0 ₃ ), polysilicate ((Na ₂ Si0 ₃ ) _n ) and pyrosilicate (Na ₆ Si ₂ 0 ₇ )] surrounding the iron oxide particles of the red mud is heated in an acidic medium, then it leaves the system in the form of silica. Thus, in a single operation the sodium silicate surrounding the iron oxide particles the red mud can be decomposed into sodium carbonate and silica, where the latter by the exit of water condenses to Si0 ₂ silicon dioxide again. Both the sodium carbonate and the silica is intimately mixed with the residual slurry, and the fac- tor hindering the digestion of the hematite ferric oxide particles will cease to exist.

Considering an average bauxite production volume, that is a logistically meaningful red mud processing, then we need to calculate with about 200 thousand tons of red mud/year processing. Calculating 300 working days annually, it is 667 tons per day that is the daily energy need is 56.7 tons of synthesis gas, or 24.4 MWh.

In two shifts with a 1-ton drum-type furnace is about 6.4 tons per day synthesis gas can be produced, which represents about 9 pieces of drum-type furnaces in the system.

The efficiency can be further increased, if the energy required to operate the plasma forge is achieved by renewable energy production run in island mode (see HU No. PI 50070 Al patent publication document).

The 358 kg of high-purity iron oxide produced per ton of red mud may be used for the production of a catalyst, iron oxide pigments, by hydrochloric or sulfuric acid dissolving a water purifying flocculant, but also can be utilized metallurgically.

However, it is appropriate to market this item at a higher price, higher stages of preparation, and the production of a query an expensive material, the Fe(CO) ₅ iron pentacarbonyl is advisable.

It is noted that the reduction of hematite content of the red mud is directed in such a way that the 48 kg/ton of sodium oxide present in sodium silicate (soluble glass) format, surrounding the iron oxide particles - partly due to the effect of the carbonic acid gas generated during the reduction of hematite into magnetite carbonic acid gas, partly to the effect of the carbonic acid gas resulting from the added carbon dioxide - is converted to sodium carbonate. Thus, the sodium mixed into the slurry (as opposed to soluble glass) does not interfere with the digestion of iron oxides.

At this point remains 582 kg/t of residue slurry, of which about 200 kg is Si0 ₂ and 38 kg is Ti0 ₂ , which do not react with a sulfuric acid (or HC1) dissolution, and the remaining slurry contains 344 kg/ton of metal oxide, in which there are the rare-earth metal oxides as well. This is dissolved in 96% industrial strenght sulfuric acid. The sulfuric acid need of the reaction is assessed in such a manner, as if the entire metal oxide mixture consisted of aluminum oxide A1 ₂ 0 ₃ . Then the industrial strenght 96% sulfuric acid need of the 344 kg of metal oxide composition resulting from 1 ton of red mud is s 1033 kg. From the 344 kg of metal oxide mixture 1201 kg of metal sulfate salt is formed, of which 29 to 86 kg is rare-earth metal sulphates. It can be seen that the rare-earth content of the red mud was concentrated to an average of sixty times (as compared to the starting concentration), which is in a single metal-sulphate salt concentrate as rare-earth metal sulphate.

The rare-earth metals can be recovered from the sulphate salt by conventional procedures. After the sulfuric acid dissolution process, the Si0 ₂ and Ti0 ₂ suspension is retained, with about 238 kg of dry matter content. From this the valuable Ti0 ₂ titanium dioxide is worth to extract, according to the PI 200075 Patent Application, wherein due to the widely differing specific gravities, the mixture is simply separated by centrifugation, or optionally by another suitable sedimentation method, the Si0 ₂ quartz sand and 38 kg/t Ti0 ₂ titanium dioxide may be separated. The titanium dioxide is commercialized. The 200 kg/t of quartz sand has been washed to high purity in the duration of the red mud processing. So it can be commercialized as high-grade foundry sand, but it is also suitable for compressing ceramic and building blocks.

According to the process of the present invention the red mud ponds occupying a lot of space, and being hazardous, can be eliminated entirely and with great profits.