CA2618170A1 - Coagulant useful in wastewater treatment and process for preparing thereof - Google Patents

Abstract

The present invention relates to a coagulant agent useful in wastewater treatment and process of transforming red mud from aluminum industry, by treatment in acid medium, to a concentrated liquid or soluble solid product rich in aluminum and iron.

Description

TITLE

COAGULANT USEFUL IN WASTEWATER TREATMENT AND PROCESS
FOR PREPARING THEREOF

FIELD OF THE INVENTION

This invention relates to a chemical process of transformation of red mud from the aluminum industry into a soluble product, rich in aluminum and iron, which could be used as a coagulant for wastewater treatment. The reagent thus generated could be specifically employed for the removal of phosphorus present in municipal and industrial wastewater.
BACKGROUND OF THE INVENTION

Red mud is a residue of the aluminum industry, generated during the alkaline extraction of alumina from bauxite in the Bayer's process. The importance of this residue is necessarily related to the significance of the aluminum industry.

In 2003, the annual world production of aluminum reached 28 million metric tons (AAC, 2005). The production of a ton of alumina generates one to two tons of red mud (Brunori et al., 2005; Agrawal et al., 2004; Hind et al., 1999). As the production of a ton of aluminum requires two tons of alumina (AAC, 2005), it is necessary to take into account two to four tons of red mud generated per ton of aluminum produced. These figures are in agreement with those of Aluminum Association (2000) which estimates that 1.5 to 4 tons of red mud are generated for each ton of the aluminum produced. The annual world production of red mud, on a dry basis, has been estimated to be more than 70 million metric tons (Aluminum Association, 2000).

The bauxite consists of approximately 75% of hydrated alumina (A12O3=3H2O and A12O3=H20). During the treatment of ore in the Bayer's process, it is initially crushed and then mixed with caustic soda and lime liquor. The mixture is then subjected to high pressure and temperature making it possible to solubilize hydrated alumina and to obtain sodium aluminate solution (equation 1), while the impurities remain in a solid state (AAC, 2005).

Al(OH)3 + NaOH --+ A1OZNa + 2 H20 [1]
The impurities are separated from the aluminate solution by decantation and filtration, followed by a washing. The solid residues thus obtained are called red mud and are mainly made up of oxides of iron, aluminum, silicon and titanium. However, despite being washed and considered as an inert solid waste, red mud remain strongly alkaline and is highly corrosive. Table 1 presents the typical composition of red mud, especially their physicochemical properties. It should be noted that the composition of red mud depend on the composition of native bauxite and that the latter varies from one mineral deposit to another.

Table 1 Composition and physico-chemical properties of red mud Composition Concentration (% w/w) a Iron oxide (Fez03) 45 - 55 Aluminum oxide (AIZO3) 15 - 25 Crystalline silica (quartz, SiO,) 4- 15 Titanium dioxide (TiOZ) 5 - 15 Sodium oxide (Na20) 2 - 10 Calcium oxide (CaO) 1-5 Sodium hydroxide (NaOH) 5 - 10 Physico-chemical properties Values pH 12- 13 Relative density (Water = 1) 1.3 Granulometry (Particle size) < 200 pm " On dry basis Various modes of disposal for red mud are utilized worldwide. In some countries, these residues are poured into the ocean by pumping. This is particularly adopted in Greece, wliere 500 000 tons of red mud per year are pumped into the Antikyra Bay, in the Gulf of Corinth (Tsakiridis et al., 2004). Currently, of the 84 alumina industries in the world, seven of them always discharge the red mud at sea due to paucity of land (Agrawal et al., 2004).
However, in North America, the disposal practices of red mud are carried out via storage in retention tanks or piling them up after thickening (Aluminum Association, 2000).

Akin to United States, the red mud in Canada is not regarded as hazardous wastes (Sante Canada, 2003; Aluminum Association, 2000). However, if poorly managed, it may become hazardous. For instance, winds can rise the dust from dried red mud, whicli could then reach neighboring communities and cause further environmental problems (Sante Canada, 2003; Aluminum Association, 2000). If dust from red mud accumulates on neigliboring grounds, such as a backyard or a playground, it may easily be carried within living quarters and may be ingested by humans. In this case, small children and pregnant women wllo could be more sensitive to the heavy metal and NaOH present in red mud could have serious health issues. The legislation differs from one country to another, and the alumina producers of Aluminum Association are conscious that this classification is not adequate to be considered as there is no universal standard. Additionally, in spite of the efforts made for better management of red mud, potential adverse impacts on the environment still lurk. Indeed, even with a clay bed present at the bottom of the storage sites, leaks can occur and greater volumes of alkaline residues are thus likely to contaminate the underground water.

Enonnous quantities of red mud is generated each year everywhere in the world, the scarcity of available spaces to store them, their threat to the environment, particularly, if they are discharged in an inadequate way, has stimulated several research teams, in various countries, to explore the potential to re-utilize these by-products. Although research efforts have been made over several decades, still Aluminum Association (2000) affirms that no mass application for the treatment of red mud at conunercial scale and at the same time profitable is known. The principal modes of value-addition that have been explored, comprise, removal of phosphorus in wastewater, metal recovery from mud, adsorption of heavy metals in water, the utilization of mud to counter the acid mines drainage and the incorporation of mud in the manufacture of materials.

Value-addition of red mud Removal of phosphorrrs Several research teams concentrated their efforts on the dephospliatation potential of red mud. Some utilized red mud as an adsorbent for phosphorus, often following a chemical treatment of mud.

Shiao and Akashi (1977) carried out the activation of red mud (dry) by reflux heating with hydrochloric acid (5 to 30 g per 100 g of mud) for 2 h. The resulting solid phase was then washed with water, dried, crushed and treated at high temperature (200 to I
000 C) for 2 h. Red mud thus activated were used to adsorb the phosphorus from dibasic potassium phosphate solutions (1.0 to 50 mg-P0437L). The dosages utilized were 0.01 to 0.1 g of red mud activated by 40 ml of solution, and the results showed an adsorption capacity of phosphates almost equivalent to that of F1 alumina, considered, during the same time as the best adsorbant for phosphorus removal.

Another research team (Pradhan et al., 1998) also carried out the activation of red mud by acid reflux using hydrochloric acid. The adsorbent obtained was able to remove 80 to 90% of phosphate solutions initially containing 30 to 100 mg-P0437L when added at a dosage of 2 g/L.

In the same context, Koumanova et al. (1997) activated red mud with concentrated sulphuric acid (10 to 30 ml of acid per g of mud) for I to 24 h, without heating. The solid phase was then washed with water, dried and crushed. This treatment made it possible to double the capacity of adsorption of phosphate by red mud. In fact, for solutions containing 50 mg-P0437L at a dosage of 10 g/L, the removal rate increased from 42 and 47%
for raw red mud to 72 and 95% for treated red mud, respectively for solutions of orthophosphate and tri-polyphosphate.

Lopez et al. (1998) studied the use of red mud as a medium for column filtration in wastewater treatment. In order to counter the tendency of red mud to form a stable suspension in water to neutralize alkalinity arising from them, a pretreatment was applied to dry mud. Thus, 8% (w/w) of anhydrite (CaSO4) was added to red mud and water was added in just enough quantity to moisten. After mixing, aggregates of 0.01 to 5 mm size were obtained. According to authors, this material proved to be a good adsorbent for phosphorus (dissolved, Pd, and total, Pt) and also for heavy metals. In particular, a column test performed on a municipal effluent, resulting from a secondary aeration treatment, made it possible to remove 100% of phosphorus (initial Pd: 0.74 mg/L; initial Pt:
0.95 mg/L) and also 56 to 100% of heavy metals present.

The research team of Roberge et al. (1999) also studied the adsorption of phosphorus by red mud during a column filtration. However, contrary to the team of Lopez et al. (1998), red mud was not subjected to any kind of pretreatment, except drying and ci-ushing, and was used as a doping agent for peat. The doping of peat by red mud, at a rate of I and 3% (w/w), made it possible to increase the adsorption capacity of phosphorus from 16.6% to 29.2 and 65.2%, respectively by using this filtration medium. The wastewater was procured from the supernatant of an activated sludge settling tank at an average PT
concentration of 2.9 mg/L.

Gnyra and Verghese (1978) patented a process (CA 1038254), developed by Alcan, aimed at transforming mud for their use as a coagulant. This process consisted of treating 100 g of red mud (on dry basis) with 65 to 120 g of a mixture of acids at a temperature of approximately 145 C for 1 h. It has been deternlined that the use of sulphuric acid in the treatment of red mud yields a product that is poorly soluble and containing insoluble impurities whereas the use of hydrochloric acid is much more efficient in solubilizing iron and aluminuin from red mud but is also 5 to 6 times more expensive than the former. Thus, the inventors have suggested the use of sulphuric acid alone or in combination with hydrocliloric acid as a solvent for the process. The acid mixture comprised 100 parts of sulphuric acid and 0 to 200 parts of hydrochloric acid. By introducing a minimum quantity of water into the reaction, that is to say, 100 g of water per 100 g of red mud (dry), this treatment resulted in a friable solid, of which 15 to 25% was not water soluble. This insoluble part is coniposed, inter alia, of active silica gel, Ti02, Fez03 and A1203, while the soluble pai-t included sodium and aluminum sulphate, ferrous sulphate, and aluminum and ferric chloride wlien hydrochloric acid is employed. At a dosage of 0.2 g/L, the product resulting from this process is able to remove 87% of the phosphorus froin wastewater containing initial concentration of 4.6 mg/L, which is comparable with the effectiveness of aluininum sulpliate under same conditions, and 100% of the phosphorus from a wastewater containing initial concentration of 8.2 mg/L.

Another team of researchers (Couillard, 1983; Couillard and Tyagi, 1986) studied the capacity of red mud without treatment to coagulate phosphorus. Red mud was diluted at a rate of 10 g/L and the tests were carried out on solutions of KH2PO4 containing 20 mg-P04 3 /L. The amount employed was 0.8 ml of red mud solution per liter of the solution to be treated. Various pH of the solution were studied and the optimal pH for coagulation proved to be between 6.5 and 7Ø However, the maximum efficacy obtained during these experiments was 20% removal of phosphorus, which was very low. These studies thus confirm the need for a novel method to transform red mud into an effective coagulant for the dephosphatation process.

Recovery of n:etals Red mud contains a certain quantity of titanium in the form of oxide. This metal possesses a great commercial value and is used in several applications.
Although studies relating to the recovery of titanium from red mud have been already carried out since 1946, there are very few publications describing the actual processes. Bhatnagar et al. (1946) developed a process comprising treatment of red mud with sulphuric acid, followed by hydrolysis and calcination. This method resulted in 70% recovery of titanium dioxide contained in mud.

In the process of Damodaran and Gupta (1956), iron and alumina are initially separated by leaching with hydrochloric acid and then mud is digested with sulphuric acid.
Likewise, Miodrage and Bratamira (1963) developed a method by which mud is reduced using coke to recover iron, and then the residue is subjected to clilorination to obtain concentrated titaniuni dioxide.

More recently, Maitra (1993) developed a process for the recovery of titanium dioxide from red mud. In this process, red mud is initially washed with water and then treated with dilute hydrochloric acid at 95 C and pH 4. After separation of the liquid fraction enriched in calcium oxide, sodium oxide and a little aluminum chloride, the solid phase is treated with concentrated hydrochloric acid, at 95 C, which makes it possible to solubilize the iron and remainder of aluminum. After separation, the solid residue is then treated with concentrated sulphuric acid at a temperature ranging from 150 to 280 C so as to solubilize titanium in the fonn of sulphate. The hydrolysis, precipitation and calcination steps make it possible to finally obtain titanium dioxide to a purity of at least 97.5%.
Although this process is effective, yet it is much more expensive than the conventional extraction of titanium dioxide from natural resources.

Kasliwal and Sai (1999) also developed a process to enrich red mud in titanium dioxide and studied the kinetics involved. Their method comprised treatment of mud with hydrochloric acid at higli temperature. The leachate obtained is rich in calcium, sodium and iron, while the solid residue is enriched in titanium dioxide, and also includes alumina and silica. A higher purity of titanium dioxide is obtained in the residue when it undergoes roasting in the presence of calcium carbonate.

Other research teams developed methods to recover several metals from red mud.
It is the case of Astukawa et al. (1967) who patented a process comprising dissolution and successive precipitation by variation of pH using sulphur dioxide. Thus, the compounds of sodium, oxides of titanium and iron, silica and alumina could be isolated from red mud.
Bamett and Mezner (2001) filed a patent based on the same principles of selective dissolution and precipitation by pH adjustment. Tliese authors managed to recover iron, aluminum, silicon and titanium from red mud.

Moreover, Cengeloglu et al. (2001, 2003) studied the recovery of ions of iron(III), aluminum(III), titanium(IV) and sodium(I) by Donnan dialysis across a polysulfone charged membrane. According to the authors, this method is faster and requires less energy than the conventional methods (chemical precipitation, adsorption on coal, ion exchange, encapsulation/chelation).

In fact, several aluminum smelters carried out research in order to recover titaniwn, iron and heavy metals from red mud, but the results were not conclusive: the purity of metal was insufficient or the process was too expensive. It seems that the extraction of metals of interest from red mud is not competitive compared to the extraction from natural sources (Aluminum Association, 2000).

Adsorption of heavy inetals Lopez et al. (1998) noted that red mud, after treatment, were able to adsorb heavy metals, copper, zinc, nickel and cadmium, in addition to phosphates.

Even, Apak et al. (1998) developed a transformation process of mud initiated with washings by water until neutrality followed by an acid treatment similar to that of Shiao and Akashi (1977). The resulting compound shows a good adsorption capacity for heavy metals such as copper (II), lead (II) and cadmium (II).

Orescanin et al. (2001) and Orescanin et al. (2002) carried out the activation of red mud by using dilute sulphuric acid (25 C, 24 h) following neutralization using waste base until pH 8. The mud thus obtained are in a gelatinous state and act as an effective coagulant for the removal of heavy metals, in particular copper, lead and zinc, from aqueous synthetic solutions (100 mg/L) and industrial effluent (pressure washings from boat). According to authors, this compound has advantages over other conunercial coagulants being already used, in particular the volume of mud generated by coagulation/flocculation is 20 times lower than those generated by aluminum and conventional iron salts.

Gupta et al. (2001) and Gupta and Sharma (2002) also subjected red mud to hydrogen peroxide treatment for 24 h, followed by drying at 100 C and then an activation at 500 C for 3 h. The product obtained was used for column adsorption tests for the removal of lead, chromium, cadmium and zinc at initial concentrations of 700, 300, 100 and 120 mg/L, respectively from aqueous solutions. The removal efficiency was satisfactory. After adsorption, metals could be eluted and recovered, and the adsorbent in the columns could be regenerated using dilute nitric acid.

Genc-Fuhrman et al. (2004a, 2004b) studied the capacity of Bauxsol to adsorb arsenic from aqueous solutions. Bauxsol is obtained by mixing red mud with sea water. For a solution containing 40.5 mol/L of initial concentration of arsenic (V), a removal rate of 89% was obtained. When Bauxsol is activated by an acid treatment (reflux using hydrochloric acid), or, by a combination of acid and thermal treatments (calcination at 500 C), the arsenic removal reaches 95 and 100%, respectively.

Reduction of the acid mine drainage Fortin et al. (2000) studied the possibility of using red mud to reduce the acid mine drainage in sulphidic mine tailings. These authors carried out leaching tests in colunms by adding cement kiln dust and 10% (in mass) of red mud to the mining residues.
Their work showed the potential of red mud to create a reservoir of alkalinity making it possible to maintain pH close to neutrality for long-term.

Doye and Duchesne (2003) also further tested this hypothesis by discontinuous leaching tests, by varying the proportions of alkaline agents.

In the same context, Paradis (2004) repoi-ted a method for the restoration of parks laden with mining residues using red mud. This method is based on the neutralization of acidogenic residues and fixation of toxic metals.

In conclusion, Konmitsas et al. (2004) studied the use of limestone and red mud as permeable barriers to decontaminate the leachate resulting from acid mine drainage. The colunin tests showed an effective removal of metals. The implied mechanisms would be the precipitation of metal ions (Fe, Al, Mn, Zn, Cu, Co and Ni) in the form of hydroxides or sulphates and coprecipitation/sorption (especially for Cd).

Manrifacture of materials Some research teams studied the possibility of integrating red mud into the manufacture of cetnent. According to Singh et al. (1997), cement enriched in iron, containing sulfoaluminoferrite, can thus be obtained. This type of cement possesses advantages over conventional Portland cenient: conservation of energy due to lower reaction temperature, re-use of by-products, good mechanical properties, good corrosion resistance, good behavior at low temperature and rapid liardening. The quantity of red mud added to the preparation of cement was varied from 0 to 50% (w/w) and the properties of material obtained were dependent on the proportion of ingredients added.

Tsakiridis et al. (2004) also studied the use of red mud during the manufacture of Portland cement. This team, however, limited the concentration of red mud to 3.5%. This experimental cement presented properties comparable with those of the Portland cement prepared for reference.

Other research teams saw an interest to use red mud in the industry of glass and ceramics. For this purpose, Puskas (1983) patented the manufacture of ceramics containing 51 to 90% of red mud.

Andrews (1989) patented the synthetic manufacture of neplielite from red mud by leaching with sulfurous acid, crystallization of leachate and calcination of crystallized residue. Nephelene-syenite is a potassium and sodium aluminosilicate used as a flux or vitrifying agent during the manufacture of glass or ceramics.

Sglavo et al. (2000) studied the manufacture of ceramics from two different types of clays of different qualities combined with various proportions of red mud (0 to 50% in one case and 0 to 20% in the otlier). The red mud contents and the reaction temperature showed effect on the physical and mechanical properties of ceramics obtained. The authors estimated that the results opened the door to a promising future for red mud as a basic material in the manufacture of ceramics.

Yalc,in and Sevinq (2000) evaluated the potential of red mud in the composition of ceramics enamel. Enamels could be manufactured successfully while using up to 37% of red mud. In addition, it should be noted that, for these various studies, red mud also play a secondary role of pigment making it possible to obtain ceramics of various colourings according to the proportion added.

Meanwhile, according to the Aluminum Association (2000), the bauxite contains natural radiation which is concentrated in red mud. This radiation is problematic for certain uses. For this reason, the aluminum smelters consider that the manufacture of building materials is an unacceptable comnlercial risk.

SUMMARY OF THE INVENTION

The present invention relates to a process of transfonnation of red inud originating from the Bayer process (transfonning bauxite in aluminum oxide and red mud), into a coagulant tlirough a treatment in acid medium. The substance can be a concentrated liquid product or solid but soluble, containing high percentages of aluminum and iron and could be used as a coagulant for wastewater treatment. The product obtained, can in particular be employed, to replace alum (hydrated aluminum sulfate) ferric chloride or fei-ric sulphate or ferrous sulphate, for the removal of phosphorus present in nlunicipal and industrial wastewater.

The present invention is to claim a chemical process of transformation of red mud from aluminum industry, comprising the steps of:

a) acidifying red mud, dried beforehand or not dried, by an acid mixture (NaC1 and sulphuric acid) preferably ranging between 500 and 3 000 kg of acid per ton (dry basis) of treated dry red mud;

b) adding water to the mixture of red mud and acid so as to adjust the red mud contents to a concentration lower than 400 g per liter of solution;

c) adding the chloride ions in the form of a chloride salt, preferably sodium chloride, to acidified mud, so as to obtain a chloride concentration preferably ranging between 150 to 800 kg per ton (dry basis) of treated red mud;

d) mixing the solution for a period of 0.5 to 8.0 h so as to effectively solubilise iron and aluminum present in red mud and this, by maintaining the temperature of solution to values lower than 2500 C and preferably at a temperature close to 110 C;

e) separating residual red mud from the liquid fraction;

0 optionally washing, residual red mud in order to recover solubilized iron and aluminum with defined volumes of water;

g) optionally, mixing the leachate from leaching step and the washings of residual red mud. This mixture is the liquid coagulant solution;

h) optionally, heating the mixture of leachate and washings so as to evaporate the liquid and to precipitate a solid (reactive solid coagulant).

The steps (a), (b), (c) and (d) of the process according to the invention can be carried out in one, two, tlu-ee or four phases of treatment, whicli can be operated easily in semi-continuous or continuous tank reactors.

The invention provides a number of advantages. For instance, the use of NaCI/sulphuric acid gives a good yield witliout increasing the cost of production. Indeed, with the use of NaCI/sulphuric acid in the process of the invention, it is possible to obtain a product comparable to the one that would be obtained if a more costly solvent such as hydrochloric acid was used instead. Furthermore, the use of the acid mixture in the process of the invention allows it to operate in temperatures lower than that traditionally used with another acid mixture (for instance 140-150 C in the process of CA 1,038,254 vs 110 C in the process of the invention). The process also allows lowering the cost of management in that it allows a lower yield of red mud impurities such as insoluble impurities.

Brief description of the drawing Fig. I illustrates a coagulant production line according to an embodiment of the invention, in which the coagulant solution or solid is produced from red mud.

DETAILED DESCRIPTION OF THE INVENTION

The invention consists of an effective and relatively inexpensive process to isolate iron and aluminum present in red mud by way of solubilizing same from red mud.
More particularly, the present invention relates to a process to obtain a product (a solution or a solid rich in iron and aluminum) which can be used as a reagent of coagulation in the processes of wastewater treatment, such product being void of any solid impurities (such as silicates quartz, Ti02 and Fe203). More specifically, the invention concerns a process for isolating iron and aluminum from red mud by the use in combination of NaCl and sulphuric acid. The acidic conditions obtained by the acidification of red mud, and the presence of high content of chloride ions makes it possible to solubilize high proportions of iron and aluniinum.

Figure 1 shows a typical diagram of the various stages of treatment constituting the invention. The first phase of the process (leaching step) includes acidification of red mud, dried beforehand or not dried, by a mixture of red mud with addition of sulphuric acid ranging between 500 and 3 000 kg of acid per ton of treated dry red mud. The red mud contents of the mixture are then adjusted with a value lower than 400 g per liter of solution by tap water addition. An addition of chloride salt, preferentially sodium chloride, is then carried out so as to obtain a chloride contribution ranging between 150 and 800 kg per ton of treated red mud. The solution is then mixed for a period of 0.5 to 8.0 h in order to adequately solubilize iron and aluminum present in red mud. The mixture is maintained at a temperature lower than 250 C, and, preferentially, at a temperature close to 110 C.

The addition of acid, chloride salt and water, can be carried out in a different order than that presented earlier. In the same way, another acid (e.g. hydrochloric or nitric acid, etc.) can be employed in substitution for sulphuric acid. The red mud content of the mixture is adjusted in a preferential way witli contents ranging between 150 and 250 g per liter of solution. The chloride salt can be, in particular, chloride of sodium, calcium, magnesium and potassium. The hydrochloric acid can also be used as a source of chloride, however, sodium chloride is preferred for economic reasons. It is also possible to operate the leaching of iron and aluininuin in two or more stages. For exainple, first period of niixing can be carried out only in the presence of red mud, acid and water, followed by a second period of mixing in the presence of chloride salt. It is also possible to carry out several successive stages of leaching, however, for economic reasons, it is preferable to carry out only one stage, which makes it possible to obtain higher solubilization yields of iron and aluminum. The previously described steps can be operated easily in semi-continuous or continuous mode in tank reactors.

The second phase aims at the separation of residual red mud from the liquid fraction. Separation can also be carried out by decantation, filtration, centrifugation or any other standard technique of solid-liquid separation. The addition of organic polymer or any other agent of flocculation for mud can also be carried out. However, solid/liquid separation of red mud generally does not require the addition of flocculating agent.

The tliird phase of the process consists of washing of residual red mud in order to recover solubilized iron and aluminum, present in interstitial water of red mud. Washing is preferentially carried out with water so as to eliminate part of acidity from residual red mud. According to the need, however, a dilute acid or an alkaline solution can also be employed as washing solution. The volume of washing solution must be minimized so as not to unnecessarily dilute iron and aluminum present. Washing can be done by rinsing of the solid residue resulting from filtration, or, by re-suspending the mixture of solid residue in the washing solution, followed by a stage of solid-liquid separation. The washing step of red mud can be repeated for one or more recoveries. The washed residue (residual red mud) can then be neutralized by addition of an alkaline agent and disposed at a disposal site for industrial wastes. The residue can also be neutralized by mixing it with untreated red mud which is highly alkaline.

The fourth pliase consists of mixing of the resulting leachate(s) of the first phase, with washings resulting from the third phase of the process. The mixture thus obtained which is rich in aluminum and iron, constitutes the liquid coagulant. The latter can also be obtained from the generated leacliate(s) during first phase of the process.
The mixture of leachates and washings is however preferable, in order to increase the total extraction yields of iron and aluminum initially present in red mud. The liquid coagulant can be used just as it is for the dephosphatation of wastewater, or for any other application requiring the use of iron or aluminum as an agent of coagulation. The liquid coagulant can also be subjected to subsequent steps of purification or concentration. For example, it can be filtered in order to eliminate the suspended matter, or, it can be heated in order to evaporate part of water to concentrate the final aluminunt and iron solution.

The fifth phase of the process, which is optional, consists of complete evaporation of water present in the coagulant solution, so as to precipitate a solid rich in iron and aluminum. According to an x-ray diffraction study, the precipitate thus obtained is mainly made up of NaFe(S04)2, NaHSO4 and A15C13(OH)12.2HZO. This solid agent, highly water soluble coagulant, contains about 8 to 10% of iron and 3 to 4% of aluminum.
The residue can be used without further treatment as a coagulant agent. Optionally, the residue obtained by evaporation can be further dried and crushed before being used.

Example 1: Tests of chemical transformation of red mud from aluminum smelter On the whole, more than 93 tests were carried out on laboratory scale in order to determine the optimal conditions for production of a coagulant derived from red mud.
These tests were carried out under various operating conditions. For example, the tests were also carried out on dried red mud, as well as wet red mud (e.g. 52% of total solids). The range of tested total solids (content of red mud) was between 150 and 400 g per liter of solution, whereas the temperature of leaching was adjusted with values ranging between 50 and 2500 C. The mixing time employed for the solubilization of iron and aluminum present in red mud ranged between 30 and 240 min. The quantities of sulphuric acid tested were in the range of 0 to 3 680 kg H2SO4 per ton of treated red mud, whereas those of the hydrochloric acid ranged between 0 and 2 190 kg HCl per ton of red mud. The use of nitric acid was also tested with values ranging between 0 and 2 520 kg HNO3 per ton of red mud.
The addition of sodium chloride was evaluated for a range between 0 and 1 250 kg NaCI
per ton of treated red mud. The red mud used during these tests contained initial iron concentrations of 26.6 to 29.6%, as well as 10.6 to 12.3% of aluminum.
Overall, the measured extraction yields of iron and aluminum comprised, respectively between 3 and 100% (Fe) and 2 and 99% (Al) during these tests.

Table 2 shows sonie examples of experimental conditions and results obtained during tests of red mud transformation by the process which forms the subject of this invention. As an example, the JI test made it possible to produce 2 385 mL of a coagulating solution containing 31.6 g Fe/L and 12.0 g Al/L. The volume of coagulating solution resulted from the mixture of the leachate collected by vacuum filtration of residual red mud (volume of 1 480 mL) and of the washings recovered during the washing of this residual mud (volume of 905 rnL). This solution was then evaporated thus producing 824 g of solid coagulant containing 91.8 g Fe/kg and 33.0 g Al/kg. The total removal yields of iron and aluminum present in red mud were 74.5 and 73.7%, respectively during the J 1 test.
Moreover, the residual red mud mass was 240 g compared to a treated mud mass of 400 g.
Table 2 Examples of tests of transformation of red mud into coagulant Parameters Tests Mass of treated red mud (g) 40 40 40 40 40 400 76.8*
Content of red niud (% w v') 18 19 19 17 17 20 20 Temperature of the leaching step ( C) 110 110 110 110 110 110 110 Duration of the leaching step (min) 240 240 240 240 240 120 120 pH of the mixture of leaching step 0.57 0.73 0.48 0.90 0.64 0.60 0.07 H2S04 consumption (kg/trm) 1 090 1 660 1 660 1 100 2 210 1 770 920 HCI consumption (kg/trm) 350 0 0 0 0 0 0 NaCI consumption (kg CI-/trm) 0 190 760 285 285 285 285 Volume of the mixture of leaching step (niL) 222 215 215 240 240 2000 200 Volume of the leachate obtained (Lx) (mL) 155 170 165 184 165 1 480 85 Volume of washing water (EI) (mL) 150 150 150 155 155 905 130 Volume of the coagulating solution (Lx + El) 305 320 315 339 320 2 385 215 Concentration of Fe in the sln coagulant (g/L) 25.8 22.2 25.7 12.6 25.8 31.6 33.9 Concentration of A1 in the sin coagulant (g/L) 11.4 10.1 8.5 8.5 9.4 12.0 12.7 Mass of residual red mud (g) 18.4 26.7 23.3 22.7 20.8 240 22.5 Mass of coagulant solid (g) - - - - - 824 50.2 Conc. of Fe in the solid coagulant (g/kg) - - - - - 91.8 106.9 Coiic. of Al in the solid coagulant (g/kg) - - - - - 33.0 41.0 Total yield of extraction of Fe (%) 71.1 64.1 73.1 35.8 69.3 74.5 59.7 Total yield of extraction of Al (%) 69.3 64.6 53.5 67.7 70.1 73.7 65.4 * Tlris test was cart-ied otit with wet red mud (total solids of 52% w 1v -1), egtiivalent to 40 g of dr-ied red mud.
rm- red ntud Example 2: Use of coagulation agent for dephosphatation of wastewater The liquid or solid coagulant obtained following the process of transfonnation of red mud can be used to replace the usual cheniicals, such as alum, ferric chloride or ferric sulphate, employed as coagulation agents in the field of wastewater treatment.
Comparison tests of dephosphatation of wastewater were carried out with the solid coagulant (JI test) for the removal of phosphorus from synthetic effluents and wastewater coming from municipal wastewater treatment plants. The cliemical composition of the given J I
coagulant was determined by X-fluorescence after dissolution in demineralized water and is presented in Table 3. A colorimetric method also showed that 97% of iron present in the coagulant is in the ferric fonn (Fe3+). The coagulant is highly water soluble.
A solution of 35 g/L was thus prepared in demineralized water.

Table 3 Composition of solid coagulant (J1 test) Elements (mg/kg) Analysis by ICP-AES Analysis by X-fluorescence Aluminium 33 000 34 700 Cadmium 0.296 -Calcium 1 830 1 570 Cliromium 347 274 Copper 5.95 -Iron 91 800 81 100 Lead 9.12 -Magnesium 143 < 603 Manganese 43.7 < 77.4 Nickel 23.8 -Pliosphorus 351 349 Potassium 129 166 Silica - < 467 Sodium 81 300 92 000 Sulfur 214 000 -Titanium - 1 200 Zinc 27.4 -Dephosphatation tests were in particular carried out on syntlletic effluents (monobasic potassium pliosphate solutions: KH2PO4) containing phosphorus concentrations ranging from 5 to 100 mg P/L. The coagulant, JI like conunercial coagulants, namely, aluminum sulphate (liquid alum, A12 (SO4)3 14H20), liquid ferric chloride (FeCI 3) and ferric sulphate (Fe2 (SO4)3) were diluted in order to obtain solutions containing roughly 0.1 molar equivalent of coagulant per liter of solution (eq./L), an equivalent referring distinctly to a mole of iron and/or aluminum. Thus, for the J1 agent, it was necessary to dissolve 35 g per liter in order to achieve the desired concentration. To make sure that there was no precipitation of metals with time, the pH was measured. As the solutions of coagulants mentioned previously possessed pH of 1.61, 2.96, 1.35 and 1.80, no addition of acid was carried out to improve their stability. The coagulants were added in order to obtain approximate ratios of 0, 0.5, 0.75, 1.0 and 1.5 of eq./mol-P.
The exact ainounts were calculated using the real concentrations of the effluents and the coagulating solutions. The tests were carried out with 100 ml volumes of synthetic solutions in which the coagulants were added. The solutions were agitated for a period of 30 min, and then letting it rest for a period of 60 min. Supernatant volumes of 15 ml were taken and filtered on a 0.4 m polycarbonate membrane. The samples were analyzed by ICP-AES.

As an example, with an amount of 1.5 eq./mol-P in an effluent containing 100 mg P/L, the phosphorus removal was 98% for the JI coagulant and 96, 93 and 95%, respectively for alum, ferric chloride and ferric sulphate. While considering the same proportion of coagulant, but for a solution containing 5 mg P/L, an elimination of 70% of the phosphoi-us was measured in the case of JI, whereas the yields with other coagulants were 66, 70 and 60%, respectively.

Finally, Table 4 shows the removal of phosphorus from synthetic effluent expressed in tenn of the molar ratios of phosphorus removed/coagulant added. This table shows that the coagulant produced from red niud has a dephosphatant capacity equivalent to the commercial coagulants.

Table 4 Average values of the molar ratios [pliosphorus removed/coagulant added] for each synthetic effluent and eacli amount of coagulant Initial concentration of P in the solution (mg/L) Coagulant Dose 5 10 25 50 l00 (eq./mol-P) Molar ratio [P removed/coagulant addedl (mole/mole) 0.52 0.53 0.71 0.73 0.74 0.82 0.79 0.59 0.65 0.71 0.69 0.79 1.05 0.52 0.62 0.65 0.71 0.76 1.57 0.46 0.51 0.56 0.54 0.62 0.50 0.54 0.64 0.70 0.71 0.80 0.75 0.56 0.61 0.65 0.72 0.77 Alum 1.00 0.51 0.60 0.65 0.65 0.73 1.49 0.46 0.54 0.56 0.57 0.63 0.48 0.58 0.57 0.67 0.76 0.86 0.72 0.52 0.46 0.66 0.70 0.76 FeCI3 0.96 0.54 0.61 0.65 0.65 0.73 1.44 0.51 0.46 0.56 0.58 0.64 0.47 0.42 0.51 0.69 0.76 0.84 0.71 0.35 0.53 0.68 0.72 0.81 Fe2(SO4)3 0.95 0.51 0.56 0.66 0.71 0.77 1.42 0.45 0.51 0.59 0.57 0.66 REFERENCES

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Claims (17)

1) A process for producing a coagulating agent rich in iron and/or aluminum, the process comprising the steps of:

a) mixing the red mud with an acid mixture (NaCl and sulphuric acid) preferably ranging between 500 and 3000 kg of acid per ton (dry basis) of treated red mud;

b) adjusting the red mud contents of the mixture by water addition so as to obtain a red mud concentration lower than 400 g per liter of solution;

c) adding a chloride salt, preferably, sodium chloride, so as to obtain a chloride contribution ranging between 150 and 800 kg per ton (dry basis) of treated red mud;
d) mixing the resulting solution for a period of 0.5 to 8.0 h in order to adequately solubilize iron and aluminum present in red mud. The mixture is maintained at a temperature lower than 250°C and preferably at a temperature close to 110°C;

e) separating residual red mud from the liquid fraction;

f) optionally washing residual red mud in order to recover solubilized iron and aluminum with defined volumes of water;

g) optionally mixing the leachate(s) and the washing (s) so as to obtain a liquid coagulant solution;

h) optionally, the coagulant solution could be subjected to a step of evaporation and drying in order to obtain a solid coagulant.

2) The process according to claim 1, characterized in that the addition of the acid, the chloride salt and water, can be carried out in a different order.

3) The process according to claim 1 or 2, characterized in that the acid used is an acid other than sulphuric acid, that is to say for example, hydrochloric and nitric acid.

4) The process according to any one of claims 1 to 3, characterized in that the content of red mud of the mixture is adjusted, in a preferential way, with contents ranging between 150 and 250 g per liter of solution.

5) The process according to any one of claims 1 to 4, characterized in that the chloride salt can be in particular sodium chloride, calcium chloride, magnesium chloride and potassium chloride.

6) The process according to any one of claims 1 to 5, characterized in that the chloride salt can be replaced by hydrochloric acid as a source of chloride.

7) The process according to any one of claims 1 to 6, characterized in that the solubilization of iron and aluminum of red mud is carried out in two steps or more.

8) The process according to any one of claims 1 to 7, characterized in that the solubilization of iron and aluminum is operated in semi-continuous or continuous mode in tank reactors.

9) The process according to any one of claims 1 to 8, characterized in that the separation of residual red mud from the liquid fraction can be done by decantation, filtration, centrifugation, or any other common technique of solid and liquid separation.

10) The process according to any one of claims 1 to 9, characterized in that the separation of residual red mud can also be carried out by an organic polymer addition, or any other agent of flocculation.

11) The process according to any one of claims 1 to 10, characterized in that the washing of residual red mud can be done with water, or a dilute acid solution, or an alkaline solution.

12) The process according to any one of claims 1 to 11, characterized in that the washing of residual red mud can be done by rinsing of the solid residue resulting from a filtration step, or, by mixture of the solid residue re-suspended in the washing solution followed by a step of solid and liquid separation.

13) The process according to any one of claims 1 to 12, characterized in that the washing of residual red mud can be carried out with one or more recoveries.

14) The process according to any one of claims 1 to 13, characterized in that washed residual red mud can be neutralized by the addition of an alkaline agent or by mixture with untreated red mud.

15) The process according to any one of claims 1 to 14, characterized in that the liquid coagulant can be only made up of the generated leachates(s) at the time of solubilization of iron and aluminum.

16) The process according to any one of claims 1 to 15, characterized in that the liquid coagulant can undergo subsequent steps of purification or concentration.

17) The process according to any one of claims 1 to 16, characterized in that the solid coagulant obtained by evaporation of the liquid fraction can be further dried and crushed before being used.