1. INTRODUCTION:

Aluminium is the earth’s third most abundant element and is one of the most widely used metals with a variety of applications including transports, construction, packaging, and other appliances due to its diverse properties. Aluminium is produced by the smelting of alumina through the Hall-Héroult process ⁽¹⁾. Alumina consecutively is produced from bauxite ore through the Bayer process, invented by Karl Bayer in 1887, which remains the most economical process till date ⁽²⁾. The process of extraction of alumina from bauxite is carried out through digestion of bauxite with sodium hydroxide at elevated temperature (106-240°C) and pressure (1-6 atm). It is a process of separating alumina from undesired components like oxides of iron, titanium, silicon, calcium, vanadium, manganese, etc. in bauxite. After extraction, the insoluble/waste product generated is known as ‘red mud’ or ‘bauxite residue’ which derives its color and name from its iron oxide content. For every ton of alumina produced, 2-3 tonnes of bauxite ore need to be processed. Fig.1 shows the schematic diagram of the Bayer process.

Fig. 1: Schematic diagram of the Bayer process

About 95% of the world’s alumina is produced from bauxite using the Bayer process, out of which approximately 90% of alumina production is of the metallurgical grade for regular production of aluminium metal. Balance 10% alumina produced is of non-metallurgical grade or also called special alumina which finds applications in different specific areas/fields. Although the total requirement of these special products is minimal, they are value-added and give multiple returns. It can be produced in an existing alumina plant by some additions, modifications, or treatment during the production stages. Special grade hydrates/aluminas differ from metallurgical grade alumina in its physical and chemical properties mainly related to grain size, surface area, degree of calcination, and purity. Aluminium hydroxides (alumina trihydrate) and calcined alumina used for these non-metallurgical purposes are termed as special grade hydrates and special grade alumina respectively ⁽³⁾. There are various types of hydrates and aluminas which have been classified according to their properties and applications.

Special hydrates find applications in chemicals, cables, pharmaceuticals, cosmetics, toothpaste, paper/ rubber/ plastics, glass, printing inks, textile, catalyst industries, whereas special alumina is used in petrochemicals, industrial gases, water treatment, artificial gems, abrasives, ceramics, refractories, electrical insulators, spark plugs, etc. ⁽⁴⁾ Alumina of 3N purity (99.9%) with very low caustic soda content (Na₂O <0.1%) are used as advanced materials in refractories and electronics having hi-tech applications and are also known as high purity alumina. Caustic soda acts as an impurity and hence it is important to know the origin, mechanism, and factors affecting its formation in alumina hydrate.

2. TYPES OF SPECIAL HYDRATE AND ALUMINA AND THEIR FIELD OF APPLICATIONS:

Special hydrate or aluminium hydroxide is produced in a wide range of particle sizes and chemical grades. It is used as fire-retardant filler in the manufacture of carpets, foams, rubber, PVC cables. It is also used in paint, paper, inks, plastic, glasses, and in manufacture of chemicals such as aluminium salts, aluminium sulfate, etc. Activated aluminas are obtained by thermal dehydration of different aluminium hydroxides in the 250-800°C temperature range. These are highly technically important alumina chemicals. These are highly porous, high surface area materials and are used as an adsorbent, catalyst, etc. in petrochemicals industries and for drying various liquids and gases. Aluminium hydroxide calcined to a high temperature is called alumina. Special hydrates are the precursor material to produce aluminas/calcined aluminas. Calcined alumina is the most widely used oxide ceramic material. Its applications are diversified being used in spark plugs, tap washers, pump seals, electronic substrates, grinding media, abrasion-resistant tiles, cutting tools, bioceramics, (hip-joints), body armor, laboratory ware and wear parts for the textile and paper industries. Large quantities are also used in the manufacture of monolithic and brick refractories. Calcined aluminas are classified according to the Na₂O content and crystal size. These are: normal soda (0.25-0.40% Na₂O), intermediate soda (0.10-0.20% Na₂O) and low soda (<0.1% Na₂O) ⁽⁵⁾. Different crystal size alumina can be generated by grinding these aluminas. Their applications depend upon their properties. Normal soda aluminas are used in abrasives, glass, electrical porcelains, sanitary wares, and the production of Tabular and White fused aluminas. Aluminas having intermediate soda content are used in electrical insulators, spark plugs, hi-tech refractories, and electronic ceramics. Low soda aluminas are used in ceramic oxides and sodium vapor lamps and advanced electronic applications. Many applications, particularly in the electrical/electronic areas, require a low level of soda to be present in the alumina. Low soda alumina is generally defined as alumina with caustic soda content of <0.1% by weight.

3. MAIN IMPURITIES IN ALUMINA AND THEIR ORIGIN:

The main impurities in alumina are Fe₂O₃, SiO₂, CaO, organic impurities, and Na₂O. About 200 ppm (0.02%) of Fe₂O₃, 100 ppm (0.01%) of SiO₂, 100 ppm (0.01%) of CaO, 3000 ppm (0.3%) of Na₂O and organic impurities in traces are associated with alumina⁽⁶⁾ . During cooling, iron, silica, and CaO precipitate from the liquor in presence of seed (aluminium hydroxide). Iron can be reduced by a pre-crystallization step using pre-seed. Due to the kinetics of precipitation of colloidal iron which is faster than that of alumina, most of the soluble iron will precipitate on this pre-seed. The pre-seed can then be removed by hydro-cyclone and aluminate liquor can be used for precipitation. Organic impurities come into the liquor from the organic substances getting dissolved during digestion and affect the whiteness of the product ⁽⁶⁾.

High purity hydrates/aluminas are mainly recognized based on one of the main impurities, caustic soda (Na₂O%) present in it. It has an undesirable effect on alumina properties when used in these specific applications. Hence it is important to know the origin, mechanism, and factors affecting the formation of caustic soda. The soda impurity varies from about 0.2 to 1% by weight as Na₂O in alumina ^(7,8). The term free soda is used to represent the sodium carbonate equivalent of caustic soda present in solution both as free caustic and in combination with alumina as sodium aluminate ⁽⁹⁾. In addition to the caustic soda (both free and combined), there is usually present some free sodium carbonate in the sodium aluminate solutions. The free sodium carbonate content varies depending on the source of the sodium aluminate solution. Thus, in the usual Bayer sodium aluminate solution the ratio of free soda i.e. caustic soda expressed as Na₂CO₃ to total soda (i.e. free soda plus dissolved sodium carbonate) may vary from 0.7 to 0.85. This ratio may be as high as 0.9 in sodium aluminate solutions from the lime-soda sinter process and almost 1 in synthetic sodium aluminate solutions prepared by digesting alumina hydroxide with caustic soda. The crystalline structure of alumina hydroxide entraps most of the caustic soda. The caustic soda content in the hydrate would be less if the growth of the crystals is slow. This can be obtained by increasing the temperature of precipitation and by lowering the supersaturation, but at the cost of drop-in productivity and in yield.

4. EFFECT OF CAUSTIC SODA ON ALUMINA PROPERTIES:

The caustic soda impurity is lethal for many applications of alumina and especially reduces the electrical insulation qualities of alumina porcelains ⁽⁷⁾. These investigations carried out at Aluminium Co. of America, Pittsburgh show that soda if present from 0.2-1% is disastrous to many applications of alumina and the impurity particularly reduces the electrical insulating properties of alumina porcelain. Sodium above about 0.20% by weight (expressed as sodium oxide) has been found to reduce the initial activity, the initial selectivity, and/or the thermal stability of the composite catalyst ⁽¹⁰⁾. As sodium even in trace amounts can adversely affect the properties of semiconductor films, soda content in alumina should be at its minimum. One of the important properties of alumina applications in electrical and electronic ceramics is its electrical resistivity. Alumina is a very good electrical insulator as it retains high electrical resistivity to very high temperatures. Lowering the caustic soda content of Bayer alumina improves their ceramic properties. Caustic soda reacts with Al₂O₃ at high temperatures to form β- alumina of the composition 11Al₂O₃. Na₂O. The presence of this compound lowers ceramic properties and affects grinding and fabrication operations. Caustic soda also volatilizes at high temperatures and creates porosity which lowers mechanical strength properties. In alumina ceramics used in electronic devices, alkali metal ions such as Na⁺ decrease the insulation (electrical resistance) properties of the ceramics. Hence alumina with low caustic soda is required to increase the electrical resistivity of these ceramics for its use as electrical insulators ⁽¹¹⁾. When aluminum oxide is used as a ceramic raw material for electrical insulators of IC boards or spark plugs, the inclusion of a soda component is not preferable, since the caustic soda component can cause insulation defects and other problems ⁽¹²⁾.

Thus, several techniques have been proposed to remove the soda component in the alumina. Low soda alumina can be manufactured by many different routes including acid washing, chlorine addition, boron addition, and utilization of soda adsorbing compounds and controlling the precipitation temperature ^(6,
13).

5. MECHANISM OF SODA OCCLUSION:

The Kossel model of precipitation states the mechanism by which solute clusters attach themselves to the surface of a seed crystal. This involves entrapment of mother liquor containing Na₂O, when super-saturation is high, and the temperature is low. Numerous clusters absorb improperly due to low surface diffusion rates forming a high caustic soda hydrate ⁽¹⁴⁾. Raising the temperature quickly and reliably controls fluctuations in the caustic soda content of the pregnant liquor. Alternatively, the A/C ratio (supersaturation) may be easily adjusted. Both practices negatively influence economic production. It is believed that the caustic soda content is related in some way to the precipitation rate and hence to the diffusion rate of sodium ion. It is also related to the solubility of sodium organic compounds usually found in the Bayer liquor ⁽¹⁵⁾. Caustic soda content in alumina trihydrate was studied by investigators and found that a high concentration of sodium aluminate liquor, ground seed, and low temperature increases the soda content in alumina trihydrate ⁽¹⁶⁾. Low temperature increases the precipitation rate thereby increasing the lattice soda content.

6. SPECIALTY ALUMINA MARKET:

The demand for the specialty alumina market is in thousands of tons and not millions of tons. The demand for low soda levels in Bayer alumina is increasing and has become stronger. Manufacturers of electro-ceramics, refractories, and various alumina salts require less than 0.1% soda content in a hydrate ⁽¹⁷⁾. Purity wise most grades require Fe₂O₃ and SiO₂ less than 0.01% and soda less than 0.35%. Some special grade products require super purity Fe₂O₃ and SiO₂ less than 0.006% and soda less than 0.1% called the low soda product ⁽¹⁸⁾. Pure-white low soda hydrates are used as flame retardant fillers in soft polyurethane foams, plastic electric parts, and as fillers in synthetic marble and onyx ⁽¹⁹⁾.

Low caustic soda alumina has excellent characteristics in chemical stability, electrical insulation, heat resistance, thermal conductivity, hardness, and mechanical strength, and is widely used in electronic parts, spark plugs, and machine parts, as well as ceramics cutting tools. Firing shrinkage and sintering characteristics are controlled to meet the requirements of applications. Prices of special grades of alumina are given in Table 1 ⁽⁴⁾.

Table 1: Prices of special grades of alumina

Variety	Number of times the special grade is costlier than smelter grade	Specific variety
Activated	5.9-8.2
Catalyst carrier	8.2-26.5
Abrasive grade	2.1-3.6
Refractory	1.5-3.3	4.9
Ceramic grade	2.1-3.6	4.9
Spark plugs	20
Electronic Ceramics	5

Source ⁽⁴⁾

7. MANUFACTURING PROCESSES:

The production of low soda hydrate/alumina is a closely guarded technology as most of the work is patented by the researchers.

A patent mentions a method of manufacture of low-soda alumina granules suitable for use as an alumina porcelain insulator and alumina catalyst carrier which comprises dehydrating aluminum hydroxide at a high temperature and then granulating it ⁽²⁰⁾. The granules thus obtained are placed in an autoclave, washed and cured with water drops in manufactured equipment, then taken out of the autoclave dried, and fired. An inventor of Alcan International Limited has developed an improved process for producing low soda alumina wherein a process has been developed for precipitating alumina hydrate from aluminate liquor which is divided into minor and major streams ^{(21, 22)}. The minor stream is seeded with fine seed for agglomeration and the major portion is charged with coarse seed for the growth stage to induce the formation of alumina hydrate product. The minor portion slurry is cooled to 45-60°C and solids are separated. The liquid after separating the solids is charged with coarse seed to generate fresh hydrate nuclei and the slurry is combined with the major portion of the stream. A similar process has been developed wherein low soda alumina hydrate (0.22%) has been produced in the precipitation process having an agglomeration stage and a growth stage ⁽²³⁾. The slurry is passed through multiple growth tanks wherein the temperature of about 75°C to 85°C is maintained in the agglomeration stage and cooled by about 2°C till 55°C in the growth stage. The process has the potential of operating at an increased yield of 85-90 g/L. The soda content in an ultrafine alumina hydrate (0.2-1 micron size) with a low bulk density (0.15-0.35 g/cc) was reduced to 0.005- 0.02% by filtering and washing ⁽²⁴⁾.

A process has been invented in which soda reducing agents such as a silica-based substance are used. These are quartzite, quartz, silica sand, a shamotte, a mulite, a silimanite, magnesium silicate, an alumina silicate ⁽²⁵⁾. They are mixed with gibbsite aluminium hydroxide or the transition alumina having γ structure and is calcined. The resultant mixture is then separated into the soda-reducing agent and alumina with low soda content. Other inventors have also followed nearly the same process wherein silica-based material is added to produce friable, high alpha alumina with very small ultimate crystal size and very low soda content ⁽²⁶⁾. Low soda alumina was produced by calcining an alumina material at 1,100°-1,400° C followed by leaching ⁽²⁷⁾. Caustic soda was reduced from 0.25% to 0.1%. A relatively inexpensive process was developed for producing alumina of low soda content (0.02-0.04%) for high-grade ceramic applications such as spark plug insulators ⁽²⁸⁾. It involves a process in which alumina of any degree of hydration is calcined with a mineralizer such as aluminium fluoride (0.5-3%), washed with water or dilute acid (dilute HCl in the concentration of 0.01% concentration) or alkali (dilute solutions of sodium hydroxide and ammonium hydroxide in concentrations of 0.01%). Alumina hydrate is calcined with boric oxide (0.9:1 molar ratio of boric oxide to sodium oxide)/boric acid solution to a temperature of 1300°C (effective volatilization temperature) to volatilize the sodium borate ⁽⁷⁾. Sodium oxide in aluminium hydroxide combines with boric acid to form sodium borate. The hot gases going out of the kiln pick up the alumina dust containing sodium borate. Hence to make alumina dust free of sodium borate, it is leached with water to dissolve the sodium borate which is then mixed with the material being converted to alumina. To make the process more effective and to produce alumina of about 0.05-0.08%, the mixture was heated at a temperature enough to solubilize the sodium oxide, but below the effective volatilization temperature of sodium borate which is then leached to dissolve the sodium borate. A process invented relates to a method of preparing low soda content alumina trihydrate and low soda content activated alumina to be used as a catalyst or catalyst support and as a processing agent such as a dehydrating agent ⁽²⁹⁾. In this invention, the supersaturation is maintained at a relatively low level. The amount of alumina supersaturation is not greater than 0.13 ratio above the equilibrium solubility of the alumina trihydrate in the aqueous caustic solution from which the alumina is precipitated and an elevated temperature of 65-82°C. In another process to get low soda content alumina (0.05-0.1%) for ceramic products, particularly insulating bodies of spark plugs and electro porcelain wares, such as porcelain insulating bodies, technical grade aluminium hydroxide is activated to 500°-600°C for 10 to 30 min, washed with water containing Ca ions (calcium fluoride) and calcined it at 1200-1350°C temperature. A mineralizing mixture (0.4-0.7%, 0.3-0.45% by weight of boric acid, and 0.1-0.15% by weight of aluminum fluoride) was used during calcination. Calcium salt was added to the mineralizing mixture to improve the properties of the product. It was found that due to the addition of calcium ions, less angular crystals are formed, and hence on sintering, a uniform shrinking that is free of deformations takes place. For alumina to be used as electrical insulators and spark plugs, along with the reduced soda content, the physical properties such as firing shrinkage, modulus of rupture, resistance to thermal shock should also be satisfactory for which improved crystal structure is required ⁽³⁰⁾. Subsequently, the investigators have developed a process for 100% alpha-alumina having a greatly reduced soda contamination and containing substantially large monocrystals having an average crystal size in the range of 5-12 microns ⁽³¹⁾. In this, calcination is done using 0.5-10% chlorine (produces a crystal size of 1-3 microns which are useful for making dense ceramic) and fluorine compounds at 1200°C and a calcining time of 0.3-3.5 hours. Chlorine-containing materials may be chlorine (Cl₂); ammonium chloride; hydrogen chloride; aluminum trichloride and phosgene.

The need for high surface area alumina in the form of alumina trihydrates or alumina monohydrates arose due to the demand for high surface area fillers and reinforcing agents in plastics, coatings, and filler for high-grade paper, catalysts and catalysts supports. Inventors have used CO₂ in precipitation processes to generate alumina to be used for catalysts application ⁽⁹⁾. In this process, low soda alumina of 0.01% is achieved only by two items of washing with CO₂ precipitated hydrate. A method was developed to produce activated alumina free of sodium for catalyst support purposes ⁽¹⁰⁾. Gamma or activated alumina is subjected to calcination temperature of 370- 600°C for 2 to 5 hours and leached with a mild acid solution of acetic acid, boric acid, formic acid, lactic acid, oxalic acid. Heat decomposable salts of strong acids and weak bases, which on hydrolysis in aqueous solution give a mildly acidic reaction, such as ammonium chloride, ammonium nitrate, aluminum nitrate can also be used.

A method for producing low soda content aluminum hydroxide by investigators had the disadvantage of requiring expensive heat exchanging means to maintain the desired alumina supersaturation by controlled cooling and uneconomical liquor productivity⁽²⁹⁾. A patent by a researcher gives an improved process for the continuous precipitation to produce low soda content alumina hydrate (about 0.12%) from aluminate liquors ⁽¹⁵⁾. In the process, alumina of spherical shape for better flowability and easy handling, and high bulk density is obtained. In this invention, a slower precipitation rate is maintained as the precipitation takes place from a comparatively lower and constant liquor ratio (alumina to caustic expressed as alumina is expressed in grams per liter Al₂O₃, and caustic is expressed in terms of grams per liter equivalent Na₂CO₃) and a high temperature of 87-94°C. In conventional precipitators, as the liquor ratio is high, the precipitation rate is high and changes with time which results in changing precipitation rate and formation of irregular agglomerates. Inventors at Aluminium Co. of America have created a low soda product by controlled slow precipitation which is achieved by controlling the residence time (longer residence time kept) to provide constant low alumina to caustic ratio and by controlling green liquor flow rate to the tank or by sizing the tank for a constant flow ⁽³²⁾. The inventors claim that low soda aluminium hydroxide can be achieved using volumetric control instead of temperature control. A process has been invented at Alusuisse Martinswerk GmBH for the production of aluminium hydroxide of improved whiteness for application as filler in paper and plastics but does not state the soda content of the product formed ⁽³³⁾. The process involves heating aluminium hydroxide to a temperature of 300-700°C, dissolving its soluble fraction in sodium aluminate liquor to achieve a molar ratio of Na₂O: Al₂O₃ of 1.40 to 2.4, and getting an un-dissolved fine boehmite. The sodium aluminate solution is then filtered, cooled to 50 to 80°C, and crystallized with aluminium hydroxide crystals to get white products. A special apparatus was used for producing low soda alumina (0.01-0.1%) by researchers ^{(31, 12)}. The method is based upon sorting the alumina-based on particle size as finer particles contain more concentration of soda in it. The apparatus consists of calciner for calcining aluminum hydroxide at 1100°C in the presence of a soda-removal agent such as chlorine-based compounds, such as hydrochloric acid, ammonium chloride, magnesium chloride, and a mineralizer. The unit collects fine alumina dust having high soda alumina and low soda content product in another unit. The low-soda alumina thus produced has wide applications in ceramics used in IC board and circuits, spark plugs, and semiconductors with an enhanced industrial value. A process has been developed for producing low soda alumina of crystal size 0.4-10 micron size and soda content of less than 0.1% by adding a soda removal agent to the alumina source material and calcining ⁽³⁴⁾. The advantage of the process is that it removes the concentrated fluorine and other mineralizing components from the system. The material is to be used in producing porcelain. Producing porcelain comprises the steps of shaping alumina source material by adding a flux followed by sintering. A process of using aluminium hydroxide having a total soda content of 0.1 mass % and precipitating aluminium hydroxide under the conditions of a supersaturated Al₂O₃ concentration of about 20g/L to produce low soda gibbsite form aluminium hydroxide having an average particle size of 5 microns or smaller have been developed ⁽³⁵⁾.

Work on using acids and other chemicals/materials for producing low soda alumina have also been undertaken as early as 1946. Low soda alumina having a value as low as 0.05% was produced by washing and calcinations ⁽³⁶⁾. The powder was kept in 4% solution of HCl at about 83°C for 5 h, washed with water several times to wash off the acid, calcined at 400°C, leached with sodium bicarbonate solution at 83°C by keeping it in the solution for 5 h, again washed several times with water and calcined at 550°C. The process involved several leaching and washing steps to get a low soda product with 0.03% of chloride remaining in it. Work carried out on high purity alumina by researchers states that soda content can be reduced by 98% in which alumina trihydrate is reacted with concentrated HCl (15-30% by weight) to form aluminium chloride hexahydrate ⁽³⁷⁾. The solid aluminium chloride hexahydrate and unreacted aluminium hydroxide are calcined in single or multiple stages for getting high purity alumina. The acid is recycled after the solid is separated. The purified product is used in the specialty ceramics field, as catalyst supports, as adsorbents, in electronic components, in prosthetic devices, or other applications. Soda levels of 0.002-0.003% are achieved by redissolution and re-solidification of aluminium chloride hexahydrate by hydrochloric acid addition or gas sparging with HCl. Investigators studied that alumina having a soda level of 0.02% is produced by calcination of gibbsite in presence of a substance containing at least 10% silica at 1260°C ⁽²⁶⁾. The caustic soda reacts with silica particles which are then separated by screening from the alumina. The product also contains about 0.02% silica in it. Low soda hydrate or alumina has also been produced by reacting aluminium metal with alcohols or amines which is then activated by moderate calcining to produce activated alumina.

National Aerospace Laboratories, Bangalore, India have patented technology of producing lightweight alumina trihydrate with low soda (<150 ppm) for filler applications in paper, plastic, rubber, paint industries, and as a flame-retardant material ⁽³⁸⁾. Altech has developed a process of producing 4N high purity alumina with ore feedstock kaolin or aluminous clay using hydrochloric acid leach process with effective acid recovery ⁽³⁹⁾. A process has been developed by JNARDDC in 2014 wherein low soda hydrate containing Na₂O<0.1% (3N purity, 99.9%) can be produced in the precipitation process by controlling precipitation parameters and adding specialized aluminium hydroxide seed. Specialized aluminium hydroxide seed is developed using calcination and several washing steps. It was established in the work that surface properties of seed hydrate change the product quality and yield in the precipitation process.

8. FACTORS AFFECTING THE FORMATION OF LOW SODA PRODUCT:

Precipitation process, washing, and addition of mineralizer affect the properties of the product formed. Various parameters in the precipitation process during hydrate formation in the Bayer process such as temperature, liquor concentration, seed surface area highly influence the caustic soda content of the alumina hydrate formed. After aluminium hydroxide formation, washing of the product is an important aspect. During calcination to form alumina, the addition of mineralizer plays a significant role.

8.1 Precipitation process:

The overall precipitation process affects product quality and research is being conducted on improving it. The factors in the precipitation step which affect the properties of the product, particularly the caustic soda content in the alumina hydrate are:

8.1.1 Supersaturation:

It was investigated as early as 1965 that the greater the degree of supersaturation, the faster will be the rate of precipitation as equilibrium solubility concentration is the driving force for the alumina trihydrate precipitation ⁽²⁹⁾. Solubilities and concentrations of alumina trihydrate in aqueous caustic solutions are given in terms of an alumina/ caustic ratio (liquor ratio). Faster precipitation accounts for more soda occlusion in alumina hydrate. Hence, the higher the supersaturation, the more will be the soda content in the product.

8.1.2 Temperature:

Low sodium oxide content is normally achieved using temperature control by raising the precipitation tank to an elevated temperature thus lowering the degree of saturation resulting in a lower precipitation rate⁽³²⁾. A considerable amount of energy would be required to maintain high temperatures in a large tank. It thus would be desirable to achieve such controlled precipitation at a low aluminum oxide to sodium oxide ratio without the need for such elevated temperatures.

Temperature also influences the rate of precipitation. Precipitation at elevated temperatures of 65-82°C produces a low soda content alumina hydrate ⁽²⁹⁾. However, a patent has stated that the reason for the low soda content of the precipitated alumina hydrate with an increase in feed liquor temperature is not fully understood⁽¹⁵⁾. It is believed that the soda content is related in some way to the precipitation rate, to the diffusion rate of sodium-ion from the alumina hydrate, and the solubility of sodium organic compounds usually found in the Bayer liquor.

8.1.3 Seed:

Increase in surface area or sites is provided by the amount of alumina trihydrate seed charge. Hence increase in seed charge increases the overall rate of precipitation⁽²⁹⁾ . The use of heavy seed charges to increase the seed surface area permits the production of a low soda product at a faster precipitation rate⁽³²⁾.

8.2 Water/acid washing:

Sodium hydroxide cannot be completely washed out of the hydrate by repeated water washing. Washing of hydrate can be made more effective by using dilute acid only once during the leach and then again washed with water although it cannot be removed by acid washing also. On the contrary, severe washing of alumina with strong acids is susceptible to dissolving a portion of alumina and seriously damage its structure ⁽¹⁰⁾. The presence of even a small amount of acid is disagreeable if it is used as a catalyst or catalyst support. Acid ions are also detrimental to the process as they affect the final strength of the particles and their resistance to erosion during calcination.

8.3 Mineralizer:

The addition of a mineralizer such as aluminum fluoride not only lowers the effective temperature of conversion to the alpha form but also results in the production of a single crystal in the distinct form of hexagonal plates which is also called recrystallized alumina. The recrystallized alumina is leached with plain water/dilute acid/dilute alkali to get the low soda content alumina ⁽²⁸⁾. The aluminium fluoride is fed to the hot end of the calcining kiln to improve the soda volatilization. A process patented uses boric acid at 1100-1200°C in an amount of 1-2 moles related to 1 mole of the sodium content expressed in Na₂O which combines with soda in the hydrate to form sodium borate and goes out with kiln gases along with alumina dust⁽⁷⁾. It was found that the volatilized sodium borate causes disintegration of kiln lining and alumina contamination with an increase in median crystal size of about 6 to 10 microns of alumina particles. Hence the mixture was washed after calcination Researchers have used 5-7% of diammonium hydrogen phosphate and 5-10% of oxalic acid respectively before calcination and a mixture of boric acid and aluminum fluoride as a mineralizer⁽³⁰⁾. A patent states that brick scraps have been used in the calcination of aluminium hydroxide at 1200-1550°C⁽²⁶⁾. Researchers using chlorine and fluorine compounds in their process expressed that at high temperatures of calcination, the soda content is reduced due to the chlorine hydro pyrolysis products formed by the reaction of the soda contaminants in the alumina to form volatilizable sodium compounds⁽³¹⁾. These compounds get volatilized during the calcining operation and go out along with the hot combustion gases of the calciner. Aluminum fluoride, hydrogen fluoride, ammonium fluoride, sodium fluoride, magnesium fluoride, and calcium fluoride are the fluorine-based compound used as mineralizer ⁽¹²⁾.

9. DISCUSSION:

Low soda hydrates are used as flame retardant fillers in soft polyurethane foams, plastic electric parts, and as fillers in synthetic marble and onyx. They are also the original materials for making activated alumina which is used as catalysts support and as oxide ceramics. In high tech applications such as the production of electro porcelains and high voltage supporting insulators, alumina having a soda content of less than 0.1% is required. They are ground or super ground according to their applications.

Different types of special hydrates and alumina and their field of applications, main impurities in alumina, and the effect of soda on alumina properties have been discussed comprehensively in the paper. As has been reviewed from the literature from the past 50 years, researchers have developed the product generally for their in–house use, and the technology developed is much guarded and has been mostly patented. Methods have been developed for the production of low soda hydrate, low soda activated alumina, and calcined low soda alumina. The paper has reviewed the manufacturing processes carried out for the development of these products which have extensive economic output. The mechanism of soda occlusion takes place during the precipitation stage. Low soda hydrate can be developed in the precipitation stages of the Bayer process by diversifying one of the aluminate liquor streams for maintaining the precipitation conditions required for it. High temperature, low supersaturation, constant liquor ratio, seed surface area is to be maintained during the process. However, low soda alumina can also be manufactured by successive washings and calcination. The mechanism of soda occlusion needs to be studied more specifically. The addition of mineralizers is favorable as sodium compounds formed during calcination volatilize or can be washed. Improvements in the alumina refinery can be made by focusing on the product quality and its market. Though several methods have been developed, one of the best options appears to be the development of low soda hydrate by making modifications in the process step, by using specialized seed, and by controlling the precipitation parameters.

10. CONCLUSION:

Different grades of low soda hydrates and alumina (3N, 99.9% purity) and having Na₂O content <0.1% can be produced in an alumina refinery by controlling the precipitation parameters, most importantly being supersaturation of liquor and temperature, calcination and washings of the product and by also using specialized aluminium hydroxide seed. Low soda hydrates are precursor materials to low soda activated alumina and low soda calcined alumina. Low soda or high purity alumina is advanced material having applications in electronics and high-end refractories and ceramics. Washing of the hydrate and calcination at high temperature with suitable and effective mineralizers yields alumina products with very low caustic soda content. Moreover, depending on the technological needs of the users, different quality aluminas can be manufactured in an existing alumina refinery.

11. CONFLICT OF INTEREST STATEMENT:

On behalf of all authors, the corresponding author states that there is no conflict of interest.

12. REFERENCES:

[1]. Hall Heroult process. http://en.wikipedia.org/wiki/Hall–Héroult_process.

[2]. Bayer Process. http://en.wikipedia.org/wiki/Bayer_process.

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Received on 10.12.2020 Accepted on 28.12.2020

Research J. Engineering and Tech. 2020;11(4):169-177.

DOI: 10.5958/2321-581X.2020.00027.6