1. Introduction

The research in the field of alumina is very interesting and encouraging as plenty of raw materials are available, which have to be exploited for newer applications as well as recovering the metal at a competitive cost. The process for the precipitation of boehmite (Al₂O₃.H₂O) from sodium aluminate liquor instead of regular gibbsite (Al₂O₃.3H₂O) precipitation is one such innovative option where reduction in the energy consumption can be investigated. The precipitation of crystalline boehmite from supersaturated sodium aluminate liquor was investigated[1-8] to ascertain its applicability. As the properties of the hydrated alumina can be steered by changing the method of preparation, more work can be done in this area. Variation in the condition of precipitation such as pH, temperature, ageing conditions etc. produces aluminium hydroxides of various compositions, structures, morphologies, etc[9-16]. Many investigators have also tried to modify the Bayer's process as well as to utilize raw materials other than the bauxite ore. Utilization of secondary sources of alumina is a search for alternate source to be used in future. Some of the attempts are already made in these directions[17-20].

The paper deals with the precipitation of boehmite. The preparation of activated alumina from waste aluminium dross, which can be considered as an alternate secondary source, is a first step in advancing towards “from waste material to value-added products.” Aluminium salts solutions are also having practical use of making different varieties of hydroxides and activated materials.

2. Experimental

2.1. Materials

Gibbsite was obtained from M/s National Aluminium Company, Bhubaneswar, India. Boehmite seed was prepared hydrothermally from the supplied gibbsite at 195°C for 4h. Aluminium granules and sodium hydroxide (AR) obtained from Merck, India. The additives like tartaric acid, oxalic acid and EDTA (Rankem, India), succinic acid, salicylic acid, glutaric acid and citric acid (Acros Chemical, India), aspartic acid, xylose, glucose and glycerol (Merck, India) were used in the precipitation process. Aluminium sulfate heptahydrate and 25% ammonia solution, used as the starting material, were supplied by Merck, India. Aluminium dross is a waste material obtained from aluminium melting plants. The <850 μm (Tyler 20) size particles were taken for the study.

2.2. Method

The supersaturated sodium aluminate liquor was prepared by dissolving required amounts of aluminium granules in sodium hydroxide solution. The liquor was prepared for different Al₂O₃/ Na₂O (A/C) ratios. In general 150g/L Al₂O₃ solution with A/C ratios of 1.0 was prepared by taking 150 g/L of Na₂O (added as NaOH). Precipitation experiments were carried out in a 300 mL capacity stainless steel reactor having a fitting lid. Stirring speed of 250 ± 25 rpm was maintained in each experiment. To the aluminate liquor (100 mL) with pre-adjusted A/C ratio 100g/L boehmite seed was added and stirred continuously at stipulated temperature. The time period mentioned 8hrs except where it was varied. The additives were added at the time of seed addition.

Aluminium sulfate solution (0.2M) was neutralized with 5% aqueous ammonia by alternate addition of the reagents lik 1. Ammonia added to salt, 2.Salt added to ammonia. Three end point pH were selected for both the cases. Half of the samples were aged with a temperature of 150℃ for 4h. The samples are named as A, B and C for pH 5, 7 and 10 of ammonia added to salt systme and D, E and F for pH 5, 7 and 10 of salt added to ammonia system.

The experiments on the dissolution of dross in H₂SO₄ were carried out in a flat-bottomed glass reactor, which was placed on a magnetic stirrer with hot plate and temperature was maintained at 90 ± 2℃. The leach liquor was analysed by conventional EDTA–ZnSO₄ method to determine the amount of alumina extracted. Precipitation and ageing were carried out exactly like the aforementioned procedure except the sequence adopted here is ammonia added to salt not the other one. After filtration each of the mentioned samples were washed with distilled water and dried in a hot air oven at 80°C for 48 h.

3. Results and Discussion

3.1. Precipitation of boehmite

Precpitation of hydrated aluminas from sodium aluminate liquor is strongly dependent on A/C ratio which refers to the degree of supersaturation as the main driving force for precipitation. Seed provides surface on which newer crystals grow. Therefore, more seed amount and lesser seed size, more is the yield as illustrated in Figure1&2. When the A/C ratios varied from 0.75 to 1.2 it was found that at A/C ratio≥1 and when temperature was ≤80℃ the precipitated boehmite was mixed with gibbsite phase because at that condition supersaturation was more and gibbsite precipitation is kinetically more favoured (Figure 3).

Figure 1. Effect of seed quantity on boehmite yield, A/C = 1.0, 8h, 62.2μm (Dash et al., Hydrometallurgy, 2009)

Figure 2. Effect of seed size on boehmite yield, A/C= 1.0, 100 g/L, 8h (Dash et al., Hydrometallurgy, 2009)

Figure 3. Lowest temperature of boehmite (only) precipitation vs A/C ratio, 8 h, 100 g/L, 62.2 μm (Dash et al., Hydrometallurgy, 2009) View Within Article

To get only boehmite precipitation at temperature ≤80℃ without any gibbsite precipitation A/C ratios less than 1.0 is mostly favoured. But under this condition the yield of precipitation is less. So, for industrial practice yield has to be increased and to increase the yield the A/C ratio has to be increased. Here comes the real problem when A/C is higher and temperature is lower, gibbsite precipitation occurs along with boehmite. So, to restrict gibbsite precipitation some additives were selected which when used only boehmite is precipitated without the gibbsite precipitation even at higher A/C ratio and low temperature as mentioned. The details of the additives are mentioned in Table-1. (Dash et al., Dalton Trans, 2010)

Only four additives such as tartaric acid, xylose, glucose and starch are successful in resisting the precipitation of gibbsite with boehmite at 60℃. The reason behind the fact may be attributed to their stereochemical structures. All these four addives have minimum four vicinal –OH group and all the –OH groups are in threo position. This arrangement makes a stable complex where it may form a boemitic template so that preferably boehmite is precipitated. Alternative reason may be attributed to the formation of hydration shell around the precipitation site at the seed surface with the additives as all the shotlisted additives are hydrotopes. The ultimate result of the use of additive in a precipitation and the yield is shown in Figure 4.

Table 1. Effect Of Additives On Boehmite Precipitation, Conditions: A/C:1, Seed: 100gL-1, Additives: 5gL-1 Each. B: Boehmite, G: Gibbsite Additives Temp./°C Yield/gL-1 Phase Tartaric acid 70 5.6 B 60 4 B Oxalic acid 70 120 B+G 60 125 B+G Succinic acid 70 17 B+G 60 24 B+G Salicylic acid 70 2.3 B 60 10 B+G Glutaric acid 70 0.35 B 60 3.15 B+G Citric acid 70 1.9 B+G 60 3.5 B+G EDTA 70 50.75 B+G 60 60 B+G Glucose 70 2.5 B 60 22.4 B Xylose 70 1.8 B 60 3.35 B Aspartic acid 70 5.15 B+G 60 6.3 B+G Glycerol 70 1.7 B+G 60 70 B+G

Figure 4. Effect of precipitation time on boehmite yield, A/C = 1.1, temp.=80℃, tartaric acid=6g/L

3.2. Precipitation of Hydrated Alumina from Sulfate Salt of Aluminium

X-ray diffraction patterns of A₀ and B₀ (without ageing) were found to be broad indicating an amorphous structure and having gel like characteristics. Thus no crystalinity was observed in these cases. However, with higher ageing temperature like 150℃, the XRD-patterns of A₁₅₀ and B₁₅₀ showed sharp peaks indicating a crystalline structure as shown in Figure 5. Thus crystalinity was found to develop on ageing at higher temperature. The diffraction patterns of the materials obtained at pH 10 (C₀ and C₁₅₀) were found to be distinctly crystalline bayerite for both aged and unaged samples.

The conditions are different in the case when salt added to ammonia. The XRD pattern of all the samples (D, E and F) was found to be crystalline as shown in Fig.6. Even the freshly precipitated samples (D_0, E₀ and F₀) i.e. the unaged ones were also crystalline. The peaks are however broad enough showing low maturity in the crystal growth. When the materials were aged at 150℃ proper crystallinity was marked in the X-ray diffraction pattern.

It was observed that in the system where ammonia is added to salt the crystallinity is either a function of pH or ageing. But if the addition is reversed precipitate crystallinity was obtained.

Figure 5. XRD pattern of samples of ammonia added to salt without ageing (subscript 0) and with ageing at 150℃ (subscript 150)

The reason behind the difference in crystallinity with respect to sequence of reagent addition is attributed to the fact that when drops of alkali falls on the salt solution there exist competition between Al³⁺ ions to take the OH^- ions. That’s why if more and more OH^- ions are added more crystalline precipitate is formed. And reverse is the case when drops of salts fall into the large excess of ammonia for each Al³⁺ ion there are number of OH^- ions. There is no competition for OH^- ions. The aluminium ions transforms into fully crystalline aluminium hydroxide. That’s why all the samples are crystalline in salt added to ammonia system.

All the six aged samples from both the systmes are calcined at different temperatures ranging from 300 to 1300°C and it was found that at 900℃ η-alumina was formed and at 1300℃ α-alumina was formed. All the samples follow same trend of calcined phases.

Figure 6. XRD pattern of samples of salt added to ammonia without ageing (Subscript 0) and with ageing at 150℃ (subscript 150)

3.3. Precipitation of Hydrated Alumina from Waste Aluminium Dross

The typical dross sample taken was found to be containing 64.8% of alumina. It was found to be containing both alumina and Al metallic phases. The leaching with sulfuric acid brings the aluminium values in any form to ionic form in the leach liquor. The following reactions are supposed to be taking place-

(1)

(2)

(3)

Figure 7. XRD pattern of the precipitated aluminium hydroxide without ageing; : aluminium hydroxide, : iron hydroxide

Figure 8. XRD pattern of the precipitated and aged aluminium hydroxide ;aluminium hydroxide, : iron hydroxide

The optimized leaching conditions are 10% pulp density, 30% acid at room temperature for 1hr. The obtained leach liquor was then neutralized with 10% aqueous ammonia upto pH 5, 7 and 10 without ageing and aged at 150℃. Similar to the earlier case the unaged samples were less crystalline as compared to the aged one. As the solution was a leach liquor the aluminium was accompanied with iron also. So, peaks of iron hydroxides are also appeared with aluminium hydroxide (Figure 7&8). To remove iron from the solution the early formed precipitate at pH 4.5 was discarded where iron was removed from the system.The percentage of alumina present in precipitate of each pH is given in Table-2.

Table 2. Composition Of The Precipitates Sample Al2O3 (%) SO42- +H2O (%) pH-5 (aged) 29.84 67.92 pH-7 (aged) 33.76 63.04 pH-10 (aged) 47.22 45.68

The precipitated hydroxide at pH 10 was subjected to calcinations in the range of 300–1300℃. The transformation to η-alumina at 1100℃ and finally to α-alumina at 1300℃ was observed.