PRODUCTION OF ALUM FROM AWASO BAUXITE

Ing. Dr. Francis Acquah, Beatrice Mensah, Yaw Obeng

Institute of Industrial Research, CSIR, Accra, Ghana

Published in The Ghana Engineer, May 1999

Reprinted with GhIE permission by the African Technology Forum

 

ABSTRACT

 

The parameters, acid strength, leaching time, temperature solid to liquid ratio, and particle size of bauxite for the production of alum from Awaso bauxite were studied. The bauxite with low iron content was found suitable for the production of alum. The alumina ratio was 34.8.

 

1.0    INTRODUCTION

 

Bauxite deposits in Ghana occur mainly at Setwi Bekwai in the Western Region, Aya-Nyinahin in the Ashanti Region, Kibi and at Mount Ejuanema, both in the Eastern Region. The Setwi Belcwai or Awaso deposits have been reported to have very low iron content with some deposit having an iron content of 4.1-6.0% as Fe2O3, [1]. This deposit was considered a possible raw material for local production of alum. Table 1 gives the chemical analysis of bauxite from Mt. Supirri, Setwi Bekwai. The estimated reserves of Bauxite in Setwi Bekwai is 19 Million tonnes, averaging 49.0% alumina.

 

Table 1: Chemical Composition of Bauxite from Mt. Supirti, Setwi Bekwai. [1]

 

COMPOSITION

%
SAMPLE A

%
SAMPLE B

SiO2

0.64

0.40

A12O3

60.68

61.08

Fe2O3

4.2

4.1

TiO2

1.11

0.87

H20

32.17

32.23

CaO

0.33

0.25

V2O3

0.04

0.02

 

Alum is produced by the reaction of bauxite or clays with sulphuric acid, [2]. The quality of bauxite with respect to its impurities mainly iron influences the choice of process technology and the economies of production. The strength of Sulphuric acid, the reaction time, and the particle size of the bauxite depend on the nature of bauxite. Separation of dissolved iron is difficult during acid leaching. A low iron bauxite is usually preferred. Alum usually has 0.5% Fe203 maximum. In this study the optimum parameters for the production of alum from Awaso bauxite were investigated. Parameters investigated were the following:

 

1.             Acid concentration

2.             Leaching time

3.             Solid to liquid ratio

4.             Leaching temperature

5.             Bauxite particle size.

 

The Awaso bauxite contains predominantly gibbsite or trihydrate alumina in which alumina is present as A1203  3 H20. The alumina and iron oxide associated with the gibbsite react with sulphuric acid forming aluminium sulphate and ferric sulphate according to the equation:

 

A1203 3 H20 + 3 H2S04 = A12(S04)3 + 6H20

Fe2O3 + 3 H2504 = Fe2(S04)3+ 3 H20

 

Iron increases the acid consumption and also contaminates the alum. The leaching of the alum was therefore controlled to maximise the yield of alum and obtain minimum extraction of iron oxide.

 

2.0    EXPERIMENTAL PROCEDURES

 

Bauxite samples were supplied by Geological Survey Department. The samples were crushed and sieved in various mesh sizes.

 

2.1    Effect of sulphuric acid concentration on leaching

 

Twenty grams of 60 mesh bauxite sample was used. Acid corresponding to acid ratio of 1 mole of alumina to 2 moles of acid was diluted to 200 m1, mixed with the bauxite and boiled for 4 hours, filtered and the residue was washed to make up the leached liquor. Various acid ratios up to 1:10 were used.

 

2.2    Effect of leaching time

 

Acid ratio of 1 mole alumina to 4 moles of acid, diluted to 200 ml was mixed with 20 grammes of 60 mesh bauxite and boiled Leaching was conducted at different periods from 1 to l0 hours.

 

2.3    Effect of solid to liquid ratio on leaching

 

The effect of solid to liquid ratio (ratio of solid bauxite in grammes to liquid acid in mls) on leaching was investigated by keeping the weight of bauxite constant and increasing the volume of the liquid phase. Twenty grammes of bauxite 60 mesh size was mixed with 100 mls of lixiviant acid ratio 1:4 and boiled for 4 hours, varying dilutions of the lixiviant up to 1:20.

 

2.4    Effect of leaching temperature

 

20 grammes of bauxite were leached at varying temperatures from 40-100 0C. The leaching conditions were, bauxite 60 mesh size, leaching time 4 hours solid to liquid ratio of l: l0 and acid ratio of l:4.

 

2.5    Effect of particle size on leaching

 

The particle size of bauxite was varied from 0.12 to 1.77 mm under the following leaching conditions: leaching time 4 hours, solid to liquid ratio of 1:10 and acid ratio 1:4.

 

TABLE 2: EFFECT OF TIME ON LEACHING

Time (Hrs.)

A1203 3 extracted

Fe2O3 extracted

A1203 3 / Fe2O3

1
2
3
4
6
8
10

56.2
71.4
89.7
86.6
92.8
92.8
92.8

46.2
57.6
63.0
59.3
68.4
71.4
76.2

24.8
24.8
25.9
27.2
28.3
27.0
26.8

TABLE 3: EFFECT OF ACID CONC. (gm. mole A1203 ; gm mole H2S04)

Acid conc.

A1203 3 extracted

Fe2O3 extracted

A1203 3 / Fe2O3

1:2
1:3
1:4
1:6
1:8
1:10

54.8
76.0
86.6
91.2
91.2
91.2

31.0
49.6
59.3
80.4
92.4
91.2

36.7
32.9
30.0
23.56
20.6
20.2

TABLE 4: EFFECT OF SOLID LIQUID RATIO (gm/ml) ON LEACHING

S/L Ratio

A1203 3 extracted

Fe2O3 extracted

A1203 3 / Fe2O3

1:5
1:8
1:10
1:12
1:15
1:18
1:20

91.2
95.8
86.6
88.6
80.6
76.0
79.0

81.6
67.2
59.3
61.8
56.4
45.6
44.4

24.4
27.0
28.5
30.4
33.2
35.3
36.0

TABLE 5: EFFECT OF TEMPERATURE ON LEACHING

Temp 0C

A1203 3 extracted

Fe2O3 extracted

A1203 3 / Fe2O3

40
60
80
90
100

8.74
31.9
66.9
68.4
86.6

5.4
26.0
43.0
48.0
59.3

29.1
27.7
30.2
30.0
30.1

TABLE 6           EFFECT OF PARTICLE SIZE ON LEACHING

Particle Size (mm)

A1203 3 extracted

Fe2O3 extracted

A1203 3 / Fe2O3

1.77
0.25
0.22
0.17
0.12

82.1
86.6
83.6
76.0
79.0

51.6
59.3
86.4
92.4
97.7

34.8
29.0
26.7
17.8
16.5

 

 

3.0    RESULTS AND DISCUSSION

 

Leached solutions were analysed to determine the aluminium and iron content using atomic absorption spectrophotometer. The ratio of the concentration of alumina to ferric oxide extracted termed as alumina ratio in the pregnant liquor served to determine the optimum parameters of the leaching process. Results of the investigation 5 are presented in Tables 2-6. The concentration of alumina and iron in solution increased sharply and leveled off with increasing time. At 6 hours an optimum alumina ratio 28.3 was obtained. The amount of alumina and iron oxide extracted increased sharply from acid ratio 1:2 to 1:4 and gradually to a ratio of 1:6 in the case of alumina. The rate of extraction of iron oxide after the acid ratio of 1:4 far exceeded that of alumina. An optimum acid ratio of 1:4 which corresponded to an alumina ratio 30 was selected.

 

With the increase of the liquid phase, the amount of alumina and ferric oxide extracted were reduced. Also the effectiveness of mixing was constrained at low solid -liquid interphase. The solid -liquid ratio 1:12 gave an optimum alumina ratio of 30.

 

Varying the leaching temperature, it was observed that the amount of both alumina and iron extracted were directly proportional to temperature. The rate of dissolution of alumina in sulphuric acid is given by the equation:

 

dc / dt = (delta S) / (d) * (Csat - C)                     (eq. 1)

 

where                  Csat - concentration of saturated alum solution.

                            C - concentration of alum solution at time, t

                            S - surface area of bauxite particles

                            D - diffusion coefficient of Al3+

                            d - thickness of the diffusion layer at the interphase, bauxite particle sulphuric acid

 

The diffusion coefficient, D of the aluminium ions, Al ~ is given by the equation:

 

D = (RT / N) * 1 / (3pdm)                      (eq. 2)

 

where                  R - gas constant

                            N - Avogadro number

                            T - temperature

                            m - viscosity of solution

                            d - particle size.

 

From equation 1 and 2, the rate of reaction increases with the rise in temperature as the diffusion coefficient of aluminium ions in solution increases and the solution becomes less viscous. At an optimum temperature, 100 0C, 86.6 % of alumina was extracted at alumina ratio, 30. Commercial leaching is reported to be in the range 100-120 °C. [2]

 

The degree of size reduction in the bauxite is a critical parameter, which influenced the optimum extraction of alumina. From equation I, the rate of dissolution of alumina increases with increase in the specific surface of the bauxite particles. There was not much variation in the amount of alumina extracted over the range of particle size examined. The amount of ferric oxide decreased sharply with increase in particle size: The increased activity of the ferric oxide at high degree of size reduction may be associated with the mineralogy of the bauxite. The surface activity of the ferric oxide increased sharply with the fineness of bauxite. The size range -7 + 14 mesh corresponding to average particle size of 1.77 mm giving an alumina ratio of 34.8 was chosen.

 

The kinetics of the reaction of bauxite with sulphuric acid was of the first order, which is expressed by the equation:

 

dc / dt = KC                           (eq. 3)

and integrating,

ln Cs/ Cs-C = Kt                   (eq. 4)

 

where     C - concentration of alumina (or iron oxide) extracted at time, t

                Cs - concentration of alumina (or iron oxide) extracted at infinite time

                K - reaction rate constant

 

The reaction rate constants were calculated to be

 

                1.45 x l0-4 sec-1       for alumina

and         1.29 x l0-4 sec-1       for ferric oxide at l00 °C.

 

 

4.0    CONCLUSION

 

Optimum parameters influencing higher yield of relatively pure alum from the reaction of sulphuric acid and Awaso bauxite were the following:

 

                Particle Size                           7 + 14 mesh

                Leaching Time                      6 hours

                Acid ratio                              1:4

                Leaching temperature          100 °C

                Solid-Liquid ratio                 1:12

 

At these optimum parameters, the alumina ratio was 34.8 which was comparable to the ratio of 34-35 for commercial grade alum [2]. The Awaso bauxite containing 62.3% A12O3 and 3% Fe2O3 is suitable for the production of alum in Ghana.

 

 

REFERENCES

 

1.   G. 0. Kesse. The Mineral and Rock Resources in Ghana. 1985

 

2.   Technical Inquiry Service, US Department of Commerce and Office of Technical Services (OTS).

 

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