Proceedings of the 8th International Alumina Quality Workshop • 2008 71

LABORATORY RESEARCH AND INDUSTRIALISATION TEST ON TWO-STAGE CONTINUOUS CARBONISATION PRECIPITATION TECHNOLOGY

FOR PRODUCING HIGH GRADE SANDY ALUMINA

Li G*, Ma D, Zhang Z, Zhang L and Fan J

Shanxi Branch of China Aluminium Corporation, China

Abstract

In order to solve the problem of the poor quality of alumina product by the traditional carbonisation precipitation and the high loss of alumina by the deep desilication for the sintering liquor, the authors put forward Two-Stage Continuous Carbonisation Precipitation technology (TSCCP). Both laboratory and industrialisation test results have proved that this process has ingeniously solved the problem.

In the First Stage of Continuous Carbonisation Precipitation (FSCCP), high-grade sandy alumina is produced. In the Second Stage of Continuous Carbonisation Precipitation (SSCCP), the alumina is charged into the slurry flash tank (130–140°C) of the high-pressure digestion unit during the Bayer process of the Combination process. In this way, the shortcoming of low A/C of the high-pressure digestion liquor from diasporic bauxite can be overcome, and will produce high-grade sandy alumina. Furthermore, the process has other advantages such as increasing productivity of two-stage carbonisation precipitation, improving the evaporation performance of spent liquor of the SSCCP (deep precipitation), reducing gallium content in product alumina during the first stage, etc. So it is proposed that this process can be applied to alumina refineries with a Sintering process.

1. Introduction

Because of the characteristics of diasporic bauxite (high alumina, high silica and low ratio of A/S) in Chinese alumina bauxite resources, either a Combination process or a Sintering process is mostly used in traditional alumina production in China. The carbonisation precipitation process is an important part of alumina production process in both Combination and Sintering processes. In the Shandong Alumina Refinery, the traditional carbonisation precipitation process, a kind of batch carbonisation precipitation technology that produces floury alumina, has been successfully upgraded to continuous carbonisation precipitation technology to produce medium-sized alumina (Xingliang, Liu, et al., 1998, p. 185). Based on the study of the technological characteristics of the Combination process, the author put forward the original model of TSCCP in 1998. The process solves a number of problems. As precipitation of seed at high A/C ratio is difficult to achieve from the hard-to-digest diasporic bauxite in the Bayer process, the strength of alumina product can be affected (Dai, Chen, 1996, p. 21). To achieve product with good chemical performance, sintering of green liquor from deep desilication and a lowered carbonisation precipitation rate are needed.

2. Research course of main technology of TSCCP

Laboratory research, pilot and industrialisation testing for FSCCP were completed during 2001 to 2003. Laboratory research, pilot and industrialisation testing for SSCCP and its product sweetening digestion were completed during 2002 to 2006.

3. Laboratory research and pilot test for producing sandy alumina by FSCCP

Through a series of laboratory and pilot tests, a technical breakthrough has been made for the transformation of the hydrate structure. Traditional carbonisation precipitation produces a branched crystal growth mode, while this new process produces crystals with an agglomeration filling growth mode (Figures 1 and 2). This has provided the key technique of FSCCP to produce sandy alumina. The industrialisation test results of the process are as follows: average 6.39% for -45 μm particle size of hydrate to calciner; max. 11.64%, min. 5.44% and average 9.46% for -45 μm particle size of product alumina from calciner; max. 14.5%, min. 7.6% and average 10.5% for Attrition Index (A.I.) of

alumina. The -45 μm particle size of product alumina by FSCCP is reduced from the previous 20–44.7% to 9.92% and A.I. from 36% to 10.5%. Figure 3 shows the -45 μm particle size and A.I. of product alumina in the pilot test, while Table 1 presents the main performance parameters of the product alumina. The physical performance of alumina reached a level of high-grade metallurgical quality alumina, thus demonstrating the ability of FSCCP to produce sandy alumina in an industrial application.

Figure 1. SEM of hydrate by carbonisation precipitation (radiation-shaped branch crystal growth)

Figure 2. SEM of sandy alumina by FSCCP (agglomeration filling growth)

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72 Proceedings of the 8th International Alumina Quality Workshop • 2008

0

2

4

6

8

10

12

14

16

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41

"-45μm (% ) A .I.(% )

Figure 3. -45 μm particle size and A.I. of product alumina by industrialisation test of FSCCP

Table 1. Main performance parameters of product alumina by industrialisation test of FSCCP

45 μm

(%)

A.I.

(%)

B.E.T. Surface

Area (m2/g)

α-Al2O3

(%) Bulk

Density

(g/cm3)

Angle of Repose

(°)

9. 64 10. 5 71. 5 6.2 0. 92 31–33

4. Laboratory research and industrialisation test of SSCCP

Through laboratory research and pilot tests (Figure 4), a technical breakthrough was achieved for the stable production of hydrate precipitated by the first-stage spent liquor in SSCCP. This has shown to be helpful in the settling, filtration and separation of hydrate. In precipitated product, there is almost no unexpected dawsonite (Na2O·Al2O3·2CO2·nH2O). Dawsonite is easily produced using the earlier technology, and causes difficulties in settling, filtration and separation. In the successful industrialisation test of SSCCP, the remaining Al2O3 content in spent liquor was lowered to an average of 2–3g/L. The SSCCP-precipitated product is gibbsite almost without crystal impurity, while settling, filtration and separation processes occurred without difficulty. The industrialisation test for sweetening digestion of hydrate by SSCCP was also successful.

4.1 Key technical breakthrough of SSCCP

Spent liquor of the FSCCP CO2 gas

Thickener

The SSCCP production to sweetening processing

Filter Evaporation

Tank

Figure 4. Illustration of pilot test of SSCCP

Figure 5. SEM of product by SSCCP before technical breakthrough

Figure 6. SEM of product by SSCCP after technical breakthrough

Note: SEM energy spectrum analysis showed that when products are fine dawsonite crystal appearing as irregular thread-like shapes (Figure 5), which independently exist or adhere to the surface of hydrate particle. These make settling, filtration and separation difficult.

SSCCP at a precipitation rate of 76.12% produced a stick-shaped hydrate that settles, filtrates and separates easily (Figure 6). Product over-precipitated by SSCCP at a precipitation rate of about 90.33% allows fine dawsonite to adhere to the surface of the product (Figure 7).

Testing proved that the technology and the product control in SSCCP of spent liquor directly affect the chemical composition of precipitated product. Improper technology produces thread-like dawsonite that is difficult for solid/liquid separation (Figure 5), while the precipitated hydrate is not suitable for use because of the high Na2CO3 content. After being washed and filtered by hot water, the main physical phase Al(OH)3, when controlled by the refined technology, still contains trace CO3

2- mainly in the form of dawsonite (Na2O·Al2O3·2CO2·nH2O).

Figure 7. SEM of over-precipitated product by SSCCP

4.2 Industrialisation test of SSCCP

Industrialisation testing of SSCCP followed the successful laboratory and pilot tests. From XRD the product of SSCCP is gibbsite without crystal impurity (Figure 8). The particle size distribution and SEM of shape of SSCCP product from this testing are shown in Figures 9 and 11. Figure 10 graphs Al2O3 remaining in spent liquor of SSCCP in industrialisation testing.

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Proceedings of the 8th International Alumina Quality Workshop • 2008 73

Figure 8. Physical phase of product by SSCCP

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13

-10μm(% )

-20μm(% )

-45μm(% )

Figure 9. Main particle size distribution of product by SSCCP in industrialisation test

02468

10121416

0 20 40 60 80 100 120 140 160 180 200 220Oprating time,hour

Al2O

3 g/L

Figure 10. Al2O

3 (g/L) remaining in spent liquor of SSCCP in industrialisation

test

During industrialisation testing, SSCCP produced dissolvable gibbsite, whose crystals mainly exist in the disperse form of a single stick-shaped crystal. It has characteristics such as large porosity, fine particle size and high activity, without thread-like dawsonite.

The filtration testing results of SSCCP product in industrialisation testing are as follows: average 25% for moisture of filtered cake; average 0.7% for Na2O; and average 0.93 g/L for suspension in filtration liquor. These results demonstrate that the filtration performance of SSCCP product is able to meet commercial production requirements.

Figure 11. SEM of product by SSCCP in industrialisation test

4.3 Sweetening digestion industrialisation test of hydrate by SSCCP

Testing shows that gibbsite does not remain in sweetening red mud with the same physical phase as is common in the Bayer process. This sweetening process resulted in total digestion, and A/C values increased 0.029–0.058 in the Bayer process pregnant liquor. The operating conditions in the industrialisation sweetening test are shown in Table 2.

Table 2. Operating index in industrialisation sweetening test

Parameter Operating Index

Digestion slurry flow rate (m3/h) 755

Sweetening temperature (°C) 138

Slurry solid content (g/L) 221

Transporting flow rate (m3/h) 17

The physical phase of red mud during the sweetening test consists of sodium aluminosilicate, garnet hydrate, calcite, perovskite and hematite.

4.4 The influence on evaporation process by TSCCP spent liquor

Relational study results (Xiancui, Lv, et al., 2002, p. 50) indicated a number of significant gains following the implementation of TSCCP. Concentrations of Al2O3 and SiO2 (which are harmful for evaporation) are considerably reduced, while the solubility of Na2CO3 in spent liquor of SSCCP and the concentration of Na2CO3 in evaporation are raised. Scaling of silica slag in the heat exchanging pipe bundle of the evaporator decreases from source, thus improving evaporation efficiency and reducing energy consumption during evaporation.

4.5 Setting of dividing point for TSCCP

To determine the defining point for TSCCP, and the selection and setting of the precipitation rate of FSCCP, the following two points should be considered: the silica index of pregnant liquor in FSCCP, and the customer’s required chemical composition for FSCCP product. Testing proved that SiO2 content in product obtained by continuous carbonisation precipitation and seed charging is much lower than that by traditional technology (Table 3).

Table 3. Corresponding relationship between silica index in pregnant liquor and precipitation rate

Silica Index (Al2O3/ SiO2 ) Precipitation Rate (%)

> 500 88-90

451-500 87-89

401-450 86-88

376-400 84-86

351-375 83-85

326-350 80-82

301-325 79-81

251-300 77-79

In addition, the customer’s requirement on the limit of gallium content in FSCCP product can be considered. The research of gallium behaviour during carbonisation precipitation (Xiaowei, Liang, et al., 2005, p. 28) has proved that in the carbonisation precipitation of sodium aluminate solution, the decreasing range of gallium in solution starts to rise slightly after a precipitation rate over 60%, and to rise further at a precipitation rate over 80%. The decreasing range of gallium content in liquor starts to be considerably raised when the precipitation rate is controlled over about 88%.

Thus, the dividing point can be set at about 80% in order to produce alumina with the same chemical composition as

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74 Proceedings of the 8th International Alumina Quality Workshop • 2008

that of Bayer process product at a silica index of 370–400 in pregnant liquor of FSCCP, but at 84% – 86% to produce common alumina.

Or CO2 gas from KilnCO2 gas Spent liquid toevaporation Pregnant liquid

from sinter processing

The FSCCPproduction to bake sand

alumina

The SSCCPproduction to sweetening processing

Figure 12. TSCCP flow sheet

5.0 Advantages of TSCCP

The main advantages of TSCCP, as demonstrated during laboratory research and industrialisation testing, are:

First of all, about 80% of TSCCP product is sandy alumina with good chemical and physical performance, which is characterised by low SiO2 and Ga content, coarse particle size and high strength. Secondly, about 18% TSCCP product has high activity and is very suitable for sweetening digestion. In a Combination process, if sweetening digestion can be carried out in the self-evaporation system of high-pressure digestion of the Bayer process, TSCCP can effectively raise A/C. The over-saturation of seed precipitation in pregnant liquor is increased and the seed precipitation rate improved. This will create favourable conditions for the Bayer process to produce high-grade sandy alumina. In a Sintering process, if sweetening digestion can be carried out by adding a medium-pressure desilication section, TSCCP will produce high-grade sandy alumina. The high impurity content in SSCCP product hydrate can be purified through re-decomposition by sweetening digestion.

In comparison with the traditional Sintering process, more Al2O3 remaining (normally 8–12 g/L) in spent liquor following carbonisation precipitation can be recovered. This allows lower circulation of alumina remaining in spent liquor of SSCCP, and increased circulation efficiency for the spent liquor of carbonisation precipitation. Thus, the alumina production capacity in the Sintering process can be improved, while energy consumption during clinker sintering can be reduced.

With this technology, the concentration of Al2O3 and SiO2 in the spent liquor of SSCCP can be considerably lowered, the solubility of Na2CO3 in liquor increased, and the scaling of silica and Na2CO in the spent liquor evaporator noticeably reduced, which provides material conditions for effective energy saving in evaporation.

The requirement for the silica index (Al2O3 / SiO2) of pregnant liquor of carbonisation precipitation in the Sintering process can be lowered, and even the deep desilication section can be eliminated. This can decrease the circulation of alumina remaining in silica slag (product of deep desilication), as well as energy consumption during sintering. Correspondingly, it can lower the water content added to the system by deep desilication agents, and reduce the relevant energy consumption.

6. Conclusions

TSCCP has good application prospects in improving product quality of the Combination process and the Sintering process, cutting down energy consumption and raising yield. The technology can be optimised by combining the flow sheet characteristics of the Combination process and the Sintering process to produce sandy alumina. This is of momentous significance in the technical upgrading of the traditional Combination and Sintering processes used in China.

Acknowledgements

The authors are grateful to many people who have contributed to the work reported in this paper; particular thanks to Shanxi Branch, Zhengzhou Research Institute and Scientific & Technological department of CHALCO and the Ministry of Science & Technology of China.

References

Guangzhu, Li, N 1998, ’Proposal of improving alumina productivity and product quality by Combination Process’, Paper Volume of 9th Chinese alumina academic meeting & 10th alumina technical information exchange, pp. 21-31.

Xingliang, Liu and Changren, Yao, N 1998, ‘Implementation of alumina quality creating project to realize leap of alumina quality’, Paper Volume of 9th Chinese alumina academic meeting & 10th alumina technical information exchange, pp. 180-185.

Dai, Chen, N 1996, ’Requirement on the quality of metallurgical-grade alumina by international aluminum industry and its developing trend’, The Chinese Journal of Light Metals, No.3, pp. 10-25.

Xiancui, Lv, Weibing, Jing, Tianmei, Bo, N 2002, ’The way of aggrandizement sintering process’, The Chinese Journal of Nonferrous Metallurgy, No.6, pp. 49-50.

Xiaowei, Liang, Kun, Quan, Qiusheng, Zhou, N 2005, ‘Behavior study of Gallium in the alumina production process by sinter method’, The Chinese Journal of Nonferrous Mining and Metallurgy, Vol. 21, No.6, pp. 27-29.

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