Determination of available alumina in bauxite using powder XRD

Introduction

Bauxite is a sedimentary rock formed by weathering of aluminium rich rocks in a wet tropical or subtropical climate that have been severely leached of silica and other soluble materials. Bauxite is the world’s principal source of aluminium and is of two types: lateritic (consisting of mostly gibbsite and sometimes gibbsite and boehmite) and karstic (comprising mainly of diaspore). Bauxite is used as an abrasive in cement, chemical, metallurgical and refractory industries. Approximately 85-90% of the world’s bauxite is mined for metallurgical purpose and the alumina is extracted from the bauxite in a refinery using a wet chemical caustic soda leaching process known as the Bayer process. The alumina is then converted to aluminium metal via electrolysis in a smelter using the Hall-Heroult process. The mineralogy is very important as it dictates the refining conditions that must be used. The typical mineral composition of bauxite is shown in table 1.

Table 1: Mineral composition of a typical bauxite

Oxide Formula Chemical composition (%wt) Mineralogy
Alumina Al2O3 35 to 65 Gibbsite (Al2O3. 3H2O), boehmite and diaspore (Al2O3.H2O)
Silica SiO2 0.5 to 10 Quartz, kaolinite
Ferric oxide Fe2O3 2 to 30 Goethite, hematite and siderite
Titania TiO2 0.5 to 8 Rutile and anatase
Calcia CaO 0 to 5.5 Calcite, magnesite, diopside and dolomite

Alumina refineries are usually classified as either high temperature (>240°C) or low temperature (~143°C-150°C).

  • Low temperature refinery: Pure gibbsite or mixed gibbsite and boehmite deposits with low boehmite content are usually sent to low temperature refineries. However, once the deposit has more than about 6% boehmite, then it must be sent to a high temperature refinery for economical extraction of alumina.
  • High temperature refinery: Bauxite containing mainly boehmite (> 6%) and diaspore is usually sent to high temperature refineries.

Bauxite is converted to alumina in the refinery following the four stage Bayer’s process:

  1. Digestion: Bauxite is heated with concentrated sodium hydroxide solution at high temperature and pressure in order to dissolve the alumina content.
  2. Clarification: The undissolved impurities are allowed to settle as fine red mud. The caustic soda is recovered and the solution of alumina in caustic soda (the liquor) is clarified by filtration with the aid of lime.
  3. Precipitation: The liquor is cooled and seeding it with previously precipitated alumina crystals, the alumina is deposited as aluminium hydrate crystals.
  4. Calcination: The hydrate crystals are washed and calcined at >1000 °C to form white aluminium oxide powder.

Not all of the Al2O3 (alumina) is available for making aluminium as some is in the clays and is lost in the Bayer process to the red mud. The total amount of alumina that is extractable in solution from bauxite in the Bayer process is called the Total Available Alumina (TAA). It is made up of two parts:

  1. THA – Trihydrate Alumina. This is the alumina that is extracted in a low temperature refinery.
  2. MHA – Monohydrate Alumina. This is the extra alumina that will be extracted in a high temperature plant. The relationship can be simply expressed as

TAA = THA + MHA

The general laboratory practice for determination of available alumina is to heat the bauxite with strong caustic soda solution under pressure at 150 °C. However at this temperature only alumina from gibbsite is available while that from boehmite or diaspore is not, which is in turn extractable at higher temperature (> 240 °C). Since the total available alumina is a combination of trihydrate form as well as monohydrate form, the alumina obtained from the above wet classical method will give low results (except only gibbsitic bauxite). Moreover the method is quite hazardous, costly and time-consuming. In addition it is essential to know the mineralogy of bauxite in order to ascertain its use in the respective refineries. To overcome these problems, we performed powder XRD studies on bauxite samples to identify not only the minerals present in it but also to quantify the phases present.

Results and Discussions

In the Bayer’s process, the hydrated aluminium oxides react with caustic soda solution to form soluble sodium aluminate

Al(OH)3 + NaOH → NaAlO2 + 2H2O (gibbsite dissolution)

AlO.OH + NaOH → NaAlO2 + H2O (boehmite/diaspore dissolution)

An undesirable side reaction occurs when kaolinite reacts with caustic soda in presence of aluminium hydroxide from gibbsite to form the sodium aluminosilicate which precipitates as red mud.

5Al2Si2O5(OH)4 + 2 Al(OH)3 + 12 NaOH → 2Na6Al6Si5O17(OH)10 + 10 H2O

This loss in trihydrate alumina can be quantified theoretically by considering the formula for kaolinite (Al2O3. 2SiO2. 2H2O) in which the Al2O3/SiO2 mass ratio is 0.85 and the formula for sodium aluminum silicate (3Na2O. 3Al2O3. 5SiO2. 10H2O) having an Al2O3/SiO2 mass ratio of 1.02. For every 1 percent of reactive SiO2, 0.85 percent Al2O3 is derived from kaolinite while an additional 0.17 percent Al2O3 has to be derived from gibbsite to form sodium aluminum silicate.

Thus from the powder XRD pattern we can quantify the aluminium phases and the kaolinite phase present and calculate the total available alumina using the formula

Total Available Alumina (TAA) = (% Gibbsite / 1.5295) – 0.17 (% Kaolinite / 2.1478) + %                          (Boehmite/1.1765 + Diaspore/ 1.1765)

Using the above formula and taking into consideration the XRD pattern of various bauxite samples, we have calculated % TAA. We compared the results with that obtained by wet classical method. However since the wet classical method was done at lower temperature (150 °C) under pressure, the alumina results obtained match with the alumina present in gibbsite. However for samples containing boehmite/ diaspore, the total available alumina calculated would be higher and more accurate than what obtained by the wet classical method.

Sample 1: For this study we took a CRM sample of bauxite and from the PXRD pattern, we calculated the % available alumina (AA) and compared with the results given in the CRM certificate. The percentage of AA calculated from PXRD pattern was found to be in excellent agreement with that given in the CRM certificate. Even though the sample contained an appreciable amount of diaspore, it was not reflected in the available alumina data as the alumina was quantified following the wet classical method.

 

Minerals Concentration (wt%) Available Alumina (wt %)
PXRD Wet classical
Gibbsite 62.68  

 

 

39.61

 

 

 

39.7 ± 0.36

Boehmite 0.20
Diaspore 8.70
Kaolinite 17.25
Goethite 5.22
Hematite 2.16
Quartz 1.73
Anatase 1.38
Titanite 0.69

Sample 2: For this sample and all the other samples the available alumina calculated from gibbsite is compared with that obtained from wet classical method, the total available alumina ( THA + MHA) given in parenthesis.

 

Minerals Concentration (wt%) Available Alumina (wt %)
PXRD Wet classical
Gibbsite 49.33  

 

 

31.38

(47.27)

 

 

 

30.97

Diaspore 18.69
Kaolinite 11.00
Goethite 2.82
Hematite 14.74
Quartz 1.73
Anatase 3.00
Titanite 0.42

Sample 3

 

Minerals Concentration (wt%) Available Alumina (wt %)
PXRD Wet classical
Gibbsite 70.32  

 

 

45.88

 

 

 

46.47

Boehmite 1.32
Kaolinite 1.32
Goethite 14.31
Hematite 4.31
Diopside 3.68
Anatase 4.73

Sample 4

 

Minerals Concentration (wt%) Available Alumina (wt %)
PXRD Wet classical
Gibbsite 86.53  

 

56.51

(62.91)

 

 

56.95

Boehmite 1.79
Kaolinite 0.75
Goethite 1.20
Diaspore 7.55
Anatase 2.18

Sample 5

 

Minerals Concentration (wt%) Available Alumina (wt %)
PXRD Wet classical
Gibbsite 59.84  

 

 

38.70

(47.93)

 

 

 

38.50

Boehmite 14.34
Kaolinite 5.26
Goethite 6.63
Hematite 3.94
Quartz 1.30
Anatase 8.68

Conclusions

The role of powder XRD method for the mineralogical quantification of bauxite was successfully demonstrated in this work. Based on comparative evaluation with chemical data, it can be concluded that the mineral estimates obtained by the PXRD method were very close to those obtained by the traditional wet classical method with the advantage of it being non-hazardous and time-efficient as only a single instrument is used. Moreover the knowledge of aluminium phases present in bauxite is imperative to ascertain the refining conditions to be used in the refinery, which can only be identified and quantified by powder XRD.

CONTRIBUTED BY DR. SATIRTHA SENGUPTA UNDER THE GUIDANCE OF PROF. BARUN KUMAR GUPTA