Bauxite: Geology, Mineralogy, Resources, Reserves and Beneficiation

Abstract

In this chapter the Authors summarize the typical features of bauxite which are believed to play roles in mining, beneficiation and mainly in alumina processing, that is, the industrial value of the raw material. Geological curiosities are neglected. General physio-chemical conditions are outlined in order to understand the ore formation being determinant of the quality (chemistry and mineralogy) and quantity of the bauxite ore. Special emphasise is given to its chemical and mineralogical composition make up and their alterations are shown in selected examples, indicating the possible inhomogeneity of the mine product processed in the refineries. In deposit geology laterite, karst and sedimentary/paralic deposits are distinguished requiring significant differences in mining methods and technology applied in processing. In laterite deposits three main types are introduced namely: (1) plateau type deposits developed on morphological terraces and low land interfluves, (2) whale back plateaus and (3) dome shaped hills confined by hilltops and bauxites on hill tops with associated slope bauxites. For estimating the possible supply furnishing of operating refineries or for establishment of green-field plants an overview is given on global bauxite reserves and resources with their grade and mineralogy, amended with their further potential established with the aid of remote sensing technics introduced by the Author in 2006. Principles of physical bauxite beneficiation of the Run of Mine (ROM) bauxite is presented together with the most common unit operations applied. A flowchart example is shown integrating many of the unit operations. To minimize transportation cost and be able to supply more than one refinery, the beneficiation takes place at the mine site before the beneficiated bauxite is exported to the refinery. Principles of thermo-chemical bauxite beneficiation/activation is presented for three distinctive purposes: (1) Reduce organic carbon in the ROM bauxite before feeding the refinery and thus reducing the organic carbon input to the Bayer process liquor before digestion; (2) Activate the content of boehmite in the ROM bauxite to enable low temperature digestion with the side benefit of significantly reducing the organic carbon in the ROM bauxite, and (3) Pyro-genic attack or sintering of the ROM bauxite with limestone and/or sodium carbonate, when the bauxite is composed of mainly mono hydrate bauxites (boehmite or diaspore) and low to high silica content in order to maximize yield of alumina, recovery of caustic and reduced energy consumption. Only the soda-lime sintering process has gained commercial status over time, and especially in China. All the thermo-chemical bauxite beneficiation/activation processes are planned to be built at the refinery site, and in the case of the soda-lime sintering process integrated with the Bayer process in the overall process flowsheet.

Benny E. Raahauge is deceased.

Appendices

Appendix: Representative sampling for elaboration of the alumina manufacturing process (Theory and practice)

Generalities

All kinds of geological sampling for analyses need comprehensive geological knowledge about the rock (geological formation) to be sampled. Representative sampling for tests serving the elaboration of technology of processing a raw material is one of the most important tasks of a geologist.

As much as 50–200 kg of representative sample for laboratory tests is needed when an operating refinery needs new feedstock to process. In case of establishing a new refinery, depending on the technology envisaged to be applied 1–10 tons, or even more may be needed when test is carried out on pilot plant scale. To take out hundred kilograms or even tons of bauxite from a deposit consisting of ten-, or even hundred million tons of raw material the geologist must be aware of its local (specific) conditions of genesis, accumulation, chemical and mineralogical make up and their variances both vertical and horizontal directions, secondary alterations, along with the shape and size of the deposits.

Representative samples are composed based on the exploration data. Bulk sample has to involve individual samples of extreme concentrations of each determinant element in proper ratio. How far a carefully taken sample is really representative in practice, that is, how far the chemical and mineralogical composition of the sample and the mine product are uniform (deviations are in acceptable ranges) it highly depends on how reliable the exploration and reserve calculation data are, and how far the mining method fits to the deposit characteristics. Reliability of the representative sample comprises all the errors of the exploration method, techniques, sampling, analyses and reserve calculation. Alumina refinery pays bonus after shipments of ore when better than agreed in contract. Mining company pays penalty after lower quality. Contract between the producer and consumer, among the others, is based on the results of the technological tests carried out on representative sample.

When no adequate data are available additional mineralogical reconnaissance is to be made for setting up a sampling plan taking into account the variances in mineralogical composition of the deposit.

What is Representative Sample?

Representative samples are taken either for:

• (a) establishing the technology in a new plant, or

• (b) satisfying the demand of an operating refinery.

In the second case the technology may need to be modified according to the new feedstock.

For satisfying the demand of the representation a composite sample, both in its chemical and mineralogical composition, should be fitted, to that of the run-off mine ore’s characteristics planned to be extracted in a long term. Quality, mineralogy of mine product changes during the mine operation period. For that reason, depending on the measure of expected changes, collecting additional subsamples may also be needed.

From the point of chemical view, the bauxite sample can be regarded as a representative one provided the differences satisfy the following deviation conditions:

• i. TAl₂O₃ ± 1.5 abs.%, when TAl₂O₃ ≥ 45% and ± 1,0 abs. % when TAl₂O₃ < 45% and,

• ii. TSiO₂ ± 0.5 – 0.8 abs.%, when TSiO₂ ≤ 5% and ± abs.%1,0 abs.% when TSiO₂ > 1%.

It is better to apply these conditions for available alumina and reactive silica, but no sufficient amount of data are always available for the deposits in this respect.

Conditions for acceptable differences must be clarified with the responsible process engineer before the sample is composed. When the deviations are somewhat looser the sample is regarded as characteristic sample.

In case of laterite bauxites, the difference for Fe₂O₃ is not so important because there is a very close negative correlation between the TAl₂O₃% and Fe₂O₃%. The correlation coefficient (R) is around 0.90–0.95. Therefore, if the bauxite is representative for alumina it must also be representative in its iron oxide content within an acceptable range (± 1–2%). In case of karst bauxites, the correlation between the alumina and iron, in contrary, is a positive one and it is not so tight. The representative sample should be taken by that way that the difference in iron oxide be less than ± 2 abs. %.

The contaminants, like C^org, ΣSO₃, MnO₂, etc. are related typically to the epigenetic (secondary) processes in karst bauxites. ΣCO₂ > 1%, in karst bauxite,  due to a secondary re-accummulation. C^org,  ∑CO₂  and P₂O₅ occur in laterite bauxites. Their variances are extremely high both vertically and horizontally. See details in Sect. 2.2.1.2. These elements are not involved in the routine analyses of the exploration, but there are, in most cases, data enough to estimate them in the deposit average. Based on geostatistical calculation extreme and most frequent values may also be known. Process engineers must be informed about these data and it is also necessary to take them into account when the bulk sample is composed, in case additional subsamples may be needed.

The sample must be representative, or at least characteristic, in its mineralogical make up, as well. Systematic quantitative mineralogical analyses usually are not available. Representative samples are composed based on the available information on mineralogy. If adequate data are not available, additional mineralogy tests are needed to make on individual samples before the bulk samples blended.

Classification of the Bauxites by Their Mineralogical Composition

Based on the fundamental processing properties of the bauxites they are classified into the following main types:

• (a) Gibbsitic bauxite: at least 95% of its free Al ₂ O ₃ content, (aluminum oxi-hydroxides) must be in gibbsite. ¹

• (b) Gibbsitic/boehmitic and gibbsitic/boehmitic + diasporic bauxite: at least 90% of free Al₂O₃ content must be in gibbsite.²

• (c) Boehmitic bauxite: ≥ 10% boehmite, at least 95% boehmite + gibbsite.

• (d) Boehmitic/diasporic bauxite 5–10% free alumina content is in boehmite and/or diaspore.

• (e) Diasporic bauxite more than 10% of its free Al ₂ O ₃ content is in diaspore.

• (f) Chamositic/diasporic bauxite: chamosite content is > 5%.

Grouping of Bauxites on Their Mineralogy and Processing Properties

2.5.1 Homogenous Bauxite Deposits

The total amount of the ore can be processed by either low (140–150 °C) or high (240–280 °C) temperature digestion, that is, the whole resource can be ranked into one of the mineralogical type of ore, as listed above, the alterations can be followed on the basis of their chemical data, so that reliable sample can be taken following their chemical makeup. Furthermore, the kaolinite/quartz ratio is, more or less, constant. Variance in alumina substitution in iron minerals is tolerable. The loss in total alumina is expressed in the value of extractable alumina. Contaminants, such as concentrations of ∑CO₂, ∑SO₃, MnO₂, P₂O₅ and C^org , in general,  can be kept at an acceptable level. What is acceptable? What is tolerable? These criteria should be discussed between the geologist and process engineer.

2.5.2 Heterogeneous Bauxite Deposits

2.5.2.1 Laterite Bauxites

Based on their aluminium bearing minerals different types of ores may occur in one deposit: so called low mono content type (a) and high mono content type (b, d) (most frequently in laterite bauxites type (a) and (b), that is, gibbsite/boehmite ratio changes horizontally and/or vertically. There are two possibilities:

• (1) High mono content type of ore cannot be extracted separately double digestion method is needed to apply when the boehmite content exceeds 8–10% in the tri-hydrate type bauxites. One representative sample may be enough + 2 separate samples, representing 10% of the bulk sample in weight are recommended to collect from the extreme parts of the deposit. (e.g., Az Zabirah—Saudi Arabia).

• (2) High mono content bauxite can be separately extracted. The run-off mine ore is stocked and supplied separately to different plants (e.g., Sangaredi—Guinea). Two representative samples are needed for plants of different types.

Based on the iron concentration some deposits can be divided into an iron rich (Fe₂O₃ typically ≥ 24–26%) and an iron poor (Fe₂O₃ typically: 12–15%) (e.g., Aya Ninahin, Ashanti Region—Ghana—Kesse [82]). In such cases process engineers are satisfied by three samples: One main bulk sample representing the average values and two (smaller) additional samples representing the two extremes. In case of higher iron concentration, the probability for higher alumina substitution in iron minerals (dominantly in goethite) is bigger.

2.5.2.2 Karst Bauxites

Karst bauxite mining is usually going ahead toward the depth. It may occur that the bauxite grade and mineralogy gradually change toward the depth. No average bauxite can be shipped to the plant but only an ore of continuously changing composition. When the difference does not result in necessity of modification of the technology, as it is most frequently, the consumer pays for bauxite according to its value, following the costs changed in processing. In such a situation one representative sample should be composed on the average values and two additional samples from the extremes are recommended to take in order to learn whether modifications in technology is needed and how far the processing costs may change.

When a deposit, regarding its alumina bearing minerals, is built up by two types of ore and they cannot be selectively extracted the bauxites are processed on the highest required digestion temperature, e.g., Kethro deposit where in boehmitic bauxite a diasporic ore body can be found (Nia 1971). In such cases, one representative sample may satisfy the demand with average boehmite/diaspore concentration, in case of need the bulk sample may be supported by two sub samples, as well.

At Manchester Plateau (Jamaica), in horizontal term the alumina substitution on goethite changes between 5 mol% and 18 mol% [8]. This is also the case when three samples are recommended to be collected.

Compilation of a Sampling Plan

2.6.1 Data Required

• (1) Capacity of the refinery, that is, bauxite demand (quality and quantity).

• (2) Concept of mine planning for the next 10–30 years in order to foresee the bauxites characteristics.

• (3) Run-off mine ore reserves (tonnage and grade by mining units; ore bodies).

• (4) Chemical and mineralogical analyses of the bauxite, accessory (contaminant) elements.

• (5) Quantity of the sample.

Overview of the geochemical, hydrological (drainage conditions) and geo-morphological features which are determinant in deposit geology: establishing the variance of each main element and mineral both vertically and horizontally.

2.6.2 Marking of the Sampling Sites on Map.

Number of the sampling points depends on:

• (1) Tonnage of the reserves

• (2) Number of the ore bodies taking also into account their size and shape.

• (3) Heterogeneity of the reserves to be processed (details in Appendix Sect. 2.4).

• ad (1) As a general rule, provided the deposit represents >100 million tons of bauxite, number of sampling points should be 1 whole section (bore hole, pit, channel sampling from mine face) for every 5–6 million tons of bauxite. In case of a smaller deposit (ore body) relatively more sampling points are recommended to be marked (e.g., in case of a deposit of about 10 million tons one sampling points should represent 2.5–3 million tons.

• ad (2) When a deposit consists of more ore bodies all of them are recommended to be sampled, provided its reserve reaches the tonnage of minimum annual production of the mine. Each ore body is to be sampled proportionally to their tonnage.

• ad (3). If the bauxite to be sampled is heterogeneous (see Appendix Sect. 2.4) it is not always necessary to mark more sampling sites, as given above in point ad (1), but in their location its heterogeneity is to be taken into account and composition of three sample(s) are advised such as: one big bulk sample which represents the average values and two smaller additional samples for the extremes.

2.6.3 Location of Sampling Sites on Laterite Deposits

• (1) Sampling points (bore holes, pits), having been marked on a map, must be checked on site to clarify the access possibilities.

• (2) In case of surface laterite bauxite, the shape and the morphology of the ore body give the guideline for sampling points location, as the mineralogy changing in the function of drainage conditions, which are determined, beside the bedrock permeability, and the surface morphology.

• Based on the surface morphology there are three main types of deposits such as:

  • flat plateau,

  • hill tops on undulating surfaces,

  • slopes as shown in Figs. 2.67 and 2.68.

• In all the three the points must be located in such a way that the possible horizontal variation of mineralogy be taken into account.

• The deposit consists of number of ore bodies developed on an undulating surface and the commercial grade ore restricts to the positive morphological forms (singe or complex heaps (e.g., Fria—Guinea, Mulanje (Malawi), Bao Loc (Vietnam, etc.). The points are to be located at the weight point(s), close to the centre and at the slopes at different topographic levels (Fig. 2.67).

• The bauxite deposits developed on an elongated, amoeba like, quasi flat plateaux such as the bauxites developed on Deccan basalt, or khondalite in India. Sampling points are proposed to be located at the “reference points” of the deposits as it is shown in Fig. 2.67.

Fig. 2.67

Bore holes showing the representative sampling sites on hill top and slopes of the Kondekoure (SW) ore body Fria—Guinea—Komlóssy [9]

Fig. 2.68

Bore holes marked for representative sampling on flat surface, Jamiraplat Plateau,—Chattisgarh State—India. Komlóssy [83]

Sampling Method and Analyses

2.7.1 Sample Taking and Preparation for Analyses

• Sampling can be made from bore holes, from pits or mining face (channelling).

• Samples must not be taken from old pits (along the wall of old pits the bauxite quality has considerably changed (kaolinite is washed out, iron precipitated, even within a half meter width even in a very short time). As it is shown in Figs. 2.8 and 2.9 in Sect. 2.3.2 the whole extractable section must be sampled in order to cover the vertical changes in mineralogy.

• Samples are advised to take in one meter intervals and analysed for TAl₂O₃, TSiO₂ and Fe₂O₃. It is useful if:

• (a) Average AvAl₂O₃ and RSiO₂ are also should be determined, if not in every interval, but at least for a composite sample of one section,

• (b) The accessory contaminants (C^org, ΣSO₃ ΣCO₂, MnO₂, etc.) are also advised to be analysed in each section in order to learn how far the expected average values of these elements fit to the reserve averages.

• Mineralogical tests of every sampled section are highly recommended to be made before the sample compilation.

• It is advantageous if the analyses are made at the existing plant’s (if any) which utilises the bauxite or at the mine’s laboratory with the same procedure as it was done during the exploration. When the analytical methods are different the correspondence between the methods should be clarified.

• Depending on the required total weight of the representative sample from one sampling interval 1–10 kg of wet sample is needed + two additional spare bulk samples representing about 2 × 20% in weight of the total representative sample. One spare sample may be needed for upgrading and another one for downgrading purposes.

• For preparation of bauxite for analyses it must be ground to:

  • In case of 1 kg: grain size ≤ 4.5 mm

  • In case of 10 kg grain size ≤ 14 mm (Richards-Cherchette’s formula)

• Before the bulk sample prepared as described above a separate bulk sample is also needed to compose for crushing and grinding test.

2.7.2 Sample Compilation

• Weighted average chemical composition of the individual samples is to be calculated.

• These results must be compared with that of the run-off mine ore planned to be extracted.

• Such a calculated (theoretical) blending is to be made which satisfies the demand by adding proportional quantities from the individual samples. It is a mathematical iteration, and the result is still a theoretical composition.

• In case of heterogeneous bauxites subsamples are also recommended to be composed (from each section) for making analyses for accessory elements (contaminants) and mineralogy. In this case the representative sample should be combined on the basis of the average values of the additional samples.

• The well blended bulk sample must be sampled by forming a cylinder which not higher than 20 cm in case of 100 kg sample and 50 cm in case of a ton. At least three channel samplings are to be made by mixing the bauxite prior to each sampling. Sample taken out in this way must be reduced by quartering and prepared for analyses. The analytical results may be still different from the required composition.

• Reaching of the desired composition can be made by proportional addition of the spare samples (up-grader or down-grader).

If we make the procedure correctly, we will have a bulk and reliable sample which is representative or at least very close to it.