Production of Ultra Clean Coal: Part II—Ionic equilibria in solution when mineral matter from black coal is treated with aqueous hydrofluoric acid

https://doi.org/10.1016/S0378-3820(01)00173-4Get rights and content

Abstract

A model for determination of the concentration of fluoride complexed aluminium and silicon species, free fluoride (F), H+ ions and molecular HF in solution when aluminosilicate compounds are treated with aqueous HF is presented. The model elucidates chemical mechanisms governing both the dissolution behaviour of the mineral matter in coal towards aqueous HF, and the unwanted precipitation of various fluoride compounds, such as CaF2, MgF2 and NaAlF4. The controlling parameter for the precipitation of fluoride compounds is the free F concentration in solution. The model has application toward the development of chemical strategies for dissolving virtually all of the mineral matter from coal and avoiding the unwanted precipitation of fluoride compounds. The model also has application toward the development of a strategy for recovering fluoride from spent leaching solutions. Ultimately, this work will assist in the development of a process for the production of Ultra Clean Coal (UCC) containing less than 0.1% by weight mineral matter.

Introduction

The two main influences on the current use of coal are environmental concerns over carbon dioxide (CO2) emissions, and the decline of oil and natural gas reserves. These two influences demand increased efficiency of electricity generation from coal, and supplementation of the current uses of oil and natural gas with coal. A technology to produce Ultra Clean Coal (UCC) with less than 0.1% by weight mineral matter satisfies these demands. UCC could be fired in high efficiency combustion apparatus, such as a direct coal fired turbine, to produce electricity. UCC could be used as a substitute for petroleum coke used to manufacture carbon electrodes in the process for producing aluminium, and as a raw material for the production of carbon based fuels, chemicals and materials.

It is generally accepted that the only way to produce UCC, without losing a significant amount of coal, is through a process of chemical leaching. As discussed in Steel et al. [1], HF is considered to be the most effective reagent for dissolving the mineral matter from coal. Unfortunately, the formation of insoluble fluoride compounds containing the alkali and alkaline earth elements, in particular Na, Mg and Ca, occurs during leaching with HF. The formation of these fluoride compounds inhibits the ability of HF to reduce the level of mineral matter in black coal to less than 0.1% by weight. In order to develop a chemical leaching sequence which is capable of reducing the level of mineral matter in coal to less than 0.1%, an understanding of the chemical mechanisms taking place is essential. In particular, the chemical mechanisms that give rise to the formation of insoluble fluoride compounds.

A novel approach has been taken in this work to gain an understanding of the chemical mechanisms taking place when the mineral matter in coal is treated with leaching reagents. The approach involves initially isolating the mineral matter from the coal using a low-temperature ashing (LTA) technique at temperatures between 300°C and 370°C. An X-ray diffraction (XRD) analysis is then performed on the isolated mineral matter residue, to identify the minerals present in the coal. A leaching study is then performed on the isolated mineral matter to study the dissolution behaviour of the minerals. This leaching study neglects any effects that the coal matrix may have on mineral dissolution. Experiments are also performed on specific model compounds, which are known to occur in coal and on combinations of model compounds, in order to confirm and obtain more knowledge on the chemical mechanisms taking place. Finally, experiments are performed on the coal to assess the effects that the coal matrix may have on mineral dissolution.

The LTA technique is presented in Steel et al. [1]. Semi-quantitative mineralogical composition of the mineral matter isolated from a bituminous coal from Queensland, Australia, using the technique is also presented. The dissolution behaviour of the isolated mineral matter towards aqueous HCl and HF is presented in Steel et al. [1]. The extent of dissolution of Al, Si, Fe, Ti, K, Na, Ca and Mg was determined as a function of HCl and HF concentrations at various reaction temperatures. It was found that when the mineral matter is treated with low concentrations of HF, precipitation of fluoride compounds, such as CaF2, MgF2 and NaAlF4, does not occur and high extractions of Ca, Mg and Na can be achieved. However, as the concentration of HF increases above a particular level, insoluble fluoride compounds precipitate. This level appears to be slightly different for each fluoride compound. The general mechanism behind this behaviour is proposed to be as follows.

HF reacts preferentially with the aluminosilicates in the mineral matter to form predominantly AlF2+, AlF3 and SiF4. The leaching solution resulting from the reaction between the aluminosilicates and HF is acidic and contains very little free fluoride (F) or anionic fluoride complexed aluminium species (AlF4, AlF52−, AlF63−). The acidity is able to dissolve the sulphates, carbonates and phosphates such as jarosite (NaFe3(SO4)2(OH)6), dolomite (CaMg(CO3)2) and apatite (Ca5(PO4)3F), liberating Na, Ca and Mg into solution as Na+, Ca2+ and Mg2+ ions. Na and Mg, which are present in the aluminosilicate clay compounds, are also liberated into solution as Na+ and Mg2+. The free F concentration in solution is not high enough to complex the Ca2+ and Mg2+ ions and precipitate CaF2 and MgF2. The concentrations of AlF4, AlF52− and AlF63− are also not high enough to complex with Na+, Ca2+ and Mg2+ and precipitate alkali and alkaline earth fluoroaluminate compounds, such as NaAlF4, MgAlF5 or K2NaAlF6. As the HF concentration increases above that required to dissolve the aluminosilicates completely, the concentrations of free F, AlF4, SiF62−, AlF52− and AlF63− increase to levels which are sufficient to complex Na+, Ca2+ and Mg2+ and precipitate fluoride compounds.

The chemical mechanisms described above for the precipitation of fluoride compounds containing Na, Ca and Mg suggest that precipitation may be avoided by controlling the free F concentration in solution. This paper presents an investigation which attempts to confirm and obtain more knowledge on the chemical mechanisms taking place in the system of mineral matter and aqueous HF. A number of steps were made in this investigation. Firstly, experiments were performed with HF on the model compound apatite (Ca5(PO4)3(OH,F)). The effect of adding Al3+ cations to the system was assessed. The effect of adding kaolinite (Al2Si2O5(OH)4) to the system instead of Al3+ was then assessed. From this leaching work, the concentrations of various species resulting in solution when kaolinite is treated with HF were modelled. The model combines theoretical knowledge on stability constants with the dissolution behaviour observed experimentally. This model was applied to the system of mineral matter and HF to explain the chemical mechanisms giving rise to the precipitation of fluoride compounds when the mineral matter in coal is treated with aqueous HF. Applications of the model toward the development of a process for producing UCC are discussed.

Section snippets

Theory on stability constants of various fluoride compounds/species

Table 1 shows the stability constant (K25°C) and heat of reaction (ΔHR25°C) at 25°C for various reactions which are relevant to this study. This data has been collated from Smith and Martell [2], Hekim and Fogler [3] and Goldstein [4]. An approximate stability constant for the formation of NaAlF4 has been calculated from the work of Grobelny [5], who determined that the solubility of NaAlF4·H2O in water at 25°C is 0.50 g/dm3. An approximate stability constant for the formation of MgAlF5 has

Experimental

The sample of apatite used in this study was sourced from the island of Nauru. An X-ray fluorescence (XRF) analysis of the sample with respect to phosphorus and fluoride content indicated that it contained approximately 90% apatite (Ca5(PO4)3(F,OH)), of which 83% was fluorapatite and 17% was hydroxyapatite. The remaining 10% of the sample was likely to be predominantly composed of calcium carbonate (CaCO3). The sample of kaolinite (Al2Si2O5(OH)4) used in this study was a synthetic sample

Dissolution of apatite (Ca5(PO4)3(OH,F)) with aqueous HF

Fig. 1 shows the amount of Ca in solution, expressed as a percent of the total amount of Ca in the initial sample, when apatite is treated with an increasing concentration of HF at 65°C. As the concentration of HF increases, the extraction of Ca increases to a small extent. A limit is reached whereby no more than approximately 20% by weight of the Ca dissolves. As the concentration of HF is increased further, the extraction of Ca drops to zero. The stoichiometric concentration of HF required to

Dissolution of apatite with aqueous HF and Al3+ cations

The following is a study of the dissolution behaviour of apatite with aqueous HF in the presence of Al3+ cations. Firstly, a theoretical prediction of the concentration of HF required to precipitate CaF2 in the presence of Al3+ cations is made. This prediction is then compared with experimental observation.

Dissolution of apatite and kaolinite with aqueous HF

Fig. 6 shows the extent of dissolution of kaolinite as a function of HF concentration. The difference in extraction between 20°C and 65°C is not great, particularly in the low HF concentration range, i.e. with less than approximately 1 M HF. The difference in extraction is more pronounced at higher concentrations of HF where the extraction is higher for experiments performed at 65°C. 95% by weight of the sample of kaolinite dissolves in 2 M HF at 65°C, and 94% by weight of the sample dissolves

Ionic equilibria in solution when kaolinite reacts with HF

The expressions for determining the fluoride complexed Al species present in solution when kaolinite dissolves in HF at 25°C are given by , , , , , , . The amount of free F in solution is given by Eq. (41). The expression for the amount of H+ in solution must be modified from that given as Eq. (42). As kaolinite contains hydroxyl groups and oxygen atoms, the H+ ions from HF will bind with these groups to form water. From a consideration of the stoichiometric reaction between kaolinite and HF,

Application of ionic equilibria model to a system containing HF and mineral matter from coal

The fact that Al and Si are by far the most abundant mineral elements in the mineral matter, constituting 16.7% and 21.5% by weight, supports the notion that the model , , , , , , , (41) and , , , may be applicable to the system of mineral matter with HF. The next most abundant mineral elements in the sample of mineral matter are Fe and Ti, constituting 2.8% and 0.85% by weight, respectively. While the effects that these elements may have on the calculation of the free F concentration in

Conclusion

The precipitation of insoluble fluoride compounds has been the focus of the work contained in this paper as it is one of the biggest problems facing the development of a process for producing Ultra Clean Coal (UCC), which uses HF. Since most of the mineral matter in black coal is composed of oxides of Al and Si, such as kaolinite and quartz, the concentrations of fluoride complexed Al and Si species, free F ions, free H+ ions, and HF in solution when the mineral matter in coal is treated with

Cited by (52)

View all citing articles on Scopus
1

Faculty of Engineering, The University of Melbourne, 3010, Australia.

View full text