Sulphur determinations

Henna Punkkinen1, Teemu Karlsson2, Markku Juvankoski1, Tommi Kaartinen1, Jutta Laine-Ylijoki1, Elina Merta1, Ulla-Maija Mroueh1, Jarno Mäkinen1, Emma Niemeläinen1 & Margareta Wahlström1; 1VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, 2Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi

Introduction

Sulphur is a key parameter in the characterization of wastes from extractive industries. A specific feature of sulphur/sulphide containing waste is the risk for acid rock / neutral rock drainage generation (ARD/NRD) (Technical Committee CEN/TC 292 2012). Sulphur determinations give important input e.g. for ABA-tests. In European legislation, a limit value for sulphur has only been given for the inert waste (European Commission 2009a).

Rikkikiisukiteitä.

Figure 1. Pyrite grains (Photo © J. Väätäinen, GTK).

Sulphur containing minerals are the main sources of acidity as they may be capable of generating sulphuric acid when subjected to weathering. The most important types of sulphur species include sulphides and sulphosalts, sulphates, organic sulphur, and sulphur species of intermediate oxidation states (Price 2009). Sulphides are considered as the most important sulphur minerals in the bedrock. Organic sulphur is usually not present in mine wastes and the same holds for elementary sulphur. (Technical Committee CEN/TC 292 2012)

Mining wastes can be divided into non-sulphide wastes, wastes with iron sulphides, and other sulphide wastes based on their sulphide mineral composition (Kauppila et al. 2013). Iron containing sulphides are prone to oxidize most effectively, pyrite (FeS2) being the most important and the primary acid producing mineral. However, not all sulphide minerals produce acidity. For example, sulphate and elementary sulphur will not usually contribute to acid production (Technical Committee CEN/TC 292 2012).

Sulphur analyses are used for the identification and measurement of the contents and compositions of different sulphur species in the sample. The information obtained via analysis is essential for the prediction of acid generation and elemental release potentials under suitable weathering conditions, and also used for the material characterization purposes. (Price 2009)

Description of the test methods

Several different methods exist for the sulphur determination from the mine waste specimens. These methods can be divided into geological and mineralogical analyses, total sulphur content determinations, and sulphur species determinations.

Geological and Mineralogical Analyses

Geological and mineralogical methods can be used for determining major, minor and trace compositions of sulphur containing minerals; identifying which sulphur species or species categories the sample contains; quantifying the spatial distribution of different species; and confirming species measurement parameters by other analyses. For example visual, petrographic, XRD and image analyses can be used for analyzing sulphur in the sample. (Price 2009) These techniques are discussed further in the “Mineralogical Characterisation” chapter. Generally, XRF technique is not suitable for the determination of total sulphur as it leads to underestimation of the sulphur content.

Total sulphur analysis

Total sulphur analysis is typically performed for material characterization purposes as well as for prediction of drainage chemistry from geological materials (Price 2009). The determination gives realistic estimation of the acid production potential of mine waste if all sulphur the sample contains is in pyritic form (Sobek et al. 1978). Total sulphur content analysis is often described as a default method, as its accuracy is adequate on many applications (Liphard 2010, Technical Committee CEN/TC 292 2011; 2012). There are both international and European standards available for the total sulphur determination. The methods for total sulphur analysis include: bomb combustion (European standard EN 14582), high temperature combustion (ISO 351 or ISO 15178), Schöniger apparatus (EN 14582) and iodine method (EN 1744-1). The bomb combustion method is the preferred one within Europe, but also the use of high temperature combustion methods is acceptable. Schöniger method and iodine method are not recommended to be used due to the sensitivity issues (Schöniger apparatus) and the use of very toxic substances (iodine method). (Technical Committee CEN/TC 292 2012)

The bomb combustion method is the most commonly used method for standard total sulphur analysis, and is usually performed using automatic equipment, e.g. Leco analyzer. In the bomb combustion the sample is combusted in a high temperature furnace in the presence of oxygen. The oxidization takes place in a bomb containing oxygen under pressure (EN 14582). The combustion transforms different species of sulphur into gaseous sulphur dioxide (SO2), which quantities can be measured by IR detection (Price 2009).

The detection of all sulphur components (including temperature stable inorganic sulphates) is possible if optimal operation conditions can be achieved. Tin capsules, increased operation temperatures (e.g. up to 1 500 °C), and extended measuring times can be used to attain the optimal conditions. In cases where the concentration of sulphur is high, extended time of combustion and/or higher operation temperatures may also be needed, as the complete combustion has to be ensured. Calibrants used should have similar range of sulphur concentration than the waste material to be tested. (Technical Committee CEN/TC 292 2011)

Sulphur speciation: Sulphides and sulphates

Total sulphur analysis gives a conservative estimate of the sulphide content of the sample. Usually, however, samples contain also other sulphur species whose acid production potential is weaker, and in these cases the estimated acid generation potential based on total sulphur is too high (Sobek et al. 1978). If speciation is of interest, a sulphur species analysis may give more realistic estimation on real acid potential (Liphard 2010). Usually the main purpose of sulphur species analysis is to determine the amount of sulphides, that is, mainly pyrite (Liphard 2010, Technical Committee CEN/TC 292 2011, 2012). However, sulphur speciation analyses also enable the determination of the other sulphide species e.g. sulphate sulphur and organic sulphur (Lapakko 2002).

For example geological and mineralogical analyses, combustion procedures, wet chemical extractions and solid phase elemental analyses can be performed for the estimation of the type and concentration of different sulphur species (Price, 2009), using either direct or indirect methods. The choice between the indirect and direct analysis method depends on the mineralogical composition of the material studied. (Liphard 2010, Technical Committee CEN/TC 292 2011, Technical Committee CEN/TC 292 2012) Direct species analysis measures the amount of desired species straight from the sample (Liphard 2010, Technical Committee CEN/TC 292 2011; 2012), whereas the basis of indirect determinations is to treat the sample in a way which removes a certain sulphur component, and the difference between total sulphur determination on the original and treated sample gives the content of the removed species (Lapakko 2002).

Although a wide variety of different methods exist, no international standards are available for the determination of different sulphur species. Table 1 presents the basics of the different method types in use for sulphur species determination (Liphard 2010, Technical Committee CEN/TC 292 2011; 2012), divided into combustion, chemical extraction and calculative methods. Sulphide sulphur is most commonly measured by combustion, whereas the basis of the wet chemical extraction methods is to add a selective solvent that isolates a major mineralogical phase in the sample (Price 2009).

Table 1. Methods for sulphur species determination (Price 2009, Liphard 2010, Technical Committee CEN/TC 292 2011; 2012).

Methods for sulphide species determination
Method name Type Principle Observations
Combustion Direct The thermal stabilities of different species vary when being combusted. Many sulphides (especially pyrite) and elemental sulphur need lower combustion temperatures in vaporization to sulphur gases than sulphates. Sulphides (+ elemental sulphur) are measured at 750°C or 810°C using automatic analyzer.
Indirect Temperatures 550 – 650°C are not hot enough to vaporize sulphate sulphur, but will vaporize sulphide sulphur. Sample is pyrolysed in a furnace at 550°C or 650 °C. The total sulphur content of the pyrolysis residue is determined. Sulphide sulphur is calculated from the difference between total sulphur in the untreated sample and total sulphur in the pyrolysed sample. Method ASTM E1915.

Limitations: the partial volatilization of some sulphides at 550°C and the partial/complete volatilization of certain non-sulphide sulphur species. The method is not recommended, if significant contents of either organic sulphur or partially volatilizable sulphides are present.
Reaction with HCl Direct Sample is treated with HCl for sulphides and for the determination of disulphides; Cr(III)chloride and HCl are added to form hydrogen sulphide (H2S). The concentration of collected H2S is determined by backtitration. Method ISO 157 (Note: Method is only applicable to coal).
Reaction with NaCO3 Direct Boiling with 10% NaCO3 removes less soluble sulphate minerals from the sample. Assuming that all sulphates are removed the result gives sulphide sulphur (+ possible elemental sulphur). If the presence of elemental sulphur is possible, carbon disulphide (CS2) leaching can be added to protocol to remove elemental sulphur.
Leaching with HNO3 Indirect The concentration of sulphide sulphur is calculated from the difference between sulphur analyzed in the HCl treated residue and sulphur analyzed in the HNO3treated residue, as HNO3 treatment oxidizes and dissolves sulphides and dissolves acid soluble sulphates, and HCl dissolves acid soluble sulphate minerals but does not dissolve sulphide minerals. “Sobek method” (EPA 600/2-78-054:1978); the so called “Modified Sobek method” uses hot HNO3instead of cold HNO3.
Calculation It is assumed that sulphur in the sample appears only in sulphidic, sulphatic and/or elemental forms. The difference between directly measured total sulphur and sulphate sulphur gives the amount of sulphide sulphur (+ elemental sulphur).
Methods for sulphate species determination
Method name Type Principle Observations
Leaching with HCl Indirect HCl treatment leaches most sulphates but leaves other sulphur species untouched. After HCl extraction sulphate sulphur can be calculated from the difference between total sulphur in the untreated sample and the sulphur in the HCl leached residue. “Sobek method” (EPA 600/2-78-054:1978)
Direct Sample is treated with HCl at 80°C temperature. H2O2 is added to the filtrate and evaporated. Water and HCl are then added to the residue and boiled. Ammonia treatment makes the solution alkaline. The precipitate is filtered off and the ammonia solution is added again. The amount of sulphate can be determined gravimetrically by adding BaCl2 solution. Method ISO 157 (Note: Method is only applicable to coal).

 
Leaching with NaCO3 Direct Sample is boiled with NaCO3, which removes insoluble sulphates from the sample, because aqueous carbonate solutions dissolve sulphates efficiently. The filtrate which contains sulphate is precipitated with BaCl2 in a dilute HCl medium. The BaSO4 precipitate is calculated as %S (of total sulphate) in the original sample.
Indirect In this approach the residue is analyzed instead of the filtrate. Boiling with 10% NaCO3 removes less soluble sulphate minerals from the sample. If the presence of elemental sulphur is suspected, carbon disulphide (CS2) leaching can be added to protocol to remove elemental sulphur. Sulphate sulphur is calculated from the difference between total sulphur in the untreated sample and sulphur in the leached sample.
Calculation It is assumed that sulphur in the sample appears only in sulphidic, sulphatic and/or elemental forms. The difference between total sulphur and sulphide sulphur (+possible elemental sulphur), determined either directly or indirectly, gives the amount of sulphate sulphur in the sample.

Appropriate applications

The suitability of the methods and the validity of the results greatly depend on the sample characteristics and the method chosen. The expected or present sulphur species; their potentiality to produce acidity, release contaminants or ability to interfere other sulphur species analysis, with the requisite precision and accuracy of the results will affect to the selection of the suitable procedure. For example, total sulphur analysis is applicable method if only one sulphur species or fraction (pyrite) is present in the sample in significant concentrations, whereas sulphur speciation analyses enable also the determination of the other species. All described methods have their strengths, but they also contain weaknesses (Liphard 2010, Technical Committee CEN/TC 292 2011; 2012). These aspects should be understood when choosing the analysis method and interpreting the result (Price 2009). Advantages and disadvantages of the most common sulphur analyses are presented in Table 2.

Table 2. Advantages and disadvantages of total sulphur and sulphur species analysis methods (Price 2009, European Commission 2009b, Liphard 2010, Technical Committee CEN/TC 292 2011; 2012).

Total sulphur analyses (Technical Committee CEN/TC 292 2011, Price 2009, Liphard 2010)
Advantages Disadvantages
Established method(s) available. Also knowledge of sample mineralogy is needed for correct data interpretation.
Automatic equipment can be used (e.g. Leco).

 

 The result gives correct acid generation potential (AP) only if all sulphur is in pyritic form, as AP will be overestimated if other sulphur minerals are present. However, the results are always “on the safe side”, that is; either correct or too high, but never too low.
Automatisation will result in: short analysis times, high throughput, good accuracy, moderate costs, and capability to analyse many samples at once. If sulphur concentrations are high, longer combustion times and/or higher temperatures may be needed to achieve correct results.
Total carbon and total sulphur analysis can be performed at the same time.  
Sulphur species analyses (Price 2009, Liphard 2010, Technical Committee CEN/TC 292 2011; 2012. European Commission 2009)
Advantages Disadvantages
Real acid potential can be assessed. Speciation is a difficult task and dependent on the sample material.
Combustion, extraction procedures, and solid phase elemental analyses are quantitative methods. Sulphur species concentrations depend on the chosen method, so choosing the appropriate method is essential.
Wet chemical procedures: quicker, cheaper and can have lower detection limits compared to mineralogical methods. Speciation analyses are generally perceived as very labour and time consuming.
  Combustion, wet chemical extraction procedures and solid phase elemental analyses all suffer from the possible contribution of several different sulphur species types to the measured sulphur amount, and the potential lack of accuracy in defining particular sulphur species or categories.
  Wet chemical procedures: Procedures are not mineral specific, additional mineral sources for dissolved sulphur and cations may occur. Also site specific differences in sulphur mineral composition may cause site specific differences in solubility. Analysis variables, mineralogical composition, solubility of mineralogical phase and physical form of a sample to be analyzed will have their influence on the results.
  Leaching with HNO3: Difficulties in repeatability have been reported.
  During extraction procedures some sulphide minerals that should not dissolve may dissolve during digestion, and on the other hand, those minerals that should dissolve may not dissolve completely. This kind of behaviour may lead to incorrect test results.
  The calculation of sulphide-sulphur from total sulphur minus non-sulphide-sulphur would lead to significant errors if significant amount of organic sulphur is present in the sample; however, in these situations HNO3leaching can be used.

Performance

As described before, the total sulphur analysis with automated analyzer is the cheapest and the most effective way in analysing multiple samples in parallel. Sulphur speciation analyses are usually more time consuming and require more labour. Nevertheless, the use of single universal procedure is impossible due to a great number of different sulphur species, the limitations of the analyses, and different needs to perform the analysis (Price 2009).

If there is uncertainty whether an important sulphur species is removed completely or not, mineralogical and elemental analysis of the residue or elemental analysis of the leachate should be used to check the situation. (Price 2009)

Method maturity

As part of the robustness study for European standard EN 15875, all together nine waste samples and one artificial sample were analysed in different laboratories by using the methods described in this chapter. The study showed that the results of total sulphur determination done by automatic analyzer had acceptable repeatability and reproducibility. Also when sample contains mainly pyrite, all different methods for sulphide speciation gave similar results and can be used. However, if the sulphidic mineral is mostly non-pyritic, there were much more variations within the results; for example some minerals such as pentlandite ((Fe,Ni)9S), galena (PbS) and sphalerite (ZnS) have higher thermal stabilities and all methods using low temperature combustion (indirect and direct combustion methods) gave them systematically lower results. If these minerals are present in the sample, it is preferred to use the other methods described. The determination of sulphate sulphur was even more difficult to perform. One reason was the lower concentration compared to sulphide sulphur. The analysis is also dependent on both total sulphur and sulphidic sulphur as the sulphate is mostly calculated as a difference to total sulphur. The two direct determinations, sulphate determination by leaching with HCl, and sulphate determination by leaching with NaCO3, gave the lowest results and are preferred methods if sulphates are to be determined. (Liphard 2010) As a conclusion, knowledge of sample mineralogy is essential for correct interpretation of data from anaysis using this standard.

References

European Commission 2009a. Comission decision of 30 April 2009 completing the definition of inert waste in implementation of Article 22(1)(f) of Directive 2006/21/EC of the European Parliament and the Council concerning the management of waste from extractive industries. 2009/359/EC

European Commission 2009b. Reference Document on Best Available Techniques for Management of Tailings and Waste Rock in Mining Activities, January 2009. Directorate-General JRC Joint Research Centre. Sustainability in Industry, Energy and Transport. European IPPC Bureau.

Kauppila, P., Räisänen, M.L. & Myllyoja, S. (eds.) 2013. Best Environmental Practices in Metal Ore Mining. Finnish Environment Institute 29en / 2011. 219 p. https://helda.helsinki.fi/handle/10138/40006

Lapakko, K. 2002. Metal Mine Rock and Waste Characterization Tools: An Overview. Minnesota Department of Natural Resources, US. April 2002 No. 67.

Liphard, K. 2010. Sulfur Speciation in waste from extractive industries, Draft Report.

Price, W. 2009. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1

Sobek, A.A., Schuller, W.A., Freeman, J.R. & Smith, R.M. 1978. Field and Laboratory Methods Applicable to Overburdens and Minesoils. U.S. Environmental Protection Agency EPA 600/278054.

Technical Committee CEN/TC 292 2011. Characterization of waste – Static test for determination of acid potential and neutralisation potential of sulfidic waste. EN 15875

Technical Committee CEN/TC 292 2012. Characterization of waste – Overall guidance document for characterization of wastes from extractive industries. CEN/TR 16376:2012.