Determination of carbon and sulphur analysis in the cement industry

Determination of Carbon and Sulphur Analysis in the Cement Industry

1. Introduction

The measurement of carbon and sulphur in cement is necessary to ensure a continuous quality of the product and is done in all cement plants.

Today´s technical requirements on the cement but also economic reasons and an environmental protection makes it necessary to analyse the products, the raw materials and the combustion materials more often and more accurate than before.

Fossil combustion materials contain sulphur which influences the properties of the cement negative; economic reasons forces the management to buy raw materials for reasonable prices, but the quality of the product must be stabile to fight the competition.

The company ELTRA is producing elemental analysers which are able to support your daily work. We will introduce our analyser with their helpful features in the following.

2. Carbon and sulphur analysis in cement

The sulphur concentration in cement is dominated by the gypsum content. This calcium sulphate is very strong so that it needs a very high temperature to crack it in a combustion analyser. Temperatures higher than 2000°C are needed to get the total sulphur out of the cement without using toxic accelerators.

The only furnace type which can manage these high temperatures is an induction furnace. Temperatures up to 2200°C can be reached which decompose also the strongest sulphur bounds and a correct result is possible.

The induction furnace works different from the usual resistance furnace with a constant temperature. The sample is given with some iron (Fe) and tungsten (W) into a crucible. This crucible is inserted automatically into the combustion area by a pneumatic lift. The induction coil heat the metal compounds with 19.5MHz and 2.2kW up until it starts burning. The combustion energy of the burning metal creates temperatures up to 2200°C and an oxidising environment. In this burning metal and molten metal oxide atmosphere reacts any sulphur containing material to sulphur dioxide which is analysed by an infrared detector.

The carbon is released as carbon dioxide at the same time and can be detected by another infrared detector.

The right instrument for this application is the ELTRA CS800. It includes an induction furnace and up to 4 different IR detectors to manage a wide range of carbon and sulphur concentrations. A state of the art design supports the user with all necessary features to get a correct carbon and sulphur result in about 50s.

3. Combustion analysis vs. X-ray fluorescence analysis (XRF)

The measuring principle of XRF instruments is very difficult for sulphur analysis.

A polychrome x-ray beam is focused on the surface of a sample. The energy of the beam hits sometime an electron of a sample atom and sends it to a higher energy level. This higher level is not stabile and the electron drops down to the previous level. The for this atom specific energy difference is send as a x-ray with certain wavelength from the sample. This fluorescence of x-rays is analysed regarding wavelength and intensity. So it is possible to calculate the containing elements and their concentrations.

This principle works well with atoms which have a lot of electrons. The smaller the atom and lower the number in the periodic system the higher the difficulties to analyse it. Electrons which are close to the centre are stronger bound than electrons which are positioned outside. It needs a higher energy to lift the inner electrons up and the emitted energy is also very high. Additional depends the possibility to hit one electron by the incoming x-ray on the number of electrons. The higher the number of electrons, the higher the number of possible targets. The intensity of emitted light of small atoms is dimensions lower than from bigger atoms at the same concentration.

These are the reasons why the XRF analysis works very well for elements with high position no. in the periodic system ( Si, all metals etc.) and why it is so difficult to analyse the first elements ( H, Li, C, S, N, O ).

Modern XRF instruments are able to analyse also sulphur. These analyser are sensitive enough to detect the sulphur concentration, but not with the same precision like the other elements.

There is still another problem of the XRF: only the surface is analysed. The x-rays can’t reach the material inside of the sample; it is absorbed by the first layer. Higher x-ray intensity doesn’t help because the emission from the first layer is much better than from inside. The precision of the analysis depends on the preparation of the sample surface.

The combustion analysis of carbon and sulphur was developed because of these problems of the XRF analysis. Only C and S is analysed, but with a very high precision. The conversion to carbon- and sulphur dioxide is done completely in an induction furnace; the detection by IR absorption is possible down to the low ppm range.

The sample preparation is more easy because the whole sample is burned and not only the surface. The analysis time is about 50s, that means it is possible to analyse the same sample 3 times and calculate the average if it is not homogeneous. There is no delay for the preparation of the next burn.

The decision which analysis principle is the right one depends on the requirement of the precision. The XRF is able to give a C and S result in a certain range together with the results of the other elements. The combustion analysis is more precise, faster and the sample preparation is easier. Quick checks of other samples (raw materials, combustion materials) are also quite easy and need no sample preparation and own calibration if only C and S is required.

4. Analysis of combustion materials

A wide range of materials is burned to heat the furnace for the cement production. Fossil combustibles like coal, coke, and oil is used, but also rubber, residual oil etc. because of the low price. All these materials contain carbon and sulphur. The carbon content is characteristic for the combustion energy which you can get out of the material. It is a main criterion for the quality.

These materials contain also more or less sulphur. Also the sulphur concentration of the combustion material is a criterion for the quality.

The analysis of these organic materials is different from the cement analysis. Here are not high temperatures important, but a long combustion area and a big moisture trap for the combustion water. A induction furnace with its high temperature in the crucible is not the best choice; a resistance furnace with a big moisture trap and long combustion tube at a constant temperature is needed.

The right instrument for both applications is the ELTRA CS2000. It includes an induction furnace for inorganic plus a resistance furnace for all kind of organic materials. With up to 4 IR cells is it possible to cover all C and S concentrations of these various materials. No compromise in sample weight, analysis precision and range is necessary.

5. Combustion accelerators

Some accelerators are used for the cement analysis, if only a resistance furnace instrument is available. These accelerators are necessary to decompose the strong sulphate bounds in the cement at temperatures of 1500°C. The toxic vanadium pentoxide is mainly use in this case. The use of this reagent for combustion analysers is forbidden in Europe because the fumes cause cancer.

The 2nd problem of accelerators is the different contact between two analyses. That means the repeatability of the results depend on the mixture, grainsize and position of sample and accelerator in the combustion boat.

Other accelerators like iron phosphate or similar are not as dangerous like vanadium pentoxide, but also not same efficient. The reaction is slower and less complete.

The use of a resistance furnace for the cement analysis is not possible as a standard method because of the poor precision and repeatability. An induction furnace instrument is the first choice to get the necessary performance.

6. Sulphite and sulphate separation

The row materials which are used for the cement production can contain next to the calcium sulphate (gypsum) also sulphite. This becomes more likely if gypsum from desulphorization processes in power plants is used. Also limestone can contain some calcium sulphite.

The separation of sulphite and sulphate is done by different extraction temperatures. As explained above, sulphate needs about 2000°C to decompose. Sulphite decomposes at much lower temperatures so that a resistance furnace is enough.

Practically is first a sample at 1000°C analysed in a resistance furnace. The sulphur coming out at this temperature was bounded only as sulphite. A second sample is analysed in a induction furnace at 2200°C. The total sulphur will decompose at this temperature. The difference between analysis at 1000°C and 2200°C is the sulphur bounded as sulphate.

This separation is only with two furnace systems like the CS2000 possible. The results are helpful to optimise the properties of the product and to higher the performance of the production.

7. Requirements of a modern analyser

The requirements of a modern analyser are increasing from year to year. As the performance of the productions is reaching the maximum also the analytical systems must be optimised. The number of analysis will be increased with a minimum of staff and service. Digital data transmission and reports are necessary for ISO 9000 and similar certifications. The analyser must fit into the modern laboratory and QM system. Therefor the following features are self-evident:

–          automatic furnace cleaning for the induction system

–          automatic data export to MS- Excel or – Word

–          TCP/IP data transmission to upper level computer

–          windows software

–          programmable LIMS connection

–          Autosampler for up to 130 samples

–          diagnostic programs

–          easy software for different operators

–          application stock for various materials

All these features are standard on ELTRA analysers.

8. Resume

Today’s competition forces all companies to work with the maximum performance. This includes the work with a minimum staff and reasonable raw materials. The requirements on the analysers become higher at the same time.

The ELTRA CS800 is a modern analyser with induction furnace which meets all specifications for the cement analysis. It is the right instrument for the production control of a cement plant.

The CS2000 is a universal carbon and sulphur analyser with two furnace systems. It is able to analyse all kind of materials without compromise. This includes the production control, the analysis of combustion materials and research analysis for the future.