Purity and free moisture content are important quality factors for both nat­ural gypsum and synthetic gypsum produced by Flue Gas Desulphurisa­tion (FGD). In the case of FGD gypsum, the purity level gives an indication of the operational performance of the FGD plant. Continuous monitoring of the purity level is therefore necessary to control the FGD process.

This application note describes how METTLER TOLEDO Halogen Moisture Analyzers expedite the analysis of gypsum by delivering accurate data on both free moisture content and crystal water content.

HR83-PHalogen Moisture Analyzer

Purity Determination in Gypsum Fast Analysis for Precise Process Control

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Gypsum is a versatile substance used in a wide range of applications such as medicine, agriculture, and the arts as well as in numerous building products e.g. plasterboards.A quality parameter for gypsum is its purity. METTLER TOLEDO’s HR83 Halogen Moisture Analyzer offers a fast solution for quality analysis of gypsum (CaSO4•2H2O) and the hemihydrate plaster (CaSO4•½H2O).

Natural GypsumGypsum is a mineral found in natural rock. It is extracted either by quarry­ing or mining. Pure gypsum contains 79.1% calcium sulfate (CaSO4) and 20.9% chemically bound water (H2O). However, since gypsum is a natural material, pure gypsum is rarely found. Natural gypsum typically contains up to 15 to 20% impurities such as lime, clay and sand2,3.Purity of gypsum is important to know, e.g. plasterboard typically requires a minimum of 70% gypsum.

Synthetic/FGD GypsumAn alternative source of gypsum is synthetic gypsum derived as a side product from the flue gas desulfuriza­tion (FGD) process – the so­called FGD­gypsum.FGD plants are typically used at fossil fuel power stations to reduce pollutants

Introduction

1 www.eurogypsum.org 2 www.rigips.ch3 www.meningiegypsum.com.au

in the exhaust gases. In this process, sulfur dioxide (SO2) reacts with lime (CaCO3) to produce high purity gyp­sum. The correct functioning of the desulfurization process can therefore be monitored by testing the purity of the obtained gypsum. Quality criteria published by Euro­gypsum, the association of European gypsum industries, states that, for synthetic (FGD) gypsum, free moisture content should be below 10% and gypsum content (CaSO4•2H2O) should be higher than 95%1.

Processing of GypsumWhen gypsum is pulverized and heated at around 150 °C, 1½ crystal water is eliminated resulting in hemi­hydrate plaster (CaSO4•½H2O). This plaster can be mixed with water, to allow molding into the desired shape (e.g. plasterboards). The wet molded product then rehardens to gypsum (dihydrate). The excess free water evaporates or can be removed by gentle drying at low temperatures (see Figure 1).

Purity Control with HR83The HR83 Halogen Moisture Analyzer quickly and easily provides two mois­ture values – free water and crystal water content. The purity of the gypsum and the hemihydrate plaster can be calculated using these values. Thanks to its step drying program the values can be determined in a single mea­surement and analysis time required is significantly reduced compared to using a drying oven. The following example shows the HR83 method used by FGD producers for determining the purity of gypsum and thus for control of the flue gas desulfurization process.Figure 1: Burning, Mixing, Shaping and Rehardening of gypsum

­1 1/2 H2O

gypsum plastergypsumslurry

wetproduct

rehardenedproduct

finalproduct

CaSO4 • 2H2O

heat (150°C)

CaSO4 • 1/2 H2O

+ H2O

+ additives

shaping

CaSO4 • nH2OCaSO4 • 2H2O+

nH2O

CaSO4 • 2H2O

­nH2O

drying at lowtemperatures

CaSO4 • nH2O

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Material and Methods

Method for determining the purity of gypsum (CaSO4 •2 H2O) with the HR83

• HR83-P Halogen Moisture Analyzer• Gypsum from FGD process

3. Press "Start" button to begin measurement.

4. The results of each step are recorded.

2. Weigh approx. 5 g of powdered gypsum into tared sample pan.

1. Press “Method” to select method for gypsum.

5. Crystal water content (CW) and purity are then calculated.

• Drying Program: Step Drying– Step 1: 25 min at 50°C (to remove free water)– Step 2: 0 min at 50°C (Step 2 not required)– Final Step: 200°C

• Switch-off criterion 5 (1 mg/140 s)

Wet weight: 5.570 gDW1 = Result from Step 1 (50 °C): 5.113 g = 8.19% free waterDWF = Result from Final Step (200 °C): 4.085 g

Molecular weight CaSO4: 136.14 g/molMolecular weight 2 H2O: 36.03 g/molMolecular weight CaSO4•2 H2O: 172.17 g/mol

Theoretical CW of pure gypsum:

Measured CW of pure gypsum:

Purity [%]:

%9272.20

17.172

03.36%100%100

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MW

%106.20113.5

085.4113.5%100%100

1

1 =−∗=−∗g

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DWDW F %106.20113.5

085.4113.5%100%100

1

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DWDW F %106.20

113.5

085.4113.5%100%100

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DW

DWDW F

%07.96%9272.20

%106.20%100%100 =•=•

lCWtheoretica

measuredCW

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To monitor the performance of the FGD process a sample is taken from the vacuum belt filters where the freshly produced FGD gypsum is washed and dewatered. Purity below 95% is an indication that the FGD process is not working optimally and must be checked for possible errors.The crystal water content of the sample is determined via the two step

The Application Note demonstrates the determination of purity of gypsum (dihydrate). Determining the purity of hemihydrate plaster is achieved using the same method, but the purity cal­culation is based on the theoretical crystal water content of 6.207% (com­pared to 20.9272% of the dihydrate). The correct purity level is important for both quality control and in­process control of synthetic gypsum produc­tion. The HR83 Halogen Moisture Ana­lyzer rapidly and easily provides the two moisture values – free and crystal

drying method with the HR83 Moisture Analyzer as illustrated above. In the example, the FGD gypsum has a free moisture content of 8.19% and a crys­tal water content of 20.106% which correlates to a purity of 96.07%. This indicates the flue gas desulfurization and the dewatering processes are run­ning smoothly yielding the best quality gypsum.

water – required for the calculation of purity of both gypsum and plaster. The halogen technology provides precise temperature control and fast, homogenous drying, and the robust construction is ideal for use in tough production environments. The HR83 significantly reduces the time and handling required for analysis com­pared to the drying oven method and therefore contributes considerably to the operational efficiency of flue gas desulfurization plants and gypsum processors.

Mettler-Toledo AGPO Box VI­400, CH­8606 GreifenseeTel. +41­44­944 22 11Fax. +41­44­944 31 70

Subject to technical changes© 03/2010 Mettler­Toledo AGPrinted in Switzerland 30004091Global MarCom Switzerland

www.mt.com/moistureFor more information

Results

Conclusion

Alternative Method for determining free and crystal water

The HR83 offers the option to add two methods. In the first method the free water content is determined at 50 °C, followed by the second method in which the crystal water is determined at 200 °C. The dry weight of the first method is used as the starting weight for the second method. The final result displayed is the crystal water content.

Method 1: 50 °C for 25 min (add method “2”), standard dryingMethod 2: 200 °C with switch­off criterion 5 (1 mg / 140 s), standard drying