Scope

This test method covers the thermogravimetric (TGA) determination of CO2 reactivities and dusting of shaped carbon anodes used in the aluminium reduction industry. Many types of apparatus are available with a wide variety of thermal conditions, sample-size capability, materials of construction and procedures for determining the mass loss and subsequent rate of reaction. This test method standardizes the variables of sample dimensions, reaction temperature, gas velocity over the exposed surfaces, and reaction time such that results obtained on different types of apparatus are correlatable.

Principle

The dusting rate and the initial, final and total CO2 reactivities are determined by passing carbon dioxide gas at a flow rate that gives a standard velocity of reactant gas around cylindrically shaped carbon artifacts under isothermal conditions for a specified length of time. The reactivity is determined by continuously monitoring the sample mass loss. The CO2 dusting rate is determined by collecting and determining the mass of carbon particles that fall off the sample during reaction.

Apparatus

The apparatus to be used should be as simple as possible and be commensurate with what is to be achieved. The principal criterion is that the reaction rate be determined under isothermal conditions and be unaffected by physical and chemical properties inherent to the apparatus (such as gas diffusion patterns, gas temperature, exposed sample surface area, and so forth). A typical apparatus that has been found to be suitable is illustrated in Figure 1.

Furnace and controller capable of maintaining constant temperature, within ± 2 °C in the 100-mm reaction zone in which the sample is centred. 

A typical apparatus, as illustrated in Figure 1, employs a three-zone heating element and associated controls to accomplish this, but other types of heaters such as tapered windings or long linear heaters are also suitable. The control thermocouple is a grounded type and shall be located within the reaction chamber near the surface of the test sample. This is to allow the furnace controller to compensate for the exothermic reactions that occur when the furnace is used for air reactivity testing. The control thermocouple shall be positioned 4 mm ± 1 mm from the side surface of the sample and within 5 mm vertically of the centre of the reaction chamber. The furnace shall be large enough to accommodate the reaction chamber.

Reaction chamber, consisting of a vertical tube constructed of a material capable of withstanding the temperature of the reaction, e.g. 960 °C ± 2 °C with a sufficiently large inside diameter to accommodate the sample and sample suspension device while not affecting the gas flow past the sample. An inside diameter of 100 mm ± 25 mm is recommended.

The reaction chamber shall be constructed with a removable dust-collection cup at the bottom capable of capturing all the dust that falls off the sample during the test. The most common materials of construction are quartz and Inconel1).

Sample suspension device, capable of supporting the sample in the reaction chamber for the duration of the test and which should be reusable.

The sample holder shall not change in mass during the test, shall not affect the flow pattern of the gas past the sample, shall not limit the gas-accessible surface area of the test sample and shall not interfere with the production of dust by the sample. A typical sample holder is illustrated in Figure 2.

*Contact us (sales@goldapp.com.cn) to get this downloaded PDF*