Carbolite Gero's ashing furnace range covers one of the most common applications for laboratory chamber furnaces: To heat combustible samples in order to analyse the resulting ash residue. As there is no single solution for all requirements, different furnaces tailored to ashing and burn-off applications are available.
INTRODUCTION TO ASHING
Ashing is the process of removing all the organic material from a sample by heating in air. The objective of the process may be to ascertain the proportional mass of the residual material (ash), or it may be an intermediate step in the preparation of the sample for other purposes, such as characterisation of the inorganic components through XRF (X-ray fluorescence), mass spectroscopy, or for other tests such as ash fusibility.
THE ASHING PROCESS
Prepared samples are heated in air until they react with the oxygen present and combust. The organic compounds within the sample are subsequently oxidised, and the residual ash comprises the inorganic, non-combustible compounds.
Some tests are defined by international standards such as ISO, EN, ASTM etc., to which both the process and the equipment used must comply. There are also processes with specific objectives, such as Loss on Ignition (LOI), which requires samples to be weighed before and after ashing to determine the reduction in mass caused by heating. It is also possible to weigh samples continuously during the heating process to record the rate at which the sample mass reduces, and to determine complete combustion, by using a furnace with an integral balance.
A specialised ashing furnace is the most appropriate piece of equipment in which to carry out this process. Carbolite Gero produce ranges of general purpose ashing furnaces, and those designed to comply with specific coal and coke testing standards.
ASHING FURNACES
Typically, an ashing furnace is designed to promote a high level of airflow through the chamber to aid sample combustion and remove the smoke created during processing.
The air is guided through the furnace insulation assembly to ensure that it is pre-heated before entering the chamber; this reduces the risk of inadvertently lowering the temperature during processing. The increased airflow also serves to carry any smoke and volatiles from the chamber and out of the exhaust.
An optional afterburner or thermal catalytic oxidiser fitted to the exhaust can further reduce the emissions escaping into the surrounding atmosphere.
Air is drawn through an inlet at the rear of the furnace and guided through the insulation where it is pre-heated before entering the chamber.
Because of the risk that exhaust fumes and volatiles emitted from samples could damage the heating elements, an ashing furnace is often fitted with silicon carbide (SiC) protection tiles that sit within the furnace chamber and provide a barrier between the samples and the heating elements.
In smaller models, the heating elements are wrapped around the outside of a ceramic muffle, into which samples are placed. The muffle contains the smoke and volatiles produced by the process, whilst protecting the heating elements from contamination.
For tests or processes where alumina/silica dust could contaminate results, or react with the sample to produce corrosive vapours, a fused quartz chamber may be necessary to protect both the furnace and the process.
Video: Thermal Catalytic Oxidiser (Catalytic Converter) reducing the amount of fumes exiting the furnace.
INDUSTRIES THAT USE ASHING PROCESSES
Both the food production and pharmaceutical industries ash samples of their products as part of their quality control checks to determine the amount of inorganic, indigestible matter contained within them. If necessary, samples can be ground down or milled to increase the surface area and enable them to be mixed with other compounds prior to heating.
Because the composition of products can vary due to their natural origin, the petrochemical industry frequently ashes samples of hydrocarbon materials to determine the amounts of metals and other inorganic chemicals present. This is important, as certain metals and minerals may cause damage to the refining equipment or later processes.
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The following example shows the change in sample weight and chamber temperature within an ashing furnace fitted with an afterburner.
VIEW INSIDE OF AN ASHING FURNACE
Cross section showing the ABF ashing furnace. Air is drawn through an inlet at the rear of the furnace, and guided through the insulation where it is pre-heated before entering the chamber. Smoke and volatiles exit into the afterburner where additional air enters to promote burning. A fan causes the fumes and fresh air to mix in the plenum and exit out of the exhaust.
Air inlet – air is preheated before entering the chamber
Air inlet into afterburner to ensure complete combustion
Air inlet into plenum to cool the gases before they enter the extraction fan
Chimney
Furnace chamber
Afterburner
Extraction fan
Two-tier perforated basket containing samples
APPLICATION EXAMPLES
Sample materials that are ashed in furnaces include food, soil and peat, plastic containers, rubber, animal feed/cereals, wool and natural fibre, paper, flour, pharmaceutical products, vegetable oils, paint, polyester and fibreglass, gelatine, sewage sludge, wood and test firing ceramics, soil to determine mineral content or plasticized PVC for calcium carbonate content.
Other processes that use an ashing furnace:
Sulphated ash determination
Combustion of coke and sulphur in spent catalysts
Loss on ignition measurement
To burn off mercury chloride residues
Filler content determination in paper
Removal of soot from samples of fumed silica grease
Cleaning ICP Torches
Volatile suspended solids in effluent analysis
Ash determination of coal samples
Resin burn-off of CFC test specimens
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