What's In Your Fly Ash and Bottom Ash?

In my last article, I discussed how coal ash – also known as fly ash, flue ash, and bottom ash – which is formed during coal combustion, can be recycled into material for Portland cement production. Formerly regarded as toxic, coal ash is now being filtered to avoid harming the environment so it can be re-used. However, the chemical makeup of fly and bottom ash varies significantly and is dependent on the source and composition of the coal being burned. Before ash can be recycled, the composition of the ash product needs to be accurately analysed to ensure the removal of toxins and the quality of the cement. Would inductively coupled plasma optical emission spectrometry (ICP-OES) be an appropriate method to analyse the coal ash? According to Xiandeng Hou and Bradley T. Jones [Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd]:

Inductively coupled plasma/optical emission spectrometry (ICP/OES) is one of the most powerful and popular analytical tools for the determination of trace elements in a myriad of sample types…. The technique is based upon the spontaneous emission of photons from atoms and ions that have been excited in a RF discharge…. The sample solution is converted to an aerosol and directed into the central channel of the plasma…. the aerosol is quickly vaporized…. Analyte elements are liberated as free atoms in the gaseous state. Further collisional excitation within the plasma imparts additional energy to the atoms, promoting them to excited states. Sufficient energy is often available to convert the atoms to ions and subsequently promote the ions to excited states. Both the atomic and ionic excited state species may then relax to the ground state via the emission of a photon. These photons have characteristic energies that are determined by the quantized energy level structure for the atoms or ions. Thus the wavelength of the photons can be used to identify the elements from which they originated. The total number of photons is directly proportional to the concentration of the originating element in the sample.

We tested the method on some coal and ash samples.

Sample Preparation

Two certified reference materials (CRMs) — 1 bituminous coal and 1 fly ash, both from the National Institute of Standards and Technology, USA (NIST) — were used in the analysis. They were prepared in triplicate using the following method:

The samples were digested by adding 6 ml of 20% hydrochloric acid (HCl) and 2 ml of 20% nitric acid (HNO3) to approximately 0.1g of solid sample.
This was then heated to 220 ˚C for 35 minutes using a microwave digestion system.
Once the samples had cooled they were made up to 25 ml with <18.2 ultrapure de-ionized water.
Calibration standards were created from 1000 mg/L and 10000 mg/L single element solutions and were matrix matched to the sample digests.

The Results

The analytical results of the two CRM samples compared with the certified and reference values. An empty sample tube was subjected to the same sample digestion method as the samples; this blank was then analysed with 10 replicates. The method detection limits were calculated by multiplying the standard deviation of these 10 replicates by a factor of 3; this was performed in triplicate and an average taken. Here are the analytical results, which are within ±5% of the certified vales and within ±15% of the reference values, demonstrating the accuracy of this method for the analysis of coal and coal ash samples.