Investigating Road Pollutants: Determining the Concentration of Fluorine Contaminants in Slag

The combination of iron and steel slag when constructing roads may cause the presence of fluorine compounds present within the slag to seep into the environment and cause potentially toxic effects. Dominik Hahn, PhD candidate, department of Technical Chemistry at the University of Koblenz-Landau has developed a method by which the concentration of fluorine contaminants present within slag samples can be determined by use of Combustion IC.

According to an estimate provided by the US Geological Survey, the steel industry accrued approximately 460-600 million tons of iron and steel slag as a result of blast furnaces and steel mill slag in 2016 alone. Slag is defined as the non-metallic substance that remains following a number of industrial processes such as the smelting of ore. It is important to note that slag itself is not considered a waste product, in fact, the repurposing of slag has formed the basis for potential alternative construction materials used in road engineering procedures. Additionally, the repurposing of slag as raw materials has found its successful application within the cement industry.

An important parameter that is required to assess the environmental compatibility of iron and steel slag involves detecting the leachability of fluorine contents within the slag sample. While there is a limited amount of information available on understanding the exact processes by which the leaching of slag products containing fluorine occurs, a number of current research projects have been focused on understanding this phenomenon. By understanding the complex processes of fluorine leaching into slag, researchers are hopeful that this newly discovered knowledge will influence changes in future metallurgical work.

The Ability of Fluorine to Leach from Slag

Assessing the leaching behavior of slag by the amount of fluorine that is leached in column or batch equates is not sufficient for analytical purposes. Fluorine concentrations in slag must be considered in relation to the original total concentration of fluorine that is present in the solid material. Current research on the leaching behavior of slag requires a quick routine method to measure the total concentration of fluorine present within a slag sample. These analytical methods must exhibit high precision and detection limits that are below 50 mg/kg.

Traditional analytical techniques that are used to measure the concentration of fluorine in slag samples often require intensive sample preparation. Following the transfer of the sample preparation, disruptive elements such as calcium, aluminum and iron(III) ions must be removed from the analyte samples. Manually performing this sample preparation procedure is often a highly time consuming and cost intensive process. Additionally, these traditional analytical techniques are often a major source of error.

Combustion IC: A Reduction in Time Requirements and Error Production

Until recently, ion chromatography with inline combustion digestion (Combustion IC) has been used primarily for the analysis of organic matrices such as fuels, polymers, pharmaceuticals and food products. The Combustion IC instrument is equipped with fully automated analytical capabilities and is therefore suitable for the determination of the total amount of fluorine present in slag samples. Since this technique has not been previously utilized for the analysis of slag, the parameters of pyrohydrolytic digestion, as well as the transferring of the analyte into the absorption solution, require optimization in order to ensure an overall reliable analysis of the inorganic oxide elements. Performing the sample preparation manually is not only time and cost intensive, but also represents a major source of errors.

Figure 1. The Combustion IC system comprising 930 Compact IC Flex, 920 Absorber Module, and Combustion Module from Analytik Jena.

Sample Preparation in Combustion IC

Pyrohydrolytic digestion, which is also described in DIN 51084, is fully automated in Combustion IC and is connected inline to the IC analysis system. Thermal digestion of the sample takes place under an argon atmosphere, whereas pyrolytic gases will require a continual addition of small amounts of ultrapure water to achieve this digestion process. The addition of ultrapure water prevents unwanted deposits and glass corrosion from occurring, while simultaneously ensuring that the fluorine concentration within the sample is fully transformed into hydrogen fluoride.

The gases produced by combustion digestion are dissolved in an absorption solution that is then transferred to the analysis module. The solution is then degassed, and prior to the chromatographic separation, inline ultrafiltration is used to release the particles and ensure the protection of the separation column. The subsequent suppression ensures a stable and low background conductivity, as well as a precise and correct determination of the fluorine concentration by means of conductivity detection.

Figure 2. Operating principle of the Combustion Module.

Optimization of Combustion IC for Analysis of Slag

To optimize Combustion IC to determine fluorine in slag, a sample weight, post-combustion time, ultrapure water rate, and the volumes and concentrations of the elution and absorption solutions must be considered.

Post-Combustion Time

Trials involving various post-combustion times have shown that extended, post-combustion times, enabled more effective digestion. Longer post-combustion times allow the fluorine to be separated more effectively from strong bonds, as are found in slag. The flattening of the curve with post combustion times exceeding 300 seconds, marks the optimum that can be achieved by adapting the post-combustion time.

Figure 3. Optimization of the post-combustion time.

Sample Weight

The attempts that were made to optimize the sample weight revealed that digestion is most effective method for samples that are approximately 10 milligrams (mg) in size. Sample sizes that are smaller than 10 mg impact the precision of the measurement, as well as causing the analyte concentrations present within the eluent to also be smaller.

A sample size larger than 10 mg will result in an adverse relationship to occur between sample surface and sample mass. The digestive effect of combustion of a small area on which energy can be transferred will lead to a temperature gradient in the sample during combustion digestion, thereby causing aggregates to form. A maximum sample surface is achieved by delicately distributing the sample on quartz wool felt.

Figure 4. Optimization of the sample weight.

Ultrapure Water Flow Rate

To improve the absorption of the combustion gases by water vapor, adapting the ultrapure water flow rate can be advantageous. Doubling the ultrapure water flow rate to 0.2 mL/min has been shown to demonstrate a more effective absorption in the analyte. Furthermore, a higher flow rate also counteracts glass corrosion and deposits that may reside within the combustion tube.

Figure 5. Optimization of the ultrapure water flow rate.

Increasing the Detection of Fluorine Volumes

To improve the transferring of the analyte the IC module, further optimizations on the volumes and concentrations required for the proper elution and absorption solutions were made. As a result of these changes, detectable fluorine concentrations increased by an average of 55%.

Figure 6. Optimization results: The detected fluorine volume increases by at least 7.5% and a maximum of 116.3%. It increases by 55% on average.

Close-up of part of the Combustion Module's sample injection system. Light from the pyrolysis oven is conveyed through the optical fiber to the flame sensor. To make it easier to see the optical fiber, the flame sensor has been removed from this image.

Fluorine Determination as the Basis for an Environmentally Friendly Process to Manage Slag

The validation, detection and determination limits for the samples of 2 mg/kg and 6 mg/kg were utilized for the examination of fluorine contents present within a slag sample. Over a range of < 10 mg/kg to 5,000 mg/kg, a standard deviation of < 2% was achieved. Combustion IC was shown to be a price and reliable technique to not only determine the fluorine concentrations present with slag samples, but also assess the environmental compatibility and usage potential of slag samples for future use as repurposed material.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

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