Back to Basic Slags as a Phosphorus Source and Liming Material

Basic slags are alkaline by-products from the steel industry or coal-fired power stations, which can be used as a liming material and soil amendment. They are mainly composed of silicates, aluminates and oxides of Ca and Mg. In the coal-fired power station, slag is also generated from fluidized-bed boiler ash and flue gas desulphurization (FGD), a gypsum-like product called FGD gypsum. It mainly contains a mixture of gypsum, calcium sulfite (CaSO₃), fly ash (FA), unreacted lime (CaO) and portlandite (Ca[OH]₂) or calcite (CaCO₃). Chemical and mineralogical composition, leachate chemistry and textural properties of basic slags are helpful for understanding their nature and assessing their impacts on the environment. The characterization of various slags can be analyzed by different instrumental methods. Inductively coupled plasma mass spectrometry and optical emission spectrometry (ICP-MS/OES) are the most commonly analytical techniques for chemical composition, and X-ray diffractometry (XRD) is used to provide further information on types of minerals and their phases. Additionally, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) could be auxiliary to analyze some specific samples, which are not easily identified or quantified by XRD. Some slags have been commercialized to be the source of nutrients and soil amendments, with the major nutrients including P, S, Ca, Mg and silicon (Si). Slags from steel industry are rich in Ca and Mg, which are beneficial to deficient soils. Slags from coal combustion are a good source of sulfur, while the presence of calcium in the form of lime and gypsum helps to mitigate the soil acidity and improves soil structure. However, they also possibly contain toxic metal contaminants, such as chromium (Cr) and vanadium (V). There have been some concerns about the toxic effects of slag materials on farm animals. Slag poisoning of animals can be avoided or minimized by increasing the dissolution of slag through soil incorporation and by avoiding overgrazing of slag treated paddocks.