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Mass Flow Analysis of Chromium and Chromium Compounds 4 Chromium as a trace constituent
Chromium occurs as a trace constituent in a wide range of raw materials and products. The most significant of these are;
Here, we focus on chromium as a trace constituent in fossil fuels and cement. 4.1 Coal and oilFossil fuels like oil and coal contain a number of heavy metals, including chromium. The average contents are shown in Table 4.1. If we base our calculations on an annual consumption of oil and coal of 182 PJ/year and 386 PJ/year, respectively, in 1999 plus residual product quantities from Danish CHP plants, we can establish a balance for chromium as a trace constituent in fuels; see Table 4.1. The figures on fuel quantities describe the gross energy consumption and cover all kinds of coal and oil products. Coal is, however, primarily used at CHP plants, which means that the residual product quantities come mainly from coal. Table 4.1
A total 0f 99% of the residual products are reused in a wide range of products, e.g. cement, concrete, lightweight concrete, asphalt, etc. The remaining 1% is sent to landfills. 4.2 CementIt is a well-known fact that cement can contain quite a lot of chromium. To ensure a safe working environment, there are rules for the content of Cr(VI). In all likelihood, chromium in cement probably comes from raw materials from specific areas, from coal used during firing, and from fly ash used as part of the cement. During the period 1998–2000, Aalborg Portland used 225,171 tonnes coal each year and 187,350 tonnes of petroleum coke/year on average (Aalborg Portland, 2001a). The chromium content of coal varies, but is often 32 mg Cr/tonne (ELSAM, 2002). If we assume that coal and petroleum coke have the same chromium content, we arrive at a Cr input of 0.013 tonnes Cr/year. During the period, an average of 204,819 tonnes fly ash/year were used. Fly ash from CHP plants contains 78–230 mg chromium/kg (ELSAM, 2002). This corresponds to chromium contributions to cement of 16–47 tonnes/year. The other raw materials used to make cement, chalk and sand, are not expected to contain large amounts of Cr. Analyses carried out at Aalborg Portland show that their "Basis" cement contains 32 ppm Cr-total, of which 5–6 ppm is chromate Cr(VI). Other types of cement – white cement and low-alkali cement – often have lower contents of Cr(VI) << 2 ppm. It is, however, assumed that the same relationship between total Cr and Cr(VI) applies (Cementfabrikkernes tekniske Oplysningskontor, 2002a). Approximately 1.5 million tonnes of cement are used in Denmark each year. Aalborg Portland sells 1.3 million tonnes of cement on the Danish market each year and exports 5–600,000 tonnes, particularly to the USA (Cementfabrikkernes tekniske Oplysningskontor, 2002a). During the period 1998–2000, the average production at the Aalborg factory was as follows; 2,487,123 tonnes cement/year, 145,963 tonnes clinker/year, and 3,450 tonnes filler/year (Aalborg Portland, 2001a). According to Statistics Denmark, the total quantity of cement produced in Denmark during this period was 2,622,168 tonnes/year on average. This is to say that almost all of this cement was produced at Aalborg Portland. As a result, their figures on chromium contents are used for general purposes. The total quantities of cement produced are divided into grey cement and other cement. In 2001, Aalborg Portland produced 2.1 million tonnes grey cement (Aalborg Portland, 2001b). As the total production figures were more or less identical for 1999 and 2001, it is estimated that 2.1 million tonnes of the cement produced in 1999 are also grey cement. The rest (2,622,168 – 2,100,000 = 522,168 tonnes) is assumed to be of a kind with lower contents of chromium (white cement, low-alkali cement). The actual chromium content is not known, but the content of Cr(VI) is much less than 2 mg/kg. If the content is set at 0.2–1 mg Cr(VI)/kg, this leads to chromium quantities of 0.10–0.52 tonnes Cr(VI)/year and 0.6–3.0 tonnes Cr(III)/year. Production of 2,100,000 tonnes grey cement/year containing 32 mg Cr/kg yields 67.2 tonnes Cr/year. Of this amount, 10.5–12.6 tonnes are Cr(VI). As far as possible, Cr(VI) is reduced to Cr(III) by adding Fe(II); this reduces the concentration of Cr(VI) to less than 2 ppm. Thus, production of 2,100,000 tonnes grey cement/year containing 1–2 mg Cr(VI)/kg yields 2.1–4.2 tonnes Cr(VI)/year and 63,0–65,1 tonnes Cr(III)/year. Table 4.2
4.3 SummaryChromium occurs as a trace constituent in, for example, fossil fuels – coal and oil – and in cement as a result of the use of coal and residual products from energy production in connection with cement production. Table 4.3 shows the estimated consumption and dispersal of chromium. Table 4.3
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