Laurent Guillot
Cement Technical Director
CHRYSO Group
European Directive 2003/53/CE, which prohibits the sale of cement containing more than 2ppm of hexavalent chromium (Cr VI) after hydration, has been officially in place since 17 January 2005. For cement manufacturers, this adds a further constraint as they look for efficient, economical and flexible solutions that enable them to comply with regulations.
Amounts of soluble hexavalent chromium found in hydraulic cements may originate from a variety of sources, including fuels, refractory bricks and cementitious additions.1 However, it most often comes from raw materials and grinding media. The raw materials contain very small quantities of elemental chromium, a common element in the earth’s crust. The increasing use of many byproduct raw materials such as metallurgical slag, spent catalyst fines, flue gas desulphurisation gypsum, lime sludge and others may also contribute additional amounts, although little data has been published in this area. Typical total chromium from the primary raw materials, according to type and origin, are shown in Table 1.2
Most quarried raw materials contain no water soluble chromium as Cr VI, and chromium is usually in oxidation state Cr III.2 If chromium alloys are used in grinding media and crushers, they may contribute metallic chromium. Klemm reports that in clinker ground with chrome alloy balls containing 17-28% chromium, the Cr VI content of the cement may increase to over twice that of the original clinker.3 However, the reduction in use of such materials over recent years makes this a less likely source of chromium. Regarding conversion to Cr VI during finish grinding, possible favourable conditions are discussed in the following sections of this article.
Cr VI Formation
Chromium from the kiln feed is primarily in the form of Cr III. The conditions in the kiln include high amount of CaO, free lime and alkalis due to the internal circulation of volatiles. Such conditions are favourable for oxidation of chromium to Cr VI, the amount of which depends on the oxygen pressure in the kiln atmosphere.
Additives to Reduce Cr VI
Additives that allow cement to be treated to reduce Cr VI are well known. They include compositions that contain Fe II sulphate. However, Fe II is unstable in aqueous solution, meaning that such additives have to be used in powder form. Precise powder dosing is awkward and requires specific equipment. Moreover, these additives rapidly lose their capacity to reduce Cr VI, due to oxidation of the Fe II ions to Fe III ions on contact with air. The efficacy of these additives is therefore reduced if they are stored for a long time.
Due to these drawbacks, CHRYSO chose to de¬velop a solution based on tin (Sn).4 In a highly basic medium (pH~13), such as in the interstitial medium of cement, Sn II reacts with the hydroxide (-OH) ions of the medium to form Sn(OH)4 2- (See Equation 1).
Sn2+ + 4OH- Sn(OH)4 2- (Eq. 1)
The Sn(OH)4 2 ion is able to reduce CrO4 2- to Cr(OH)3- by a redox reaction (See Equation 2).
2CrO4 2-+8H2 0+3Sn(OH)4 2- 2Cr(OH)3 + 4OH- + 3Sn(OH)6 2- (Eq. 2)
The main aim of the CHRYSO invention was therefore to propose an additive to reduce Cr VI ions to Cr III ions in the form of a solution that is stable during storage. The term ‘stabile’ herein refers, primarily, to the absence of precipitation, even at a pH of greater than 12.
The outcome of a research and development project was the launch of CHRYSO® Reductis 50, a patented solution based on tin complex technology to reduce Cr VI reliably in cement.
Case study 1: Sustainability of Cr VI reduction
In the following case study, the ability of CHRYSO® Reductis 50 to maintain Cr VI over a period of months after treatment of the cement was shown. A CEM I cement sample was treated with CHRYSO® Reductis 50 and compared with an otherwise idential but untreated sample. The dosage of CHRYSO® Reductis 50 was 400ppm, according to untreated cement Cr VI content.
From an initial Cr VI content of 10ppm, the level fell to less than 1ppm immediately upon injection of CHRYSO® Reductis 50 at a dosage of 400g/t of cement. As shown in Table 2, the main properties of the cement were not affected by using CHRYSO® Reductis 50.
The sustained performance of the Cr VI reducing agent was also studied over an entire year by picking cement samples from the same bag for analysis. The amount of Cr VI in the treated cement was roughly stable and always below 1ppm over the 12 month period (See Figure 1).
Case study 2: Way to extend Cr VI reduction
The tin-based liquid solution was also compared to regular iron sulphate and equivalent powders. In order to reach the same level of performance, the powder requires higher dosages. An example is given by the following laboratory study showing a comparison of Cr VI reduction by iron sulphate and CHRYSO® Reductis 50 treatments (See Figure 2). This study was conducted under laboratory conditions with CEM I cement. The initial Cr VI concentration was 5ppm prior to treatment.
In order to reach 1ppm Cr VI content in cement, liquid CHRYSO® Reductis 50 required 40ppm/ ppm of Cr VI, whereas powdered iron sulphate needed a dosage of 650ppm/ppm of Cr VI, more than 16 times more. Despite this high dosage, the longevity of the reduction effect with powder is much more limited. After just three months of storage, the Cr VI level approaches the 2ppm limit. On the other hand, despite its very low dosage (200g/t of cement), CHRYSO® Reductis 50 led to a constant Cr VI concentration of close to 0.5ppm over a six month monitoring period. This demonstrates the high added value of its longer shelf life.
A high level of optimisation can be obtained with a combination of both liquid and powder reducing agents. Indeed, the powder’s poor long-term reducing ability can be offset by use of CHRYSO® Reductis 50. This means that the shelf life of a powdered Cr VI reducing agent can be extended. Industrial trials have been conducted with this aim in mind.
A comparison between powder iron sulphate (600ppm/ppm of Cr VI), CHRYSO® Reductis 50 (50ppm/ppm of Cr VI) and a combination of both products (600ppm + 30ppm) were investigated. CHRYSO® Reductis 50 was injected on cement mill feed conveyor belts to complement an existing iron sulphate treatment.
The cement’s Cr VI content was monitored over six months and the three options were compared. The initial Cr VI concentration was 4ppm prior to treatment. The addition of CHRYSO® Reductis 50 at a very low dosage (20ppm/ppm of Cr VI or 80g/t of cement) alongside the powdered agent (600ppm/ppm of Cr VI) leads to a significant improvement in cement shelf life.
This opens up different possibilities for the cement plant to optimise iron sulphate dosage at lower level, while maintaining cement shelf life.
Conclusion
The CHRYSO Group, through its Cr VI reduction expertise, brings added value to cement producers by:
• Extending cement shelf life;
• Reducing the cost of Cr VI treatment;
• Improving cement quality.
CHRYSO® Reductis 50 is a unique patented Cr VI reduction solution based on tin complex chemistry that can be used either alone or in combination with an iron sulphate powder. As every cement plant has its own specific process, material, product and cost characteristics, CHRYSO can provide customised solutions to help cement producers achieve their target key performance indicators and monitor their daily technical performance.
Sources: 1. ‘Hexavalent Chromium in Cement Manufacturing: Literature Review,’ Hills, L. & Johansen, V. C.; PCA R&D Serial No. 2983. 2. ATILH Center for Information and Documentation, ‘Chromium in Cement, origin and possible treatments,’ September 2003. 3. Klemm, W. A.; ‘Hexavalent Chromium in Portland Cement,’ Cement, Concrete and Aggregates, Vol. 16, No. 1, 1994, pages 43-47. 4. ‘Additive for reducing chromium (VI) to chromium (III) ions,’ US patent No. 8361221, CHRYSO.