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Mass Flow Analysis of Chromium and Chromium Compounds

1 Introduction

1.1 Objective of the analysis

Due to the health risks associated with chromium(VI) compounds (hexavalent chromium), efforts have been made to identify all flows of this substance.

1.2 Method and scope

A mass flow analysis is based on the principle of substance balance over a given period of time. This principle states that:

Input + formation = accumulation + output + degradation

Chromium is an element, which means that it will neither form nor be degraded. As a result, the mass flow is simple: the quantities entering the community (input) equal the sum of the quantities leaving the community (output) and the accumulated mass. See Figure 1.1.

Figure 1.1
The principle of mass flow analysis

The survey of the mass flow for chromium is based on statistics from Statistics Denmark about imports and exports (Statistics Denmark, 1999a; 2000a; 2001a) and sales of domestically produced goods in Denmark for 1998–2000 (Statistics Denmark, 1999b; 2000b; 2001b). This information has been supplemented with information from the Product Register about the supply and composition of products containing chromium and their use within various functions. Among other things, the information from the Product Register is used to identify CN¹ numbers for chromium and products containing chromium for which Statistics Denmark has compiled figures on import, export and production. Efforts have been made to confirm the statistic information by contacting relevant industry organisations, companies and the like.

The mass flow analysis looks at the flows of chromium and chromium compounds in Denmark during a single year. In order to take fluctuations in sales into account, average figures from Statistics Denmark have been used. These average figures are for import, export and sale of domestically-produced goods in Denmark in the years 1998, 1999 and 2000. More years have also been included in the analysis in order to determine development trends as regards the use of chromium.

When we look at the data basis from Statistics Denmark, we see examples of significant variations in the sales of goods between the years 1998–1999 and 2000. One of the reasons for this is that in 2000, Statistics Denmark made an extra effort to have companies report quantities which they had not previously felt able to report information on (Statistics Denmark, 2002).

This means that 2000 may be a more accurate reference year than the preceding ones, as reports on quantities of chromium and chromium compounds have been made for more items in 2000 than ever before. On the other hand, the reports on these hitherto unreported quantities may not be entirely accurate. This uncertainty has to do with the fact that they cannot be compared with previous reports, and the people supplying the new information may not be used to doing so, and may make mistakes as a result of their lack of experience.

If, for example, we look at item no. 74122000, "Pipe fittings made from copper alloys", the statistics tell us that a total of 54,325 tonnes of this product were sold in Denmark in 2000. This seems unlikely, as it would mean that every citizen in Denmark bought approximately 10 kg of pipe fittings. One explanation for this mistake might be that the report has been made using the wrong unit, e.g. "pieces" instead of "tonnes". In cases where the quantities stated for a year (primarily 2000) appear obviously unrealistic, the year in question has been disregarded in the calculations of the average quantities for 1998–2000².

1.3 Chromium and chromium compounds

1.3.1 Occurrence and use of chromium

Chrome is a shiny, very hard and brittle metal. It belongs to the group of heavy metals and is the 13^th most common element on Earth. It occurs in nature as red lead ore (PbCrO₄, crocoite) and as chromium ironstone (FeO, Cr₂O₃). Metallic chromium can be produced by reducing chromium(III) oxide (Cr₂O₃) with aluminium.

Chromium is used in many contexts, either in metallic form or in chemical compounds. The Danish Product Register includes records of approximately 130 chromium compounds, and chromium(III) oxide, chromium (VI) oxide, metallic chromium and lead(II) chromates account for more than 95% of the total consumption if we take a worst case scenario view³.

As metallic chromium is very chemically stable, it is often used to cover the surface of less durable metals. This is known as chromium plating. Chromium is also widely used as an ingredient in metallic alloys. Among other things, it is used in ferrous chromium, a carboniferous 60% chromium-iron alloy, in chromium steel with 12–13% chromium, in 18/8 steel (18% chromium and 8% nickel), and in particularly heat-resistant special steel (25–30% chromium and up to 15% nickel).

Among other things, chromium compounds are used as pigments in paints, printing colours, artists' colours, and similar products. Particularly popular are lead chromate and zinc chromate – both of them yellow – and the greenish chromium oxide. Tanning of leather involves chromium in the form of chrome alum, a double salt consisting of potassium sulphate and chromium sulphate. Within the chemical industry, the reactive chromium(VI) is used as an oxidising agent, particularly in the form of chromic sulphuric acid (potassium dichromate and concentrated sulphuric acid) and in catalysts.

1.3.2 Physical/chemical characteristics

The environmental and health-related characteristics of chromium and chromium compounds depend on the relevant oxidation level. As a result, a brief description of the electrochemical characteristics of chromium in the environment is given. The most significant physical-chemical characteristics of chromium and a number of inorganic chromium compounds are presented in tables 1.1 and 1.2.

Table 1.1
Physical-chemical characteristics for chromium and selected inorganic chromium compounds

Table 1.2
Physical-chemical characteristics for chromium and selected inorganic chromium compounds

1) Alkaline lead chromate (18454-12–1; 1344-38-3) also occurs.

1.3.3 Electro-chemicals characteristics

Chromium occurs within the oxidation levels 3, 6 and 2, listed in order of decreasing stability. Cr(VI) can be reduced to Cr(III) by Fe(II). In all likelihood, soluble Cr(III) complexes are formed with the organic ligands. Similarly, Cr(VI) is reduced in soil with high humus contents and in connection with microbial activity.

The conditions for reduction of Cr(VI) are not good in surface water, in sea water, in aerobic soil and in sediment. Cr(VI) will, however, often be mobile. As a result, it may reach anaerobic areas (such as lower layers of sediment) where reduction can occur.

It is not very likely that oxidation of Cr(III) to Cr (VI) will occur in nature. Oxidation is only to be expected under aerobic conditions and when MnO₂ is present.

At pH< 1, Cr(VI) will appear as H₂CrO₄; at 2< pH <6 it will appear as an equilibrium between HCrO₄ and Cr₂O₇^2–, and at pH>7 it will appear as CrO₄^2–. Similarly, in acid liquids Cr(III) will appear as Cr³⁺, Cr(OH)²⁺, Cr(OH)₂⁺, Cr(OH)₃ and Cr(OH)₄^– as pH values increase. At pH>5, however, Cr(III) is deposited as Cr(OH)₃, even though complex formation with organic ligands may compete with this process, thereby increasing solubility. Chromium compounds are not volatile. In the atmosphere, they will mainly occur in association with aerosols and particles.

1.3.4 Classification of chromium compounds

The classification of chromium compounds on the basis of their inherent characteristics is of great significance to the use – and limitations on use – of the substances in question. Table 1.3 shows the classifications of the most frequently used chromium compounds.

Table 1.3
Classification of the most frequently used types of chromium and chromium compounds

Due to their classification as carcinogenic and mutagenic, chromium(VI) oxide, lead(II) chromate and the other chromates are all subject to a number of limitations on sale and use (The Danish Ministry of the Environment, 2000). The substances are not available in retail shops and may only be sold to buyers who submit requisitions in accordance with specific rules. Limitations have been introduced for the use of products which contain hexavalent chromium compounds. These limitations include a requirement for notification of the Danish institute for occupational safety and health, the National Working Environment Authority (the Ministry of the Environment, 1997). The limit value for chromic acids and chromates in the working environment is 0.005 mg/m³. (National Working Environment Authority, 2000).

1.4 Prioritisation of the survey

1.4.1 Hazardousness and potential exposure

Chromium primarily occurs as metallic chromium and as chromium compounds, where chromium has an oxidation level of 3 or 6. The toxicological and eco-toxicological properties of chromium depend on the chemical combinations in question. Due to their high bio-availability and highly oxidising properties, Cr(VI) compounds are far more toxic to biological systems than Cr(III) compounds. This means that Cr(VI) compounds are far more hazardous than Cr(III) compounds (EU, 2000a). Generally speaking, metallic chromium is not available for absorption in organisms in nature and is generally held to present little hazard and little potential for exposure.

The risk of being exposed to chromium is particularly high for people working in industries where chemicals containing chromium are used. Cigarette smokers are also at risk⁴. For most people, however, food will be the greatest source of chromium intake (ATSDR, 2000). Occurrences of chromium in products can, however, lead to direct human exposure which is not work-related. Examples include chromium in dust from brake linings and cement dust. People working with wastewater from chromium-plating industries, tanning, textile industries and waste containing chromium also risk exposure. Chromium might occur in drinking water as the result of pipes made from alloys containing chromium in the supply system; this does not, however, seem likely, as such chromium alloys are very stable. Contamination from landfills is another possible source of chromium in drinking water. Finally, exposure to chromium may occur due to incineration of products which contain chromium, e.g. impregnated wood and fossil fuels (ATSDR, 2000).

As was the case for human exposure, the main source of potential environmental problems is exposure to Cr(VI) due to the toxic effects and high bio-availability of this substance. Cr(VI) is relatively stable in water, a fact which increases the risk of critical exposure. It is estimated that the aquatic environment is mainly subjected to exposure through wastewater from companies using chemicals which contain chromium, but household wastewater and atmospheric deposition can also be sources of exposure. Alloys containing chromium are not expected to cause significant exposure as far as the aquatic environment is concerned, as the chromium is firmly bound in these alloys. Small amounts of chromium may be released into the aquatic environment in connection with oxidation of steel alloys in water (rust).

Significant chromium exposure must be expected for organisms in soil due to leaching of chromium from impregnated wood, waste deposits/landfills and spills of chemicals containing chromium. Atmospheric deposition and wastewater sludge will also give rise to more diffuse exposure. It is expected that Cr(VI) in soil is rapidly reduced to Cr(III), and this reduces the risk of impacts (EU, 2000a). As was the case for the aquatic environment, release of chromium from alloys is held to be of little significance in terms of soil exposure.

To sum up, chromium exposure from alloys which contain chromium is regarded as unproblematic for human beings and the environment. It is estimated that environmental exposure mainly comes from impregnated wood and wastewater from industries using chemicals which contain chromium, as well as from disposal in the form of incineration and landfilling.

1.4.2 Chromium supply via goods and products

Based on the information about use of chromium and chromium compounds appearing in various materials and products, the list of CN numbers (the Combined Nomenclature, Statistics Denmark) has been studied with a view to identifying product groups which may contain chromium.

Information on import, export and production has been obtained for the selected CN numbers. This information covers the years 1998, 1999 and 2000.

As far as possible, "tonnes" have been used as the preferred unit when collecting information on product quantities. For some product groups, however, Danish production is measured in other units (pieces or m²). In such cases, the figures for production in tonnes are estimated on the basis of the assumption that values per weight unit are the same for goods manufactured in Denmark as for exported/imported goods. This approach could not be used with a few product groups. In those cases, weight per unit or m² has been estimated on the basis of actual testing and weighing or studies of the available literature. Worst-case scenarios were applied in all cases to make sure that chromium contributions from the relevant product groups were not underestimated.

The results of the preliminary data processing are presented as the supply of chromium within product groups (Table 1.4). The table also states the oxidation level for chromium within the various product groups and provides estimates of the potential exposure and hazardousness.

Table 1.4
Supply of chromium within product groups, oxidation levels (stated as 0, II or VI) and estimates of the potential exposure and hazardousness (ranked as L for Low, M for Medium or H for High).

1) Chromium-plated iron and steel products are included in this product group
2) Preliminary calculation of the chromium supply in Denmark

The results of the preliminary data processing indicate that chromium in alloys is the most significant category: iron and steel, copper, aluminium and goods made from these metals. Next in line is chromium in the form of chemical compounds: accelerators etc., pigments and chromium compounds. The ranking of specific products indicates that special attention should be paid to stainless steel sheets (>600 mm>), pipes and hollow profiles, tanks, vats and similar containers, products made by means of hot rolling, wire mesh, and copper and copper products.

1.4.2.2 The Product Register

Three different searches of the Product Register were made:

1. A search for substances containing chromium
2. A search for import, export and production of substances containing chromium (inventory as at 1999, 2000 and 2001⁵); the supply was calculated and minimum and maximum values have been given
3. A search for the contents of specific chromium compounds in specific goods/functions

The first part of the search resulted in a list of approximately 900 names of substances which contain chromium. The second part of the search revealed that notification of use has been submitted for approximately 130 of these substances. Statistics on the supply of the 130 substances in question have been converted into a value for chromium supply, based on maximum chromium content in the relevant compounds. Thus, the figure for chromium supply through chromium compounds represents the maximum quantities possible according to the information in the Product Register. Table 1.5 illustrates the supply for the 15 chromium compounds accounting for the greatest supply, ranked in order of the quantities supplied. We see that chromium(III) oxide, chromium(VI) oxide, chromium, "not specified"⁶ and lead(II) chromate account for more than 95% of the chromium supply registered in the Product Register.

Table 1.5
Supply of the 15 chromium compounds accounting for the greatest supply in Denmark, calculated as an average of the supply during the period 1998–2000. The figures are based on statistics from the Product Register and reflect the maximum possible content.

The grey areas have been blanked out due to confidentiality. This is to say that they have been reported within less than 3 functions.

These are followed by a number of less frequently seen hexavalent chromium compounds: potassium dichromate (0.51 tonnes of chromium), zinc chromate (0.35 tonnes of chromium), sodium dichromate dihydrate (0.17 tonnes of chromium) strontium chromate (0.13 tonnes of chromium), ammonia dichromate (0.12 tonnes chromium), and more.

1.4.3 Summary

The survey has been prioritised as illustrated in Table 1.6. This prioritisation is based on the discussion on hazardousness and exposure outlined above, as well as on information from Statistics Denmark and the Product Register.

Table 1.6
Survey priorities

Electronics and glass are examples of product groups which were not accorded high priority on the basis of the initial survey. It may be relevant to include these product groups in connection with any subsequent studies.

1.5 International market and development trends

According to information from the international industry association for chromium (International Chromium Developing Association – ICDA), approximately twenty countries in the world extract chromium today. Of these, South Africa accounts for almost half of all the chromium extracted (47%), Kazakhstan accounts for 18%, and India accounts for 13%. The total extraction carried out in Brazil, Finland, Turkey and Zimbabwe amounts to 16%, and a total of 12 other countries account for the remaining 6%. In total, approximately 15 million tonnes of chrome iron ore were extracted in 2000 (ICDA, 2002).

According to the ICDA, chromium consumption worldwide is distributed as follows: 85% of all chromium is used within the metal industry, 8% is used within the chemical industry, and 7% is used for fireproof products and foundries.