| Front page | | Contents | | Previous | | Next |

The Elements in the Second Rank

6 Indium

6.1 Identity

Table 6.1
CAS No., EINECS No. and molecular weight for indium

6.2 Physico-chemical properties

Indium is a very soft, silvery-white metal. It belongs to the same group in the periodic table (IIIB) as boron and aluminum. +3 (In(III)) is the typical valence, but valences +1 and +2 can also be seen [1]. With a density of 7.3 g/cm³, indium is a heavy metal belonging to the transition elements together with e.g. cadmium and lead. Indium dissolves slowly in diluted acids and fast in hot, concentrated acids. The metal is not attacked by bases [11].

Table 6.2
Physico-chemical data for metallic indium. Data from [1, 3]

6.3 Uses and consumption

6.3.1 Uses

The most important use of indium in USA today is in liquid crystal displays (LCD) [17].

Indium is used in high-efficient solar cells such as CuInSe₂ and InP, in electronics in combination with phosphorus, selenium, copper, tellurium, tin, arsenic, nitrogen, mercury, silver, sulphur, zinc, lead, gallium and bismuth (especially semi conductors [13]) and in selected metal alloys of e.g. tin, bismuth, gold, lead and zinc [2, 13, 15]. The gallium-indium alloys are important since they are liquid at ambient temperature and are used in for transmission of both heat and electricity. Lasers, cameras and IR-detection equipment are other areas where indium is used [15, 17].

Indium is used in sodium lamps, as an additive in chemical products, and in mercury-free dry cells. [14]. As a substitute for mercury in alkaline batteries, indium compounds such as indium hydroxide, indium acetate, and indium sulphate are used [15].

To avoid thermal damages when materials with different coefficient of conductivity are combined, indium is often in the soldering material to decrease the thermal stress [15].

By use of galvanic processes, metal surfaces can be covered with indium in order to achieve surface hardness and resistance to corrosion [16].

In dentistry, as much as 10% of indium is used in gold alloys.

6.3.2 Consumption

The world production of indium was between 200 and 240 tons/year in 1995-1997 [6]. The consumption of indium has increased more than any other metal in the last 15 year. The expansion of the electronics industry is assumed to account for the major part of this increment [8].

From the per capita consumption in USA, the Danish consumption is estimated to be approximately 1 ton/year.

Table 6.3
The relative distribution of USA's consumption and the Danish use of indium in 1997 based on [8]

In Sweden, the recycling of indium is low and 0.4-1.0 tons of indium is disposed of yearly. This corresponds to a recycling of 25-62 %. With the Swedish conditions applied in Denmark, the disposal of indium will amount to approximately 0.2-0.6 tons yearly.

6.4 Emissions to and occurrence in the environment

As seen in Table 6.4, the concentration of indium is very low in seawater and earth crust.

The high increase in indium consumption during the last decades will be reflected in emission from use and waste handling. Only sparse information on emissions is available from Danish and Swedish investigations. High concentrations were observed in sewage sludge from waste water treatment plants receiving waste water from industries using indium [8].

Usage of indium in electronics will probably in the future be reflected in increased indium concentration in the waste streams. A Danish investigation from 1996 showed that the concentration of indium in waste water and sewage sludge was low [4]. This was confirmed with the results from the present study, see Table 6.5. The concentration of indium was relatively low compared to the concentration of the other metals that were measured. A Swedish study showed that cinder and fly ash from waste incineration were rich in indium (0.8-3.1 mg/kg), whereas the concentration is sediments and sewage sludge was low [8].

During waste incineration indium will be in the liquid phase due to its low melting point, and indium will adsorb on the smallest particles. Indium compounds with low boiling point such as e.g. indium chloride will be on gaseous form during waste incineration and therefore constitute a potential risk of indium dispersion.

The low concentrations of indium in coal results in low indium emission related to coal combustion [8].

Table 6.4
Typical background concentration of indium in the environment. Data from [2, 10]

Table 6.5
Levels of indium in selected emissions and waste products from measurements conducted as part of this study in the autumn of 2001.

6.5 Danger classification

Indium or inorganic indium compounds are not listed on the Danish list of dangerous compounds [9].

6.6 Toxicology

The toxicological data for indium is limited. No data was found in the standard works of reference [7, 12]. Indium can impede protein synthesis, thereby affecting numerous essential physiological processes, including detoxification of organic carcinogens [8]. Intravenous indium chloride exposure to mice had a toxic effect on the kidney and resulted in necrosis of the liver [18]. As for gallium, data regarding therapeutic use of indium compounds is available. However, these data can not be used to describe the toxicology of indium.

6.7 Environmental properties

The information on the environmental properties of indium is sparse. It is expected that its environmental properties are similar to the properties of gallium.

6.7.1 Environmental chemistry

Distribution of indium in the soil between particle-bound and mobile forms of the element is not known. Indium is probably found as hydroxides, whose solubility depends on the pH.

In(OH)₂⁺and In(OH)₃⁰ are the typical compounds in the aquatic environment. It is possible that indium is biologically reactive and can form stable complexes with nitrogen and sulphur in living organic material. In the aquatic environment, indium is expected to be assimilated by e.g. algae [8]. A fraction will, however, be sorbed on particulate matter.

6.7.2 Environmental toxicology

No data on environmental toxicity was found for indium by searching in the AQUIRE database [5].

6.7.3 Bioaccumulation

It is not known if indium can bioaccumulate.

6.8 Conclusions

Indium is used in e.g. the electronics industry, and the consumption has increased significantly in recent years. This increase has resulted in higher amounts of indium in the waste. However, concentrations are still low in e.g. wastewater and compost. The available data regarding environmental fate of indium is not sufficient for proper evaluation.

6.9 References

| Front page | | Contents | | Previous | | Next | | Top |