Occurrence and Uses
Elemental chromium (Cr) is not found free in nature, and the only ore of any importance is the spinel ore, chromite or chrome iron stone, which is ferrous chromite (FeOCr2O3), widely distributed over the earth’s surface. In addition to chromic acid, this ore contains variable quantities of other substances. Only ores or concentrates containing more than 40% chromic oxide (Cr2O3) are used commercially, and countries having the most suitable deposits are the Russian Federation, South Africa, Zimbabwe, Turkey, the Philippines and India. The prime consumers of chromites are the United States, the Russian Federation, Germany, Japan, France and the United Kingdom.
Chromite may be obtained from both underground and open cast mines. The ore is crusted and, if necessary, concentrated.
The most significant usage of pure chromium is for electroplating of a wide range of equipment, such as automobile parts and electric equipment. Chromium is used extensively for alloying with iron and nickel to form stainless steel, and with nickel, titanium, niobium, cobalt, copper and other metals to form special-purpose alloys.
Chromium forms a number of compounds in various oxidation states. Those of II (chromous), III (chromic) and VI (chromate) states are most important; the II state is basic, the III state is amphoteric and the VI state is acidic. Commercial applications mainly concern compounds in the VI state, with some interest in III state chromium compounds.
The chromous state (CrII) is unstable and is readily oxidized to the chromic state (CrIII). This instability limits the use of chromous compounds. The chromic compounds are very stable and form many compounds which have commercial use, the principal of which are chromic oxide and basic chromium sulphate.
Chromium in the +6 oxidation state (CrVI) has its greatest industrial application as a consequence of its acidic and oxidant properties, as well as its ability to form strongly coloured and insoluble salts. The most important compounds containing chromium in the CrVI state are sodium dichromate, potassium dichromate and chromium trioxide. Most other chromate compounds are produced industrially using dichromate as the source of CrVI.
Sodium mono- and dichromate are the starting materials from which most of the chromium compounds are manufactured. Sodium chromate and dichromate are prepared directly from chrome ore. Chrome ore is crushed, dried and ground; soda ash is added and lime or leached calcine may also be added. After thorough mixing the mixture is roasted in a rotary furnace at an optimum temperature of about 1,100°C; an oxidizing atmosphere is essential to convert the chromium to the CrVI state. The melt from the furnace is cooled and leached and the sodium chromate or dichromate is isolated by conventional processes from the solution.
Technically, chromium oxide (Cr2O3, or chromic oxide), is made by reducing sodium dichromate either with charcoal or with sulphur. Reduction with sulphur is usually employed when the chromic oxide is to be used as a pigment. For metallurgical purposes carbon reduction is normally employed.
The commercial material is normally basic chromic sulphate [Cr(OH)(H2O)5]SO4, which is prepared from sodium dichromate by reduction with carbohydrate in the presence of sulphuric acid; the reaction is vigorously exothermic. Alternatively, sulphur dioxide reduction of a solution of sodium dichromate will yield basic chromic sulphurate. It is used in the tanning of leather, and the material is sold on the basis of Cr2O3 content, which ranges from 20.5 to 25%.
Sodium dichromate can be converted into the anhydrous salt. It is the starting point for preparation of chromium compounds.
Chromium trioxide or chromium anhydride (sometimes referred to as “chromic acid”, although true chromic acid cannot be isolated from solution) is formed by treating a concentrated solution of a dichromate with strong sulphuric acid excess. It is a violent oxidizing agent, and the solution is the principal constituent of chromium plating.
Chromates of weak bases are of limited solubility and more deeply coloured than the oxides; hence their use as pigments. These are not always distinct compounds and may contain mixtures of other materials to provide the right pigment colour. They are prepared by the addition of sodium or potassium dichromate to a solution of the appropriate salt.
Lead chromate is trimorphic; the stable monoclinic form is orange-yellow, “chrome yellow”, and the unstable orthombic form is yellow, isomorphous with lead sulphate and stabilized by it. An orange-red tetragonal form is similar and isomorphous with lead molybdate (VI) PbMoO4 and stabilized by it. On these properties depends the versatility of lead chromate as a pigment in producing a variety of yellow-orange pigments.
Compounds containing CrVI are used in many industrial operations. The manufacture of important inorganic pigments such as lead chromes (which are themselves used to prepare chrome greens), molybdate-oranges, zinc chromate and chromium-oxide green; wood preservation; corrosion inhibition; and coloured glasses and glazes. Basic chromic sulphates are widely used for tanning.
The dyeing of textiles, the preparation of many important catalysts containing chromic oxide and the production of light-sensitive dichromated colloids for use in lithography are also well-known industrial uses of chromium-containing chemicals.
Chromic acid is used not only for “decorative” chromium plating but also for “hard” chromium plating, where it is deposited in much thicker layers to give an extremely hard surface with a low coefficient of friction.
Because of the strong oxidizing action of chromates in acid solution, there are many industrial applications particularly involving organic materials, such as the oxidation of trinitrotoluene (TNT) to give phloroglucinol and the oxidation of picoline to give nicotine acid.
Chromium oxide is also used for the production of pure chromium metal that is suitable for incorporation in creep-resistant, high-temperature alloys, and as a refractory oxide. It may be included in a number of refractory compositions with advantage—for example, in magnetite and magnetite-chromate mixtures.
Compounds with CrIII oxidation states are considerably less hazardous than are CrVI compounds. Compounds of CrIII are poorly absorbed from the digestive system. These CrIII compounds may also combine with proteins in the superficial layers of the skin to form stable complexes. Compounds of CrIII do not cause chrome ulcerations and do not generally initiate allergic dermatitis without prior sensitization by CrVI compounds.
In the CrVI oxidation state, chromium compounds are readily absorbed after ingestion as well as during inhalation. The uptake through intact skin is less well elucidated. The irritant and corrosive effects caused by CrVI occur readily after uptake through mucous membranes, where they are readily absorbed. Work-related exposure to CrVI compounds may induce skin and mucous membrane irritation or corrosion, allergic skin reactions or skin ulcerations.
The untoward effects of chromium compounds generally occur among workers in workplaces where CrVI is encountered, in particular during manufacture or use. The effects frequently involve the skin or respiratory system. Typical industrial hazards are inhalation of the dust or fumes arising during the manufacture of dichromate from chromite ore and the manufacture of lead and zinc chromates, inhalation of chromic acid mists during electroplating or surface treatment of metals, and skin contact with CrVI compounds in manufacture or use. Exposure to CrVI-containing fumes may also occur during welding of stainless steels.
Chrome ulcerations. Such lesions used to be common after work-related exposure to CrVI compounds. The ulcers result from the corrosive action of CrVI, which penetrates the skin through cuts or abrasions. The lesion usually begins as a painless papule, commonly on the hands, forearms or feet, resulting in ulcerations. The ulcer may penetrate deeply into soft tissue and may reach underlying bone. Healing is slow unless the ulcer is treated at an early stage, and atrophic scars remain. There are no reports about skin cancer following such ulcers.
Dermatitis. The CrVI compounds may cause both primary skin irritation and sensitization. In chromate-producing industries, some workers may develop skin irritation, particularly at the neck or wrist, soon after starting work with chromates. In the majority of cases, this clears rapidly and does not recur. However, sometimes it may be necessary to recommend a change of work.
Numerous sources of exposure to CrVI have been listed (e.g., contact with cement, plaster, leather, graphic work, work in match factories, work in tanneries and various sources of metal work). Workers employed in wet sandpapering of car bodies have also been reported with allergy. Affected subjects react positively to patch testing with 0.5% dichromate. Some affected subjects had only erythema or scattered papules, and in others the lesions resembled dyshidriotic pompholyx; nummular eczema may lead to misdiagnosis of genuine cases of occupational dermatitis.
It has been shown that CrVI penetrates the skin through the sweat glands and is reduced to CrIII in the corium. It is shown that the CrIII then reacts with protein to form the antigen-antibody complex. This explains the localization of lesions around sweat glands and why very small amounts of dichromate can cause sensitization. The chronic character of the dermatitis may be due to the fact that the antigen-antibody complex is removed more slowly than would be the case if the reaction occurred in the epidermis.
Acute respiratory effects. Inhalation of dust or mist containing CrVI is irritating to mucous membranes. At high concentrations of such dust, sneezing, rhinorrhoea, lesions of the nasal septum and redness of the throat are documented effects. Sensitization has also been reported, resulting in typical asthmatic attacks, which may recur on subsequent exposure. At exposure for several days to chromic acid mist at concentrations of about 20 to 30 mg/m3, cough, headache, dyspnoea and substernal pain have also been reported after exposure. The occurrence of bronchospasm in a person working with chromates should suggest chemical irritation of the lungs. Treatment is only symptomatic.
Ulcerations of the nasal septum. In previous years, when the exposure levels to CrVI compounds could be high, ulcerations of the nasal septum were frequently seen among exposed workers. This untoward effect results from deposition of CrVI-containing particulates or mist droplets on the nasal septum, resulting in ulceration of the cartilaginous portion followed, in many cases, by perforation at the site of ulceration. Frequent nose-picking may enhance the formation of perforation. The mucosa covering the lower anterior part of the septum, known as the Kiesselbach’s and Little’s area, is relatively avascular and closely adherent to the underlying cartilage. Crusts containing necrotic debris from the cartilage of the septum continue to form, and within a week or two the septum becomes perforated. The periphery of the ulceration remains active for up to several months, during which time the perforation may increase in size. It heals by the formation of vascular scar tissue. Sense of smell is almost never impaired. During the active phase, rhinorrhoea and nose-bleeding may be troublesome symptoms. When soundly healed, symptoms are rare and many persons are unaware that the septum is perforated.
Effects in other organs. Necrosis of the kidneys has been reported, starting with tubular necrosis, leaving the glomeruli undamaged. Diffuse necrosis of the liver and subsequent loss of architecture has also been reported. Soon after the turn of the century there were a number of reports on human ingestion of CrVI compounds resulting in major gastro-intestinal bleeding from ulcerations of the intestinal mucosa. Sometimes such bleedings resulted in cardiovascular shock as a possible complication. If the patient survived, tubular necrosis of the kidneys or liver necrosis could occur.
Carcinogenic effects. Increased incidence of lung cancer among workers in manufacture and use of CrVI compounds has been reported in a great number of studies from France, Germany, Italy, Japan, Norway, the United States and the United Kingdom. Chromates of zinc and calcium appear to be among the most potent carcinogenic chromates, as well as among the most potent human carcinogens. Elevated incidence of lung cancer has also been reported among subjects exposed to lead chromates, and to fumes of chromium trioxides. Heavy exposures to CrVI compounds have resulted in very high incidence of lung cancer in exposed workers 15 or more years after first exposure, as reported in both cohort studies and case reports.
Thus, it is well established that an increase in the incidence of lung cancer of workers employed in the manufacture of zinc chromate and the manufacture of mono- and dichromates from chromite ore is a long-term effect of work-related heavy exposure to CrVI compounds. Some of the cohort studies have reported measurements of exposure levels among the exposed cohorts. Also, a small number of studies have indicated that exposure to fumes generated from welding on Cr-alloyed steel may result in elevated incidence of lung cancer among these welders.
There is no firmly established “safe” level of exposure. However, most of the reports on association between CrVI exposure and cancer of the respiratory organs and exposure levels report on air levels exceeding 50 mg CrVI/m3 air.
The symptoms, signs, course, x-ray appearance, method of diagnosis and prognosis of lung cancers resulting from exposure to chromates differ in no way from those of cancer of the lung due to other causes. It has been found that the tumours often originate in the periphery of the bronchial tree. The tumours may be of all histological types, but a majority of the tumours seem to be anaplastic oat-celled tumours. Water-soluble, acid soluble and water insoluble chromium is found in the lung tissues of chromate workers in varying amounts.
Although it has not been firmly established, some studies have indicated that exposure to chromates may result in increased risk of cancer in the nasal sinuses and the alimentary tract. The studies that indicate excess cancer of the alimentary tract are case reports from the 1930s or cohort studies that reflect exposure at high levels than generally encountered today.
Safety and Health Measures
On the technical side, avoidance of exposure to chromium depends on appropriate design of processes, including adequate exhaust ventilation and the suppression of dust or mist containing chromium in the hexavalent state. Built-in control measures are also necessary, requiring the least possible action by either process operators or maintenance staff.
Wet methods of cleaning should be used where possible; at other sites, the only acceptable alternative is vacuum cleaning. Spill of liquids or solids must be removed to prevent dispersion as airborne dust. The concentration in the work environment of chromium-containing dust and fumes should preferably be measured at regular intervals by individual and area sampling. Where unacceptable concentration levels are found by either method, the sources of dust or fumes should be identified and controlled. Dust masks, preferably with an efficiency of more than 99% in retaining particles of 0.5 µm size, should be worn in situations above non-hazardous levels, and it may be necessary to provide air-supplied respiratory protective equipment for jobs considered to be hazardous. Management should ensure that dust deposits and other surface contaminants should be removed by washing down or suction before work of this type begins. Providing laundering overalls daily may help in avoiding skin contamination. Hand and eye protection is generally recommended, as is repair and replacement of all personal protective equipment (PPE).
The medical surveillance of workers on processes in which CrVI compounds may be encountered should include education in toxic and the carcinogenic properties of both CrVI and CrIII compounds, as well as on the differences between the two groups of compounds. The nature of the exposure hazards and subsequent risks of various diseases (e.g., lung cancer) should be given at job entry as well as at regular intervals during employment. The need to observe a high standard of personal hygiene should be emphasized.
All untoward effects of exposure to chromium can be avoided. Chrome ulcers of the skin can be prevented by eliminating sources of contact and by preventing injury to the skin. Skin cuts and abrasions, however slight, should be cleaned immediately and treated with 10% sodium EDTA ointment. Together with the use of a frequently renewed impervious dressing, this will enhance rapid healing for any ulcer that may develop. Although EDTA does not chelate CrVI compounds at room temperature, it reduces the CrVI to CrIII rapidly, and the excess EDTA chelates CrIII. Both the direct irritant and corrosive action of CrVI compounds and the formation of protein/CrIII complexes are thus prevented. After accidental ingestion of CrVI compounds, immediate swallowing of ascorbic acid may also quickly reduce the CrVI.
Careful washing of the skin after contact and care to avoid friction and sweating are important in the prevention and the control of primary irritation due to chromates. In previous years an ointment containing 10% sodium EDTA was applied regularly to the nasal septum before exposure. This preventive treatment could assist in keeping the septum intact. Soreness of the nose and early ulceration were also treated by regular application of this ointment, and healing could be achieved without perforation.
Results from research indicate that workers exposed to high air concentrations of CrVI could be monitored successfully by monitoring the excretion of chromium in the urine. Such results, however, bear no relation to the hazard of skin allergy. As of today, with the very long latent period of CrVI-related lung cancer, hardly anything can be said regarding the cancer hazard on the basis of urinary levels of Cr.