Chromium chromate, from

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

Generally, water is used in this plant to cool, leach, filter wash, scrub, heat, and washdown. The unreacted ore is slurred and sent, along with chromium and other impurities originally present in the ore, to the treatment plant. The boiler blowdown, which is sometimes contaminated with chromium escaping from the process area, adds to the volume of wastewater coming from the plant. The non-contact cooling water from the plant contains dissolved sulfate, chloride, and chromate thus it is sent to a wastewater treatment plant. The scrubber water may be used to slurry the ore or discharged. 

Elwood, J.W., J.J. Beauchamp, and C.P. Allen. 1980. Chromium levels in fish from a lake chronically contaminated with chromates from cooling towers. Int. Jour. Environ. Stud. 14 289-298. 

Another example of where ultrasound influences the discharge rate of chromium is in the chromium electroplating of steel plates. When cold-rolled steel plate is elecUo-lytically chromated from a silent aqueous solution containing chromates and dichromates, a chrome coating of 13 mg m is obtained after 1 s, whilst in the presence of ultrasound a coating of 45 mg m is obtained [25] in the same time. The product is also accompanied with an increase in the brightness of the metal. Similar results have been found by other workers [26]. 

Chromate, dichromate, permanganate, chlorate and hypochlorite and other oxidants are readily reduced hy hydrazine for example, removal of chromate from wastewater may he achieved fuUy hy converting water-soluhle chromate to insoluble precipitate of chromium hydroxide, Cr(OH)3 ... 

Pankow, J.F. and Janauer, G.E. (1974) Analysis for chromium traces in natural waters. 1. Pre-concentration of chromate from parts per billion levels in aqueous solutions by ion exchange. Anal. Chim. Acta, 69, 97-104. 

An alternative method which can also be automated by the use of the Titertek supernatant harvester (see Appendix 3) involves the measurement of radioactive chromium released into the culture medium from killed cells. The harvester consists of a set of absorbent cylinders aligned so that they may be inserted into the wells of a microtitration plate (Appendix 3). Once the supernatant in the wells has been absorbed the cylinders are transferred to counting vials and the amount of radioactive chromium released from the cell monolayer is estimated. Cells take up 51 Cr sodium chromate rapidly and the excess is readily washed away by rinsing in culture medium. 

Cardiovascular Effects. Information regarding cardiovascular effects in humans after inhalation exposure to chromium and its compounds is limited. In a survey of a facility engaged in chromate production in Italy, where exposure concentrations were 0.01 mg chromium(VI)/m3, electrocardiograms were recorded for 22 of the 65 workers who worked in the production of dichromate and chromium trioxide for at least 1 year. No abnormalities were found (Sassi 1956). An extensive survey to determine the health status of chromate workers in seven U.S. chromate production plants found no association between heart disease or effects on blood pressure and exposure to chromates. Various manufacturing processes in the plants resulted in exposure of workers to chromite ore (mean time-weighted concentration of 0-0.89 mg chromium(ni)/m3) water-soluble chromium(VI) compounds (0.005-0.17 mg chromium(VI)/m3) and acid-soluble/water-insoluble chromium compounds (including basic chromium sulfate), which may or may not entirely represent trivalent chromium (0-0.47 mg chromium/m3) (PHS 1953). No excess deaths were observed from cardiovascular diseases and ischemic heart disease in a cohort of 4,227 stainless steel production workers from 1968 to 1984 when compared to expected deaths based on national rates and matched for age, sex, and calender time (Moulin et al. 1993). No measurements of exposure were provided. In a cohort of 3,408 individuals who had worked in 4 facilities that produced chromium compounds from chromite ore in northern New Jersey sometime between 1937 and 1971, where the exposure durations of workers ranged from <1 to >20 years, and no increases in atherosclerotic heart disease were evident (Rosenman and Stanbury 1996). The proportionate mortality ratios for white and black men were 97 (confidence limits 88-107) and 90 (confidence limits 72-111), respectively. 

Figure 059. Set-up for the preparation of lead-vi-chromate from potassium dichromate. The positive lead electrode will slowly be corroded in this operation. The Lead-VI-Chromate will form a flaky precipitate that will collect in the anode compartment. The negative lead electrode can be replaced with titanium, chromium, or graphite.

Chromium can also be obtained by reduction of the chloride or of chromates, from the boride, and by electrolytic methods. 

Atomic Weight.—From a consideration of the vapour densities of volatile compounds of chromium, and from the application of Dulong and Petit s Law, it is obvious that the atomic weight of chromium is about 52—that is, three times the chemical equivalent of chromium in chromic salts, or six times its combining weight in derivatives of chromium trioxide. Chromium thus exhibits di-, tri-, and hexa-valency in the chromous salts, chromic salts, and chromates and diehromates respectively. 

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