Chromate chemicals

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

Steffee CH, Baetjer AM. 1965. Histopathologic effects of chromate chemicals. Arch Environ Health 11 66-75. 

To produce the desired hexavalent, water-soluble chromate chemicals, the ore is bail-milled to less than 100 mesh, mixed with soda (Na2C03) and lime, and roasted in rotary kilns at 1100-1150°C in air. While the mixture does not fuse, the molten soda ash reacts with the chromite to form water-soluble sodium chromate. The lime reacts with any aluminum impurities in the ore to form water-soluble compounds. The resulting yellow mixture is then leached with water and filtered the aluminum impurities are precipitated as aluminum hydroxide the solution is then acidified and concentrated to yield sodium chromate. From the sodium chromate, chromium can be obtained by electrolytic reduction or reduction with carbon. 

It has been recognized over the past 15 years that the continued use of chromate chemicals is undesirable because of their toxicity and carcinogenicity (US Public Health Service 1989). The obvious proteetive properties of the R oxides suggest that, once applied to a metal surface, they too should be able to aet as a protective coating, in a manner similar to a chromate conversion coating. 

Rare earth compounds have been proposed as alternatives to chromate chemicals on the basis of being relatively benign. For example. Table 7.1 shows the LDjo for some simple salts. Clearly the LDj for the chromium chlorides is much lower than the other compounds. Importantly, the for cerium nitrate is... 

Chromate chemicals used in metal finishing are derived from the mineral chromite. The consumption of chromium is dominated by ferrochrome and stainless steel industries. According to O Driscoll in 2011, these industries consumed some 95% of refined chromite production (13Mt) and hence drive the market price for this material. These figures have not changed much over the decade, since, in 2002, total consumption for ferrochrome and stainless steels was 90% of total demand (13.4Mt). The price of chromite traded between US365 and US450/t (Fig. 10.1). 

Organochromium Catalysts. Several commercially important catalysts utilize organ ochromium compounds. Some of them are prepared by supporting bis(triphenylsilyl)chromate on siUca or siUca-alumina in a hydrocarbon slurry followed by a treatment with alkyl aluminum compounds (41). Other catalysts are based on bis(cyclopentadienyl)chromium deposited on siUca (42). The reactions between the hydroxyl groups in siUca and the chromium compounds leave various chromium species chemically linked to the siUca surface. The productivity of supported organochromium catalysts is also high, around 8—10 kg PE/g catalyst (800—1000 kg PE/g Cr). 

Insoluble lead chromate can be chemically treated where appHcable, then filtered, washed, dried, and ground. 

Chemical Properties. The valence states of chromium are +2, +3, and +6, the latter two being the most common. The +2 and +3 states are basic, whereas the +6 is acidic, forming ions of the type CrO (chromates) and (Cr203 [ (dichromates). The blue—white metal is refractory and very hard. 

The prices of some important chromium chemicals are given ia Table 4, and production and shipment data for sodium chromate and dichromate are given ia Table 5. Data for the productioa and shipment of chromic acid have not been available siace 1972. However, traditionally CrO has held at about 30—35% of sodium dichromate production. The estimated capacity for domestic production of sodium dichromate is 150,000 to 200,000 t/yr. 

Table 11. Chemical Composition and ASTM Specifications for Chromate Color Pigments ...

Table 12. Chemical Compositions and Analytical Specifications for Chromate Corrosion Inhibiting Pigments...

Drilling Muds in the Petroleum and Natural Gas Industry. Since 1941, chromium chemicals have been used in the drilling of wells to combat fatigue corrosion cracking of drill strings, with about one metric ton of sodium chromate being used aimuaHy for an average West Texas well. Other early uses were in gas-condensate wells in Louisiana and East Texas. 

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

For exposure of reasons of observable discrepancy of results of the analysis simulated experiment with application synthetic reference samples of aerosols [1]. The models have demonstrated absence of significant systematic errors in results XRF. While results AAA and FMA depend on sort of chemical combination of an elements, method of an ashing of a material and mass of silicic acid remaining after an ashing of samples. The investigations performed have shown that silicic acid adsorbs up to 40 % (rel.) ions of metals. The coefficient of a variation V, describing effect of the indicated factors on results of the analysis, varies %) for Mn and Fe from 5 up to 20, for Cu - from 10 up to 40, for Pb - from 10 up to 70, for Co the ambassador of a dry ashing of samples - exceeds 50. At definition Cr by a method AAA the value V reaches 70 %, if element presences an atmosphere in the form of Cr O. At photometric definition Cr (VI) the value V is equal 40%, when the element is present at aerosols in the form of chromates of heavy metals. 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

Chemical Designations - Synonyms Calcium Chromate (VI) Calcium Chromate Dihydrate Gelbin Yellow Ultramarine Steinbuhl Yellow Chemical Formula CaCr04-2H20. 

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