26 октября, 2015 
Pokraskin Chromium(III) oxide can be obtained by thermal decomposition of ammonium dichromate. Above ca. 200 °C, a highly voluminous product is formed with elimination of nitrogen [3.58]. The pigment is obtained after addition of alkali salts (e. g., sodium sulfate) and subsequent calcination [3.59].
In the industrial process, a mixture of ammonium sulfate or chloride and sodium dichromate is calcined [3.60]:
Na2Cr2O7 2 H2O + (NH4)2SO4 ^Cr2O3 + Na2SO4 + 6 H2O + N2
The workup is then carried out as described above. A chromium oxide pigment
|
obtained |
by this process typically contains (wt.%) |
|
G2O3 |
99.0-99.5 |
|
SiO2 |
0.05 (max.) |
|
Al2O3 |
0.1 (max.) |
|
Fe2O3 |
0.05 (max.) |
|
S |
ca. 0.02 |
|
Water |
ca. 0.3 |
Chromium oxides with minimal sulfur content are preferred for metallurgical applications. These are obtained by reacting sodium dichromate with ammonium chloride or sulfate in a deficiency of 10 mol% [3.61]. Chromium(III) oxides with a low sulfur content can also be obtained by thermal aftertreatment [3.62]. Thermal decomposition of chromic acid anhydride (CrO3) yields high-purity chromium(III) oxide [3.63].
The pigment properties of chromium oxides can be modified by precipitation of hydroxides (e. g., of titanium or aluminum), and subsequent calcining. This treatment changes the color to yellow-green, and decreases the flocculation tendency [3.64]. Aftertreatment with organic compounds (e. g., alkoxylated alkylsulfonamides) is also used [3.65].
Опубликовано в рубрике 