Ammonium dichromate decomposition

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

A number of manufacturers around the world are using the decomposition of ammonium dichromate to produce chrome oxide (eq. 5) (78). Generally, an excess of finely ground ammonium sulfate is mixed with sodium dichromate, and the dry mixture is heated to form chrome oxide and sodium sulfate, evolving nitrogen and steam. 

Chromia—alumina catalysts are prepared by impregnating T-alumina shapes with a solution of chromic acid, ammonium dichromate, or chromic nitrate, followed by gentie calciaation. Ziac and copper chromites are prepared by coprecipitation and ignition, or by thermal decomposition of ziac or copper chromates, or organic amine complexes thereof. Many catalysts have spiael-like stmctures (239—242). 

In the previous examples, the calculated enthalpies of decomposition are taken. The enthalpies of formation of the decomposition substances come from the corresponding chapters in Part Two. The published values of enthalpies of formation are favoured and use of the values estimated is only made when there is no experimental data. A few inorganic compounds have been added which are noted for their instability eg ammonium dichromate and ammonium nitrate. 

Methanol is sometimes used to prepare dichromium trioxide by the reduction of chromium oxide (Vi). Since these attempts lead to spontaneous ignition and detonation, it is preferable to use the thermal decomposition of ammonium dichromate, provided it is carried out with caution (see p.200). 

Catalytic elfects on the thermal decomposition and burning under nitrogen of the nitrate were determined for ammonium dichromate, potassium dichromate, potassium chromate, barium chloride, sodium chloride and potassium nitrate. Chromium(VI) salts are most effective in decomposition, and the halides salts during burning of the nitrate [1]. The effect of chromium compounds soluble in the molten nitrate, all of which promote decomposition of the latter, was studied (especially using ammonium dichromate) in kinetic experiments [2],... 

Sodium chromate is produced as part of the process for obtaining Cr203, and it is probably the most important chromium compound. Although there are other oxides of chromium, Cr203 is very important because of its catalytic properties. One way to obtain this oxide is by the decomposition of ammonium dichromate,... 

In a Downstream Decomposing facility not the subject of this example, intentional chemistry is performed as ammonium dichromate is heated to decomposition to make chromium dioxide, which is used in the production of magnetic tape products. In the Upstream Feeds facility under study, physical processing is performed as ammonium dichromate is fed through a screw conveyor. The Preliminary Screening Method is to be used as a first-cut determination whether chemical reactivity hazards will need to be managed in the physical processing facility. 

In the laboratory, nitrogen may be generated by several methods, such as thermal decomposition of ammonium dichromate ... 

Reduction of Ammonium Dichromate. 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 [3.48]. The pigment is obtained after addition of alkali salts (e.g., sodium sulfate) and subsequent calcination [3.49]. 

Azobenzene, 340/13/ 6.5 azoformamide, 260/194/925 diketene, 125/140/— 4-nitroisopropylbenzene, 250/ > 182/ 830 4-nitrophenol, 280/ > 199/ 1,030 4-nitro-sophenol, 120/ 23/ 5. It is concluded that the first and last compounds are of relatively low hazard [1]. Improved equipment has provided more accurate and detailed results for a further 7 compounds and has shown the effect of variations in the initial stage of decomposition on the final pressure attained, and of the increase in pressure causing a reduction in the rate of pressure rise. At 0.2 g/cm3 loadings, comparable results are — A diazonium salt, 112/ 200/ 66 azoisobutyronitrile, 80/ 130/ 8,800 1,3-diphenyl-triazene, 140/ 95/ 420 2-nitrobenzaldehyde, 200/ 945/ 8,700 3-nitrobenzaldehyde, 190/ 830/ 4,100, 4-nitrobenzaldehyde, 200/ 960/ 4,700. Solids which deflagrate give substantially higher rates of pressure rise because the rate of pressure rise is not depressed by increase in pressure, e.g. ammonium dichromate, 227/ 510/ 68,000 [2]. 

The thermal decomposition of ammonium dichromate is an impressive reaction. When heated with a Bunsen burner or propane torch, the orange crystals of ammonium dichromate slowly decompose to green chromium(III) oxide in a volcano-like display. Colourless nitrogen gas and water vapour are also given off. 

The intermediate precipitate obtained by the reaction of copper nitrate with ammonium dichromate and ammonia has been shown to be Cu(0H)NH4Cr04,122 and the decomposition of the precipitate to give the catalyst to be formulated as in eq. 1.6, by an X-ray diffraction study by Stroupe, although the catalysts obtained by decomposition at sufficiently controlled low temperature (350°C) are amorphous.123 Catalysts previosly used in liquid-phase hydrogenation below 300°C often show crystalline cupric chromite to have been largely reduced to the cuprous chromite... 

Ammonium dichromate decomposes on heating, but is not an explosive. It is a component of pyrotechnical charges, and is an effective additive which is introduced into ammonium nitrate-based propellants in order to catalyze the decomposition reaction. 

Thermal decomposition of a polymer with the structure HS(-R-S-S)n-R-SH with R = -(CH2)2-0-CHy-O-(CH2)2- cured with various oxidizing agents such as ammonium dichromate also has been reported [17]. Flash pyrolysis was performed at 420° C and 470° C generating a mixture of compounds where 1,3-oxathiolane, 2-mercaptomethyloxirane, 1,3-dioxa-6-thiocane and 1,3-dioxa-6,7-dithionane were the main components. 

The presence of fluxes such as NaCl or Na2S04 during the decomposition of ammonium dichromate ensures that Cr203-pigments are formed in addition to nitrogen and water, instead of extremely fine powders. The isolation of ammonium dichromate can therefore be dispensed with by calcining mixtures of sodium dichromate and ammonium chloride or ammonium sulfate at 800 to 1000°C. 

Thermoanalytical measurements distinguished four stages in the decomposition [99] of ammonium dichromate. At 508 K ... 

In decompositions of other reactants, initially solid, molten intermediates have been recognized or suggested. Examples (28) include the interventions of acetate and copper in copper(II) malonate decomposition, of Cr03 in the thermal reactions of ammonium dichromate, and of N02C104 in NH4C104 breakdown. It appears that the... 

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