Starting materials, purity

To facilitate learning, we have chosen a very clean chemistry with only one reaction step. Other factors, particularly those that are more physical in nature were assumed to have no effect. So, for example, agitation levels, mixing times, starting material purity, pH, and so on would not affect the reaction. It is of course assumed that this information is unknown when starting the experimental effort. These factors would most certainly be included early in the design discussions. 

Higher starting material purities are being achieved by the use of guard catalysts that remove catalyst poisons such as sulfur and halogen compounds, metals, and organic impurities. Here the zeolites have advantages over conventional adsorbents such as activated carbons [8]. 

Like silicon nitride, aluminum nitride is not easy to sinter. The high covalent bonding character (60% covalent-40% ionic (13)) of AIN limits the atomic mobility and prevents complete densi-fication. The condition of the starting materials—purity, particle size, particle-size distribution, oxygen content and specific surface area—influences the sinterability and properties of AIN ceramics (14). 

Two-Step Process. The significant advantage of the two-step process is that it only requkes commercial-grade methyl formate and ammonia. Thus the cmde product leaving the reactor comprises, in addition to excess starting materials, only low boiling substances, which are easily separated off by distillation. The formamide obtained is of sufficient purity to meet all quaUty requkements without recourse to the costiy overhead distillation that is necessary after the dkect synthesis from carbon monoxide and ammonia. 

Activated carbon of high absorptive capacity is suitable for use as a catalyst it need not be treated with metallic salt or other substances. If starting materials of high purity are employed, excellent and economic catalyst efficiency is obtained. 

Precipitation of a hydrated titanium oxide by mixing aqueous solutions of titanium chloride with alkaU forms the precipitation seeds, which are used to initiate precipitation in the Mecklenburg (50) variant of the sulfate process for the production of pigmentary titanium dioxide. Hydrolysis of aqueous solutions of titanium chloride is also used for the preparation of high purity (>99.999%) titanium dioxide for electroceramic appHcations (see Ceramics). In addition, hydrated titanium dioxide is used as a pure starting material for the manufacture of other titanium compounds. 

Because no process has been developed for selectively removing impurities in vanadium and vanadium alloys in the metallic state, it is essential that all starting materials, in aggregate, be pure enough to meet final product purity requirements. In addition, the consoHdation method must be one that prevents contamination through reaction with air or with the mold or container material. 

Another commercial process yields high purity boron of greater than 99%. In this process boron hydrides, such as diborane, are thermally decomposed (4). Because only boron and hydrogen are present in the starting material, contamination is minimal, and a very uniform, submicrometer powder is formed by the gas nucleation process. 

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

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