Powders reactivity, testing

Data reported in the literature indicate that sodium compounds are considerably more reactive than calcium compounds when injected into flue gas as dry powders, although reactivities of the calcium compounds (particularly lime) are increased when they are injected via a spray dryer system. The study described in this paper was confined to dry-powder injection tests with sodium bicarbonate, nahcolite (a sodium bicarbonate mineral), and trona (a sodium carbonate/bicarbonate mineral). 

Nevertheless, in some cases the [2+2]-cycloaddition reaction under ultrasonic irradiation does not proceed or the yield is low. Different zinc powders were tested for the [2+2]-cycloaddition of dichloroketene with 1-methyl-l-cyclohexene. In line with the results of Takai et who reported the importance of catalytic amouts of lead on zinc reactions, we observed that zinc dust free of lead impurities was indeed poorly reactive, affording only ca. 10% of the cycloadduct. However, the reactions carried out under the same conditions, but with zinc powders containing lead as an impurity (from Union Mini re, Belgium), gave the cycloaddition product in 65-75% yield. The addition of a catalytic amount (0.5 mol%) of PbCl2 afforded highly reproducible results (80% yield after distillation). 

The relative reactivity of thermosetting powders can be easily deterrnined by the gel time or stroke-cure test. A small amount of powder is placed on a hot plate, usually at 200°C, and the time until the coating composition gels, or no longer forms fibers, is deterrnined. Powders are characterized by relative gel times (cure rate) as shown in Figure 1. 

It was apparent from the very earliest tests that control of thin moisture films on the surface of reactive particles was the key to success. The main three competing arrangements, as compared by Statnick et al. [4th Annual Pitt. Coal Conf. 1987)] involved slurry spray dryers, where lime and water were injected together, versus systems where the gas was humidified by water injection before or after injection of limey dry powder reagents. It turns out that there are tradeoffs among the costs of hardware, reagent, and water dispersion and reagent purchase and disposal. Systems where water evaporates in the presence of active particles are usually less expensive overall. 

The very finely divided anhydride prepared by precipitating and drying silicic add is more reactive than the most finely powdered quartz, and it finds use as a reagent in certain analytical tests. 

AIO.5.4.2 The transformation/dissolution tests are to be carried out within the pH range described in AIO.2.3.2 and AIO.5.1.6. The test solution pH should be recorded at each solution sampling interval. The pH can be expected to remain constant ( 0.2 units) during most tests, although some short-term pH variations have been encountered at 100 mg/1 loadings of reactive fine powders (reference 7, this annex), due to the inherent properties of the substance in the finely divided state. 

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