Mechanical particle size reduction

Agglomerates are broken down through mechanical shearing forces Particle size reduction Desagglomeration. 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

DSC experiments were conducted up to 500 and 600°C for the ball-milled mixtures of NaBH with 12, 55, 73 and 92 wt%Mg (DSC traces not shown here). The temperatures of melting and decomposition peaks of NaBH are plotted in Fig. 3.46a. It is seen that the data points for the equivalent content of 12, 55 and 73 wt%Mg follow quite closely the data points for MgH. Only data points for the equivalent content of 92 wt%Mg show slightly higher decomposition temperature than their MgH counterpart and lie slightly above the ROM line for MgH. This behavior is probably related to different mechanical properties of Mg and MgH which results in different abilities to particle and grain size reduction during the milling process as can be seen in Fig. 3.47. The particle size reduction of ductile... 

Mechanical methods of powder production, also known as mechanical attrition, typically require a brittle material, or at least one that becomes brittle during processing. There are various types of equipment that perform particle size reduction. 

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Impact Milling Particle size reduction by high-speed mechanical impact or impact with other particles (also known as milling, pulverizing, or comminuting)... 

Cutting Particle size reduction by mechanical shearing... 

Screening Particle size reduction by mechanically-induced attrition through a screen (commonly referred to as milling or deagglomeration) Tumble Milling Particle size reduction by attrition, using grinding media... 

FRACTURE MECHANICS] (Volll) particle size reduction [SIZE REDUCTION] (Vol 22) in plastics [PLASTIC TESTING] (Vol 19)... 

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Mahajan and coworkers [85] studied the impact of abrasive size at different particle concentrations on the oxide removal rate. It was found that the removal rate was a direct function of the particle concentration for monosize abrasives of size 0.2 pm, thereby supporting the contact-area mechanism. The mechanism shifted to indentation for a monodispersed system at 1.5 pm, resulting in reduced removal rates. At 0.5 pm, the removal rate initially increased and then decreased with particle concentration, suggesting a shift in the removal rate mechanism. Particle-size distribution [86] has an equally important effect as the particle size. A larger number of oversized particles in the distribution also cause a shift in the mechanism of material removal. Mahajan and coworkers conducted studies to evaluate the impact of size distribution on oxide removal rates. Baseline commercial slurry was spiked with different concentrations of impurities in the range of 0.5-1.5 pm. The size at different concentrations resulted in removal rates lower than that obtained with the original slurry. Slurry spiked with 1.1 % of 1.5pm particles resulted in a removal rate equal to the baseline slurry, suggesting the predominance of indentation mechanism. Slurries spiked with other concentrations and sizes resulted in a decrease in the removal rate explained by the reduction in the contact area of the abrasives with the oxide substrate. 

Somewhat similar measurements could be based on solid disruption [18], polymer degradation [7], or accelerated dissolution. These well-known mechanical effects of ultrasound also derive from cavitation. Thus one might measure the rate of particle size reduction under sonication of some standard solid dispersed in a given fluid. Alternatively one could measure the rate of dissolution of a standard solid in a solvent, or the reduction in molecular weight of polymer chains. Here again the initial particle size and surface conditions, together with pressure and temperature, should be carefully monitored. 

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