Analysis of the grain-size composition of refractory powders using the VSA-500 mg automatic sedimentation balance

Abstract

The grain-size compositions of finely milled powdered refractories, raw materials, and suspensions are determined as a rule by the method of sedimentation analysis with periodic sampling [i]. These methods do not require complicated apparatus, but they are very laborious and yield results only after 12-24 h from the start of the test. The completion of sedimentation analysis by collecting and continuously weighing residues makes it possible to automate the measurement process, to cut the time to 1-6 h, and to increase the volume of information obtained about the graln-size composition of the sample.The All-Union Institute of Refractories has developed and implemented a sedimentation method for powders and suspensions using the VSA-500 mg automatic sedimentation balance manNfactured hy the "Gosmetr" factory. This balance, which has a maximum loading of 500 mg, is an automatic device designed for continuous weighing and recording of masses of suspension particles deposited on a pan. The design of the apparatus consists of two blocks --the scales themselves, and the control and recording panel.The working principle of the halance ks based on the compensation of the weight imbalance hy the power action of an electric circuit of the automatic equilibrium type. The scheme is shown in Fig. i. In addition to the beam i, the suspension 2 with the stirrup 3 and the insulator 4, necessary to provide rigid fixing of the beam during placement and removal of the suspension, the scales have a built-in gyro mechanism 5 (50, i00, 200, and 500 mg) for checking the metrological characteristics and matching parameters of the circuit, and also a gyro for approximate balancing of the beam during immersion of the suspension in the liquid. Furthermore, the beam is fitted with a differential inductive sensor 6 controlling the displacement, and a magnetoelectric compensator 7.All-Union Institute of Refractories. "Gosmetr" Factory.

Modeling of Sedimentation Processes for Calculating the Particle Size Distributions of Disperse Systems

“…Tables 2 and 3 show that, in the case of discontinuous fractional composition (when one or two fractions are absent), the accuracy of calculations both by expressions (8)- (11) and by expression (4) is not reduced. Since the calculations by expressions (4) and (8)- (11) are more accurate than graphical differentiation [6], the approach considered provides the researcher with additional opportunities, e.g., the possibility of determining the particle size distribution of bimineral composites. For this purpose, particle size analysis is performed in two liquids of different densities and the sediment is weighed (or the sediment mass values are specified) at moments of time determined from individual expressions for each liquid: where t i(j) are the times of weighing the sediment in the first and the second liquids, respectively; µ i and µ j are the viscosities of the first and the second dispersion liquids, respectively; H is the height of the liquid column over the scale pan; ρ A, 1 and D A, i are the density and the size of particles of the ith fraction of mineral A, respectively; g is the gravitational acceleration; ρ 1 and ρ 2 are the densities of the first and the second dispersion liquids, respectively; S Ä = (18H/g)µ 1 /(ρ A -ρ 1 ); and = (18H/g)µ 2 /(ρ A -ρ 2 ).…”

“…Then, by the moment of time t j , the fractions with the sedimentation times t i ≤ t j have settled and the portion of settled particles of the fractions with t i ≥ t j is m i t j /t i . Table 5 lists the theoretical values of the mass M j of the sediment that has settled by the moment of time t j (for simplicity, only six fractions with a total mass of 100 mg were taken) that were calculated by the conventional algorithm and also by expressions (2)- (6) and (13), (14). It is seen that the conventional approach gives rise to essential deviations from the given masses of the fractions (with a decrease in ∆t j , the calculated error decreases but is not completely eliminated).…”

“…The accuracy of sedimentation analysis of chemical engineering processes depends on the method for processing experimental curves, which can involve both approximate empirical or graphical techniques [1,6] and cumbersome procedures of computational mathematics [7,8]. Therefore, it is expedient to refine algorithms for processing sedimentation curves.…”