Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

92
reliable processing and stable values of end-product. Peschl verified his method
testing products of pharmaceutical and coal industry [133] and recommended its
use for quality determination. The obtained parameters are: consolidation pressure
(1 1 ), angle of internal friction (3) and unconfined compressive strength (1 c ).
The only parameter of free flowing material is the angle of internal friction. To characterize
a cohesive material two parameters are necessary – the angle of internal
friction and cohesion. Consolidation pressure should be set close to the pressure
that acts on the material under the conditions of technological process considered.
The angle of internal friction is a function of stresses 1 1 and 1 c . As the nondimensional
parameter the author proposed “absolute flowability – FLA:
FLA
where:
! w – density of water,
!– density of bulk material.
( σ −σ
)
1
⋅ ρ
(13.1)
σ ρ
=
2
c
Bell et al. [16] examined a number of methods of quality determination of
powders regarding their usefulness in industry including the method of Peschl.
Two directions in simplification of powder quality assessment were explored:
automation of the Jenike approach and elaboration of a new method. The Peschl
method is an example of the first approach. Bell et al. admitted that the method
gave repeatable results in the range of intermediate and low normal pressures. The
possibility of performing several measurements on one sample of powder is an
advantage of this method, however special care should be taken in the case of
materials susceptible to damage in shearing. Automation applied in the tester
results in its relatively high price that limits the number of units that can be placed
along production lines. Moreover, the advantage of automation became dubious
when determination of flowability with time consolidation is required. Such
examination requires separate samples consolidated under different pressures and in
controlled ambient conditions, thus the same sample cannot be examined without
conditioning for every single test.
13.1.2. Johanson’s apparatus and indices
J.R. Johanson separated from Jenike & Johanson in 1985 and before long
proposed his own method of determination of mechanical properties of powders [77].
The basic unit of this equipment is an indicizer test cell shown in figure 13.1. The
sample is consolidated in a cylindrical mould using a two-piece piston that measures
the consolidation pressure directly on the inner piston while the inner and outer
w