Not so long ago in our publication we considered one of the methods for measuring quality of grinding material when using grinding balls. This indicator was based on the definition of «residue on the sieve «. In this article we will consider another method, which involves the calculation of the «specific surface area value». Both of these indicators fully characterize the concept of »grinding quality» and the grinding process optimality in ball mills.
The specific surface, or specific surface area, is the total surface of all the crushed material (usually dispersed or porous ) particles divided by the mass of the test material. The material specific surface consists of the total surface areas of all its grains. The smaller the size of these grains (grinding is thinner), the greater the area of their total surface. If the particles of the crushed material have a developed porous surface, the surface area of a single grain gets bigger compared to its absolutely smooth analog, which is equal in geometrical parameters.
The physical properties of the grinding material can be defined by the indicator of specific surface area (measured in cm2/cm3 or cm2/g). This indicator of grinding material is very important in the preparation of raw materials for the production of cement, aerated concrete or the minerals processing.
How is this indicator calculated? We assume that all particles (grains) in the crushed material have the same size and spherical shape. The value of the specific surface S shall be determined as follows:
n — number of particles, S — the outer surface, V — volume of particles having a diameter δср.
Today, all calculations are executed automatically using special instruments. Mostly, our customers use the instruments of PSC series (Khodakov’s instruments).
Dispersive analysis is used in instruments of the PSC series through the determination of the gas permeability. The gas permeability index of the crushed material is determined by the duration of filtration time of air passing through it. At the same time, the initial and final thin air (pressure) is taken as a constant. The specific weight (density) of the test material, the mass and height of its layer in the ditch are used to calculate the specific surface area and the average particle size.
All processes are fully automated, the device itself calculates the specific surface value, gas permeability and the average particle size, which should coincide with the results of screening this material. The device controls the following parameters: time of air passing through the loaded sample, air temperature, pressure in the working chamber, air viscosity, etc.
These devices significantly reduce the duration of such analyzes. Since all processes are fully automated (besides the taking a sample), the measurement and calculations accuracy is ensured by reducing to zero the «human factor» action. The only condition for obtaining accurate results is continuous monitoring of the device’s operability and regular testing.
Each industry has its own requirements for the specific surface area of the crushed material. So, for the production of aerated concrete blocks, the specific surface of the finished sludge should fluctuate between 2800-3300 cm2 / g. For cement production this range is 2500-3500 cm2 / g (depending on the brand of cement produced).
To sum up, we remind our readers once again that correctly select of steel grinding balls is the main factor that directly affects the quality of grinding, which in turn leads to cost minimization for the grinding one ton of material.