Detachment of particle

The removal rate of particles or the rate of flotation from pulp is essentially governed by (i) collision between particles and bubbles (ii) adhesion of particles to bubbles and (iii) detachment of particles from bubbles. Keeping these factors in mind, one can arrive at the following relationship ... 

Fig. 9.3 The proposed four steps of the mechanism of particulate mucoadhesion. Step 1, administration. Step 2, initial adsorption of the particles. Step 3, mucoadhesion of particles and further transit in the lumen. Step 4, detachment of particles, further transit and further fecal elimination. Adapted from Ponchel et al. (1998)...

Through a recharge of the bubble surface not only the electrostatic barrier is removed, but also the near potential well becomes deeper, so that a subsequent detachment of particles becomes impossible. In this case microflotation perfection reduces to the control of the transport stage... 

The term "small particles" in microflotation covers a very wide range of sizes and also the values of different factors vary strongly having an effect on transport, attachment process and detachment of particles. A general analysis of the elementary microflotation act without a unified theory is impossible. Such theory needs to take into account the role of all these factors within the whole range of particle sizes, and in addition a unified DAL theory. To bypass this difficulty two regions of particle sizes are discussed separately in subsequent sections micron/submicron particles, and decimicron particles (see Appendix lOE, lOF). 

Eq. (10.63) can be hilfilled at low and high surface activity of surfactant. It is not recommended to use a surfactant of very high smface activity (cf. condition (8.72)) in the absence of a rear stagnant cap since the adsorption layer is carried away to the rear pole of the bubble. Addition of such surfactant does not enhance particle deposition on the leading surface of bubbles and does not prevent their detachment from the rear surface. The surfactant molecules are concentrated in the close neighbourhood of the rear pole of the bubble and detachment of particles can occur from any part of the surface in the vicinity of which the normal velocity components are directed to the liquid. 

The greater the equilihrium thickness of the liquid interlayer in a contactless flotation, the higher is the probability of detachment of particles from the Inibble and the more important is the decrease of the bubble size, which reduces the detaching forces. 

The possibility of detachment of particles is critical when millimeter bubbles are used. If this cannot be prevented, centimicron particles should obviously be used. Millimeter bubbles can be combined with reagents whose adsorption on the particles surface prevents their detachment. 

Consequently, clay particles do not disperse at low values of pH. At high pH, both the clay particles and pore walls become negatively charged. This produces a significant increase in the repulsive force and leads to the detachment of particles from the wall. 

Freitas A, Sharma M (2001) Detachment of particles from surfaces an AFM study. J Colloid Interface Sci 233 73-82... 

Sometimes, particularly in the detachment of particles by an air or water stream (Chapters X and XI), we do not use the adhesion number, but rather the reciprocal of the adhesion number. This quantity is termed the coefficient of removal ... 

Knowing the probabilities of detachment and removal of particles, we can use Eq. (1.33) to determine the coefficient of removal. In the detachment of particles under the influence of an air stream, when the probability of removal is equal to unity, the following calculated values are obtained for the coefficient of removal in the case of spherical and irregular particles from a steel surface with a Class 5 finish ... 

Detachment of Particles. The force acting on an adherent particle in the direction perpendicular to the dust-covered surface determines the magnitude of adhesive interaction. If this force is directed tangential to the surface, we are measuring the static friction in the detachment of particles. Under real conditions, the detaching force may be directed at an angle to the dust-covered surface [30-32]. 

The conditions under which detachment of particles is possible are expressed by the following inequality ... 

As would be expected, there is no detachment of particles under the influence of a compressive force (position 10). When a detaching force is acting at an angle of 30,60,90,120, or 150° to the dust-covered substrate in air (positions 2-6), the detached particles are immediately removed from the surface. 

In an air medium, y > An analogous relationship was observed by G. I. Fuks [12] in the adhesion of quartz particles with diameters from 0.8 to 15 jum in water and in solutions of certain electrolytes. In this case, is equal to 1.2 to 2.0 times 7, depending on the electrolyte concentration. After detachment of particles by a force directed tangential to the dust-covered surface, they execute a complex movement rolling, sliding, and intermittent shifts. 

Experimental Determination of Contact Area. The direct measurement of the area of contact between particles and a surface involves great experimental difficulties that have not been resolved successfully thus far. The area of actual contact of the particles with the surface can be judged if we start from the conditions of detachment of particles by a pulse technique [19]. 

Centrifugal Method. The centrifugal method for determining adhesive forces is based on the detachment of particles by rotating the dust-covered surface around a horizontal or vertical axis. The magnitude of the detaching force can be found from the formula... 

The following comparative data refer to the detachment of particles 40-60 jLtm in diameter from steel surfaces with a Class 9 finish, by vibration and centrifuging techniques [11] ... 

Through the use of simulation methods, it is possible to study the adhesion of two bodies and to obtain reproducible results expressed in terms of the width and properties of the gap separating the contiguous bodies [56,68,95-97]. Simulation, however, cannot replace methods for the determination of adhesive forces by actual detachment of particles [99]. 

In particle deposition, the force F is understood to be the force of interaction between particles and surface in the detachment of particles (for example, in a centrifuge), the symbol F will denote the magnitude of the centrifugal force. In the case of contact of plane surfaces, the force F is responsible for pressing these surfaces together. This force is sometimes termed the contact pressure. The force F determines the equilibrium thickness of the gap between the contact surfaces,... 

Slope angle (in deg) for detachment of particles in following liquids ... 

A reduction of particle adhesion has been found in aqueous solutions of a complexing agent sodium hexametaphosphate (SHMP). Our studies with sodium hexametaphosphate with a labeled phosphorus atom have shown that this compound, like certain condensed polyphosphates, may be sorbed on the surface of the contiguous bodies and thereby will facilitate detachment of particles. 

Dependence of Adhesive Force on Particle Size in Detachment by Inclination of Surface. Detachment of particles by tilting the surface (see Section 11) has been used to investigate the adhesion of quartz particles of various sizes in an aqueous medium [4] and also in certain organic solvents and alcohols [190, 197]. 

In support of this statement, let us now turn to experimental data obtained by detachment of particles by centrifuging, with the adhesion being characterized in terms of the minimum, median, and average forces of adhesion. Using calculated values for the width of the gap between the contiguous bodies and taking a value = 3, we can use Eq. (11.53) to calculate the possible minimum values of adhesive force for spherical glass particles on a metal surface, when the constant is 2 10" erg. The calculated values of the minimum forces of adhesion are listed below in comparison with the experimental values ... 

First, let us examine the detachment of particles under the influence of a dc field that has been established by converting low voltage into high voltage (in this particular case, 50 kV at a current of lO A) in a converter based on a semiconductor triode of the P4B type [198]. In studying particle detachment in vacuum or in an atmosphere of some particular gas such as nitrogen, we used a specially designed unit in which the electrode and the dust-covered surface were placed inside a bell jar connected to a vacuum pump to remove the air and to a source of gas. 

The absolute value of the charge measured after switching off the electric field (Table VII. 1) was very small and little different from the charge observed upon detachment of particles from a surface that had not been exposed to an electric field. On this basis, we can assume that at the time of measurement there was no longer any polarization charge of the particles. Apparently, elastic rather than bulk polarization takes place during the time of field application (3 min), and the lifetime of this effect after the field is switched off is only 10" sec i. e., the polarization charge on the particle surface exists only in the electric field. 

The detachment of particles may be prevented by sorption of ions on the surface. The charge acquired by a particle as a result of ion sorption (see Fig. VII. 2.c) should be determined by the residence time of the particle in the high-voltage field. According to experimental results reported in [130], particles acquire a maximum charge after being held in the field of a corona discharge for 0.1 sec, or in an electrostatic field for 5-10 min. 

Equation (VII.6) can be used to determine the forces and conditions for detachment of particles of different sizes. The force required to detach spherical molybdenum particles adhering to an iridium surface, using a metal screen as the electrode, were found to be as follows [200] ... 

TABLE Vlil.5. Detachment of Particles, by Centrifuging, from Surfaces Painted with Conventional and Modified Chlorinated PVC Enamel... 

Features of Particle Adhesion to Paint and Varnish Coatings with an Oil Layer. The presence of an oil layer changes the magnitude of adhesive interaction. The conditions for detachment of adherent particles will depend on the ratio between the adhesion of the particles to the oil layer and the cohesion of this layer. If the adhesion is greater than the cohesive interaction, detachment of particles will require that the cohesive forces of the oil layer be overcome. If the adhesion is less than the cohesion, the adherent particles will be detached as a result of overcoming the adhesive interaction. 

The particle weight is considerably smaller (by several orders of magnitude) than the force of adhesion and can be neglected. In the boundary layer (see Fig. X.l, p. 308), the drag force drops off sharply (hence, detachment of particles takes place at velocities considerably greater than the velocities for particle deposition). After contact, the force of inertia disappears. Then the condition for attachment of particles, Eq. (IX. 1), if Ffj = 0, can be represented in the following form ... 

The deposition of particles, the same as the detachment of particles already adhering (see p. 307), takes place in the boundary layer and depends on the nature of the movement in this layer. In [249], this situation is taken into account, and the coefficient Kji is related to the flow structure as follows ... 

As would be expected, increasing flow velocities gave smaller values for the number of adherent particles since the higher flow velocities increased the probability of detachment of particles previously sticking to the surface. Fewer particles adhered to the lower side of the plate since part of the particles can be detached from the lower surface by the force of gravity. 

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