Interaction of chemical and physical parameters in the removal of aggregates in specific technical separation units

As indicated earlier, coagulation is frequently used in a technical context for the improvement of the liquid-solid separation. Separation techniques employed today are sedimentation, flotation, and filtration. Sedimentation and flotation, which might be looked at as analogous unit processes, are considered separation methods with a large capacity in terms of solids loading. They also appear as highly economical due to their favorable loading characteristics. The following discussion will therefore focus on sedimentation, or flotation, respectively, as a separation step following the coagulation process.

The notion of technically successful sedimentation describes the course of a sedimenting particle or aggregate through the sedimentation reactor in the way depicted schematically in Fig. 9. It is postulated that the particle which reaches the bottom of the tank before it enters the outlet zone is removed permanently. By the same arguments, a particle whose travel time for horizontal movement through the basin is shorter than its sedimentation time is not removed at all. This concept of the sedimentation process, which can be adjusted for different reactor geometries, has been confirmed by many observations from large-scale technical systems. The most significant conclusion from this concept is that only the size and density, i.e., the Stokes' velocity of spheres controls the removal rate. In an analogous way it is formulated and proven useful for flotation.

If a suspension is destabilized by a given amount of metal ions then the coagulation reaction described as destabilization ratio — or for defined energy input as reaction rate constant — and the result in terms of aggregate size and density should be identical. And then the removal rate should also be identical. However, it is frequently observed that some destabilizing reagents produce better sedimenting aggregates than others with very

horizontally oriented rectangular sedimentation tank

Fig. 9 A simplified model for the removal of solid matter in a sedimentation tank horizontally oriented rectangular sedimentation tank

Fig. 9 A simplified model for the removal of solid matter in a sedimentation tank

Dosage And Removal Rate

0 150 300 450

CHEMICAL DOSAGE (umol/1)

Fig. 10 Observations of the different removal efficiency of suspended solids coagulated with different chemicals at identical dosages in a sedimentation tank (after [10])

REMOVAL EFFICIENCY

IRON CHLORIDE IR0N(II)SULPHATE POLYALUMINUM ALUMINUMSULPHATE

0 150 300 450

CHEMICAL DOSAGE (umol/1)

Fig. 10 Observations of the different removal efficiency of suspended solids coagulated with different chemicals at identical dosages in a sedimentation tank (after [10])

similar chemical characteristics applied at comparable concentrations (see Fig. 10 drawn after [10]). The data indicate that for these boundary conditions aluminum sulphate produces the most effectively settled floes, while iron chloride generates floes which do not all reach the bottom of the tank in a sufficiently short time to be removed completely. Other investigations have shown that Al3+ coagulated floes sediment less readily than iron coagulated floes at identical concentrations [11].

Such deviation from the simple concept of sedimentation — the analogy holds for flotation as well with only the direction of motion changed — can be explained by the interaction of aggregate properties with the flow structure in the separation unit: if there are regions with higher shear stress in a separation apparatus then the more stable floe will be separated more effectively than the shear-endangered one. Or, similarly, the less heterodisperse suspension will be separated more effectively in sedimentation tanks than suspensions with a broader spectrum of aggregate sizes.

This concept of interdependence of the coagulation step and the separation step is confirmed by data from systematic investigations on the separation effectivity of geometrically different units for systematically varied boundary conditions of the coagulation process. Figure 11 (after [12]) illustrates some results of these studies. Suspensions were coagulated quite differently using aluminum or iron or a combination of iron and polymeric flocculants. This is described quantitatively by the observed collision efficiency factor "alpha". Increasing values of "alpha" indicate more rapidly coagulating systems. The resulting aggregates are described in more detail in terms of first and second moment of the statistical distribution of the aggregate diameters in the insert in Fig. 11. The removal of such aggregates has been studied in reactangular sedimentation tanks and in a

REMOVAL EFFICIENCY

Fig. 11 Removal efficiency of aggregates in different separation reactors showing the interdependence of the coagulation step with the separation step. The X-axis describes the different reactor types investigated, the Y-axis the various aggregate types generated and separated, and the Z-axis the observed removal efficiency

COLLISION EFFICIENCY ■¡¿/fl. A/S 4.6% total. A/S 2.8% sediment, tank D'

beactor^.35 qa7 o.59 0.84

Fig. 11 Removal efficiency of aggregates in different separation reactors showing the interdependence of the coagulation step with the separation step. The X-axis describes the different reactor types investigated, the Y-axis the various aggregate types generated and separated, and the Z-axis the observed removal efficiency flotation unit of a geometry similar to one of the sedimentation units. It has been recorded as (Cin-Cout)/Cin, i.e., as removal efficiency. We conclude from these data that there is a very clear effect of the floe type upon the separation efficiency, even if the chemical conditions are kept constant, i.e., for unchanged "alpha" values. More rapidly aggregating systems, characterized by a higher collision efficiency factor "alpha" are sedimenting better than a slowly coagulating one. This is not the same for flotation as a separation process as is shown by the data (the geometry of the flotation tank is the same as that of the sedimentation tank). For practical purposes of designing and operating a liquid-solid separation step, one can deduce that

1) flotation is more effective than sedimentation, under conditions of geometrically similar reaction units;

2) floes sedimenting relatively effectively cannot in all instances be separated with good results by flotation (the difference results from the type of aggregation chemical), and

3) very dense and/or heavy floes are sedimented and floated equally well.

It has been described elsewhere [12] that the flow pattern in the tanks investigated and described by the dimen-sionless dispersion number (DN = Z)/(u.L], where D is the turbulent dispersion coefficient, u the convective transport, and L a characteristic length) has, in conjunction with the floe properties, a more pronounced effect upon the separation effectivity than can be concluded from the coagulation rate alone.

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