Solid/liquid separation problems

The steps in solving a solids-liquid separation problem, in general, are ... 

Hardly any solid-liquid separation problems due to draining by gravity and separately discharging the liquid residue and the remolten solid layer, hence, the product. 

Direct treatment of oxide, silicate and carbonate zinc ores is difficult if conventional zinc technology is used (leaching, purification by means of cementation with zinc powder and zinc electrowinning). There is only one case (Padaeng, Thailand) where conventional zinc flowsheets were successfully modified to treat zinc silicate ores. This was possible given the high-grade zinc ore treated (30% Zn) and the fact that this plant was able to solve Ae solid-liquid separation problems. 

Ernst M., Talcott R.M., Romans H.C. and Smith G.R.S., 1991. Tackle solid-liquid separation problems, Chem. Eng. Prog., 87, 22-28. 

Elutriation and fluid classification methods are also highly relevant to solid-liquid separation problems—they use the same or similar mechanisms for analysis as many separators. Use is made here of the size-dependent nature of dynamic separation processes and most of these methods are based on the analytical cut size defined in chapter 3, Efficiency of Separation . 

Thus, methods are now becoming available such that process systems can be designed to manufacture crystal products of desired chemical and physical properties and characteristics under optimal conditions. In this chapter, the essential features of methods for the analysis of particulate crystal formation and subsequent solid-liquid separation operations discussed in Chapters 3 and 4 will be recapitulated. The interaction between crystallization and downstream processing will be illustrated by practical examples and problems highlighted. Procedures for industrial crystallization process analysis, synthesis and optimization will then be considered and aspects of process simulation, control and sustainable manufacture reviewed. 

Many industrial processes begin with a leaching step, yielding a slurry that must be clarified before solvent extraction. The solid-liquid separation is a costly step. The solvent extraction of unclarified liquids ( solvent-in-pulp ) has been proposed to eliminate solid-liquid separation. The increased revenue and reduced energy cost make this an attractive process, but many problems remain to be solved loss of metals and extractants to the solid phase, optimization of equipment design, effluent disposal, etc. 

The calorimetric measurements in metal oxide-aqueous electrolyte solution systems are, beside temperature dependence of the pzc measurements, the method for the determination of the enthalpy of the reaction in this system. Because of the low temperature effects in such systems they demand very high precision. That is why these measurements may be found only in a few papers from the last ten years [89-98]. A predominant number of published measurements were made in the special constricted calorimeters (bath type), stirring the suspension. The flow calorimeters may be used only for sufficiently large particles of the solid. A separate problem is the calculation of the enthalpy of the respective reactions from the total heat recorded in the calorimeter. A total thermal effect consists of the heat of the neutralization in the liquid phase, heat connected with wetting of the solid, heat of the surface reaction and heat effects caused by the ion solvation changes (the ions that adsorb in the edl). Considering the soluble oxides, one should include the effects connected with the transportation of the ions from the solid to the solution... 

If a saturated solution is cooled, the solubility of the solute generally decreases in order for the cooled solution to return to equilibrium, some solute must come out of solution as solid crystals. The crystallization rate may be slow, however, so that a metastable condition can exist in which the concentration of the solute is higher than the equilibrium value at the solution temperature. Under such conditions, the solution is said to be supersaturated and the difference between actual and equilibrium concentrations is referred to as supersaturation. Ail problems involving solid-liquid separations in this text assume that equilibrium exists between the solid and liquid phases, so that supersaturation need not be considered. 

As the MBR process does not need the secondary clarifier for the solid-liquid separation, it can prevent these inherited problems that occur in the conventional processes for the wastewater reclamation (29). The resulting high-quality and completely disinfected effluent means that the MBR process can be used for many purposes, i.e., industrial and municipal wastewater treatment and reuse (30). More important, the land requirement of MBR is much less than that of conventional wastewater reclamation processes. 

FIG. 18-145 Coding the problem specification. Purchas, Solid/Liquid Separation Equipment Scale-Up, Uplands Press, Croydon, England, 1977, p. 10, by permission.)... 

A good solids-liquid separator performs well in service, both initially and over time. It operates reliably day after day, with enough flexibility to accommodate to normal fluctuations in process conditions, and does not require frequent maintenance and repair. Selection of such a separator begins with a preliminary listing of a number of possible devices, which may solve the problem at hand, and usually ends with the purchase and installation of one or more commercially available machines of a specific type, size, and material of construction. Rarely is it worthwhile to develop a new kind of separator to fill a particular need. 

The participants in this symposium addressed many of these problems. Methods for characterizing size and chemical composition by size were discussed, and several models of metal and virus adsorption were presented. Modeling particulate dynamics in rivers and the ocean provided new insights into the fate of contaminants associated with particulates. Papers on applications of size distribution measurements for selection, process modeling, and control of solid/liquid separation processes demonstrated the analytical value of particle counting compared to cumulative measurements of particulate concentration. 

Average values for the liquid flow are 10-30 m m h , and for the gas flow 300 1000 m m h . Solid-liquid separation is not necessary. Disadvantages are the poor heat removal and the occurrence of hot spots with potential instabilities. However, since the reactors are generally operated adiabatically, the relatively poor heat removal is not necessarily a problem. 

Precipitation technology is not yet very advanced, and many industrial companies experience problems with small crystals that cannot be separated easily. Furthermore, for some purposes, it is necessary to produce very small crystals (e.g., pigments, paper-coating materials, and paper fillers) and, therefore, solid-liquid separation becomes inevitably difficult. In the paper industry, submicron-sized paper-coating agents may be produced in a process in connection to the paper mill so that the suspension is transported as slurry and no filtration is required. If the filtration of small crystals is needed, the problem is the need to be handled by filtration specialists. 

---