Wet processes, separators

Figure 20.6 Atomic force microscopy image of wet process separator (Setela E20MMS, courtesy of Tonen Specialty Separator Codo Kaisha).

There are numerous variations of the wet process, but all involve an initial step in which the ore is solubilized in sulfuric acid, or, in a few special instances, in some other acid. Because of this requirement for sulfuric acid, it is obvious that sulfur is a raw material of considerable importance to the fertilizer industry. The acid—rock reaction results in formation of phosphoric acid and the precipitation of calcium sulfate. The second principal step in the wet processes is filtration to separate the phosphoric acid from the precipitated calcium sulfate. Wet-process phosphoric acid (WPA) is much less pure than electric furnace acid, but for most fertilizer production the impurities, such as iron, aluminum, and magnesium, are not objectionable and actually contribute to improved physical condition of the finished fertilizer (35). Impurities also furnish some micronutrient fertilizer elements. 

A flow sheet of the basic TVA process for granular diammonium phosphate is given in Figure 12. The raw materials are wet-process phosphoric acid and anhydrous ammonia. Feed acid concentration of at least 40% P2 5 required to give a satisfactory water balance. This average concentration usually is provided by two separate feed streams, one of 54% P2 5 concentration and one of about 30% P2 5 arrangement shown, the 54% acid is... 

The formation of the metallic salts is a pyrometaHurgical process, and is commonly referred to as the dry process. The separation of the salts from each other is accompHshed by selective dissolution in water, and is named the wet process. 

For fine pulverization, both dry and wet processes are utilized, but increasingly the dry process is more popular because wet grinding ultimately requires drying and is much more energy intensive. A sensitive fan swirls the dust sizes into the air separator and permits coarse particles to recycle to the grinding mill or be rejected as tailings the fines are drawn into cyclones where the dust is collected. 

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

Wet-process acid is manufactured by the digestion of phosphate rock (calcium phosphate) with sulfuric acid. Depending on availabiHty, other acids such as hydrochloric may be used, but the sulfuric-based processes are by far the most prevalent. Phosphoric acid is separated from the resultant calcium sulfate slurry by filtration. To generate a filterable slurry and to enhance the P2O5 content of the acid, much of the acid filtrate is recycled to the reactor. 

Apphcations include ka olin clay dewatering, separation of fish oils from press Hquor, starch and gluten concentration, clarification of wet-process phosphoric acid, tar sands, and concentrations of yeast, bacteria, and fungi from growth media in protein synthesis (14). 

A wet-process plant maldug cement from shale and hmestoue has been described by Bergstrom [Roc/c Prod., 64—71 (June 1967)]. There are separate facilities for grinding each type of stone. The ball mill operates in closed circuit with a battery of Dutch State Mines screens. Material passing the screens is 85 percent minus 200 mesh. The entire process is extensively instrumented and controlled by computer. Automatic devices sample crushed rock, slurries, and finished product for chemical analysis by X-rav fluorescence. Mill circuit feed rates and water additions are governed by conventional controllers. 

In the wet process the clay minerals are crushed and slurried with water to allow pebbles and other rock particles to settle out. The limestone is also crushed and slurried. Both materials are stored in separate bins and analyzed. Once the desired ultimate composition is determined, the slurry blend is ground and then partially dried out. 

The processes for manufacturing microporous membranes can be broadly divided into wet processes and dry processes. Both processes usually employ one or more orientation steps to impart porosity and/or increase tensile strength. Figure 2 shows scanning electron micrographs of surfaces of separators made by each process. 

In the case of a high-intensity wet magnetic separation process, the principal force acting to resist particles from being captured in the matrix is the fluid drag force, FD, which is given by the expression ... 

The typical properties of some commercial microporous membranes are summarized in Table 4. Celgard 2730 and Celgard 2400 are single layer PE and PP separators, respectively, while Celgard 2320 and 2325 are trilayer separators of 20 and 25 fim thickness. Asahi and Tonen separators are single layer PE separators made by the wet process. Basic properties, such as thickness, gurley, porosity, melt temperature, and ionic resistivity are reported in Table 4. These properties are defined in section 6.1.3. 

This feature will be increasingly Important as battery manufacturers continue to increase the cell capacity with thinner separators. The pore structure is usually influenced by polymer composition, and stretching conditions, such as drawing temperature, drawing speed, and draw ratio. In the wet process, the separators produced by the process of drawing after extraction (as claimed by Asahi Chemical and Mitsui Chemical) are found to have much larger pore size (0.24—0.34 fixxi) and wider pore size distribution than those produced by the process of extraction (0.1—0.13 after drawing (as claimed by Tonen). ... 

Figure 5 shows the surface SEM and cross-section SEM of Celgard 2325. The surface SEM only shows the PP pores while the PE pores are visible in the cross-section. It is clear from the image that all three layers are of equal thickness. The SEM of separators made by wet process are shown in Figure 6. The pore structure of all of these membranes is very similar.

Figure 6. Scanning electron micrographs of separators made by wet process and used in lithium-ion batteries (a) Setela (Tonen), (b) Hipore-1 (Asahi), (c) Hipore-2 (Asahi), and (d) Teklon (Entek).

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