Particle leaching

Uranium ores are leached with dilute sulfuric acid or an alkaline carbonate [3812-32-6] solution. Hexavalent uranium forms anionic complexes, such as uranyl sulfate [56959-61-6], U02(S0 3, which are more selectively adsorbed by strong base anion exchangers than are other anions in the leach Hquors. Sulfate complexes are eluted with an acidified NaCl or ammonium nitrate [6484-52-2], NH NO, solution. Carbonate complexes are eluted with a neutral brine solution. Uranium is precipitated from the eluent and shipped to other locations for enrichment. Columnar recovery systems were popular in South Africa and Canada. Continuous resin-in-pulp (RIP) systems gained popularity in the United States since they eliminated a difficult and cosdy ore particle/leach hquor separation step. 

The stability and durability of Pt alloys, especially those involving a >d transition metal, are the major hurdles preventing them from commercial fuel cell applications. "" The transition metals in these alloys are not thermodynamically stable and may leach out in the acidic PEM fuel cell environment. Transition metal atoms at the surface of the alloy particles leach out faster than those under the surface of Pt atom layers." The metal cations of the leaching products can replace the protons of ionomers in the membrane and lead to reduced ionic conductivity, which in turn increases the resistance loss and activation overpotential loss. Gasteiger et al. showed that preleached Pt alloys displayed improved chemical stability and reduced ORR overpotential loss (in the mass transport region), but their long-term stability has not been demonstrated. " These alloys experienced rapid activity loss after a few hundred hours of fuel cell tests, which was attributed to changes in their surface composition and structure." ... 

Initially, smaller particles leached less than larger ones, but eventually the cumulative amount of metals leached from the smaller particles became greater then that from the larger particles (Fig. 9). This generally occurred when the acid flowing by the smaller particles was no longer totally neutralized and the leachate pH dropped. 

Table 1 shows a summary of the percentage of metals leached after the addition of 25 meq acid/g solids. This shows that the small particles leach much more metal than the larger ones, once the leachant pH is reduced. A comparison of the percentages leached from large particles in the sequential extraction tests with those in the column tests shows that the results from the 2 procedures are very similar. 

Solvent casting/particle leaching [41, 45] Controlled porosity Controlled interconnectivity (if particles are sintered) Structures generally isotropic Use of organic solvents... 

Typical soils exhibiting distinctive layers with increasing depth are called horizons (Fig. 9.7). The top layer, normally several centimetres in thickness, is known as the A horizon, or topsoil. This is the layer of maximum biological activity in the soil and it contains most of the soil organic matter. Metal ions and clay particles in the A horizon are subject to considerable leaching. The next layer is the B horizon, or subsoil. It receives material such as organic matter, salts, and clay particles leached from the topsoil. The C horizon is composed of weathered parent rocks from which the soil originated [1]. 

Various fabrication methods have been developed in order to attain the 3D scaffold characteristics. In the case of synthetic polymer or polymer-matrix composite scaffolds, the methods include [47] solvent casting and particle leaching, phase separation, extrusion, gas foaming, and free form fabrication. Each method presents certain advantages with respect to others, ranging from ease of manufacture to control of the microstructure/nanostructure. Solvent casting and phase separation methods have been studied at our laboratory. 

Fig. 7 SEM image of a composite scaffold produced by solvent casting and particle leaching. The black arrows indicate glass particles. The magnification bar corresponds to...

The development of foams for medical purposes was described by Radusch [193], To eliminate thermal degradation, as well as to avoid the problems associated with the low melt viscosity, a process was developed based on solvent casting and subsequent particle leaching. Well-defined porous structures can be formed in such a way [197-200], for example, PHB was dissolved in chloroform, the ensuing solution was mixed with water-soluble particles like NaCl, films were then prepared by solvent casting and evaporation and, subsequently, the salt particles were washed out with water. Various foamed structures were thus prepared, depending on the particle size and amount, as well as other parameters, such as the PHB type and its concentration in the chloroform solution. 

Figure 20.1 SEM image of polymer scaffolds synthesized via (a) solvent casting (Boateng et al., 2009), (b) gas blowing, (c) emulsion templating (Janik and Marzec, 2015), (d) particle leaching (Janik and Marzec, 2015), and (e) electrospinning (Tseng et al., 2013).

Figure 20A Summary of the processes used in ca eating porous scaffolds by means of particle leaching.

Cai Q, Yang J, Bei J, Wang S. A novel porous cells scaffold made of polylactide-dextran blend by combining phase-separation and particle-leaching techniques. Biomaterials 2002 23 4483-92. 

---