Hydrogen precipitation processes

Several geochemical processes are responsible for the accretion of metals onto Fe-Mn nodules. In the case of hydrogenous precipitation, the metal ions are supplied by... 

The observation that nodules grow at widely varying rates provides further support for the existence of multiple formation mechanisms. The nodules that accrete most slowly (1 mm per million years) appear to have formed primarily by the process of hydrogenous precipitation. This accretion rate is equivalent to the annual deposition of a layer that is only one atom deep. These slow rates cause a significant amount of metal-rich seawater to become occluded between the Fe-Mn oxide layers. 

Phosphorites are hydrogenous precipitates with phosphorus concentrations greater than 5% w/w P2O5. Concentrations as high as 40% have been observed. In comparison the phosphorus content of most sediments is aroimd 0.3%. Phosphorites represent an important economic ore deposit as shown in Table 18.3, supplying phosphorus for fertilizer use. The United States is the leading supplier of processed phosphates in the world, accounting for about 45% of world trade. 

Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

The recovery or removal of metals from solutions derived from the leaching of minerals is an important step in any hydrometallurgical process. Precipitation by reduction to the metallic state in electrochemical cells will be discussed in Section 63.3.5 this section will cover the use of chemical reagents to control the precipitation process. Therefore, although the production of metallic powders by the reduction of metal ions with hydrogen or less-noble metals (cementation) is electrochemical in nature, it will be discussed under this heading. 

One feature that surprises many people is the high water content of precipitated silica. The actual silica content is only about 90%. The rest of the mass is made up of three types of water that combined as surface silanols, strongly hydrogen bonded water and loosely held water. The latter is about 5% by mass of the filler and can be removed by drying at 105 °C. The effects of this water are not often considered, but it can obviously be important, especially in reactions with surface modifiers, such as organo-silanes. Normal products also contain significant amounts of sodium and sulfate from the precipitation process. 

Nanosilica is commercially available under the trademark of AEROSIL (product of Evonik Industries) [27]. Nanosilica powder is industrially produced by both the fuming method and the precipitation method. In the fuming method it is manufactured by a high temperature vapor process in which SiCU is hydrolyzed in a flame of oxygen-hydrogen. Precipitated silica, in turn, is manufactured by a wet... 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

The removal of copper from the pregnant nickel solution in the Sherritt-Gordon process is an example of purification by precipitation of a fairly insoluble compound. First, in the copper boil step, ammonia is driven off by heating the solution, and some copper sulfide precipitates. The residual copper is removed by a dding hydrogen sulfide for the chemical precipitation of mote copper sulfide. 

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