Activation of MnO

Lime is added to the reaction to increase the activity of MnO and reduce the activity of Si02. This allows greater extraction of manganese in equilibrium with less than 2% Si in the alloy. 

The right hand expression can be graphically integrated to obtain the value of G. This expression is the area under the curve obtained by plotting NJ - NJ) against G. The calculation process can best be illustrated through the following estimation of the activity of MnO from the activity values of FeO presented in Fig. 8.4. 

Figure 8.7 Estimation of activities of MnO in FeO-MnO system by Gibbs Duhem integration...

Because of the relative instabiUty of FeO, the reduction to metallic Fe occurs at a much lower temperature and appreciable CO2 is present in the product gas. The high temperature required for the reaction of MnO and C results in the formation of essentially pure CO the partial pressures of CO2 and Mn are <0.1 kPa (1 X 10 atm). The product of this reaction is manganese carbide (7 3) [12076-37-8J, Mn C, containing 8.56% carbon. Assuming immiscibility of the metal and carbide, Mn should be obtainable by the reaction of MnO and Mn C at 1607°C. However, at this temperature and activity of Mn, the partial pressure of Mn vapor is approximately 10 kPa (0.1 atm) which would lead to large manganese losses. 

Figure 4.5 Phase diagram of FeO-MnO at 1 bar. The solid lines represent experimental observations [3]. The activity of iron is kept constant and equal to 1 by equilibration with liquid Fe. Dashed lines represent calculations assuming that the solid and liquid solutions are ideal.

The relative positions of the H20—02 boundary and the Mn2+—Mn02 boundaries for both 10"3 and 10"7M activities of aqueous Mn2+ indicate that for pH values greater than about 4, Reaction 58 is spontaneous. Similarly, the pure solid phases MnCOa and Mn(OH)2 are unstable with respect to oxidation to MnO >. Extensive interpretations of manganese chemistry in terms of the thermodynamic properties of the oxides and on other solid phases and solution species of manganese can be found in the recent literature (3,14, 24). 

The most common technique of measuring activities of oxide constituents (e.g., FeO in solid solution with MnO) has been through equilibration with suitable gas mixtures. In case of FeO-MnO, solid solutions, Foster and Welch (1956) equilibrated them with H2/H2O gas mixtures in the temperature range 850 to 1150 °C. 

Engell (1962) studied the activities of FeO in FeO-MnO solid solutions, by setting up the cell... 

The acceleration of solid state reactions under mechanical activation is promoted by the formation of the molecular-dense mechanocomposites. XPS and Li NMR spectroscopy revealed two different types of mechanocomposites initially formed in the activated mixtures of MnO with LiOH and Li2C03, depending upon structural and mechanical properties of lithium reajents. 

Li NMR spectra of the mixtures of MnO with LiOH and LijCOj, activated for a short time in corundum jars, were found to be considerably different from each other [97], In the first case, strong shifts to weaker magnetic fields are observed (Fig. 6.24). Such behavior is characteristic of a paramagnet, while lithium hydroxide is diamagnetic. Moreover, the value of the shift increases with the time of activation (see Table 6.1) and does not coincide with that in the final paramagnetic crystalline product, LiMn204 (470 ppm). 

Specific catalytic activity in the reaction of CO and butane oxidation was found to be a function of the calcination temperature with its maximum at 800°C, SIMS data demonstrate that the middle- and high-temperature samples differ by the transition metal surface concentration. In some cases, the high level activity of middle-temperature disordered samples can be assigned to segregation of MnO, on the surface of the particles. 

MnOi oxidizes allylic and benzylic alcohols to the corresponding aldehydes. Oxidation of primary saturated alcohols wdth Mn02 is also possible but much slower. Therefore it is possible to oxidize an allylic or benzylic alcohol in the presence of an unprotected primary alcohol w ith MnO. The yields of this procedure. strongly depend on the activation grade of MnO. Other oxidation methods are Dess-Martin periodinane, IBX," TPAP or DMSO/QOiCB. ... 

Decomposition is suggested [63] to involve the formation of Mn " ions at the salt-oxide interface and subsequent bond redistributions to involve electron transfer steps, resembling those in solution, thereby regenerating active Mn " ions at the interface. In the absence of oxygen, the carbon monoxide evolved reduces the oxide product to a finely divided and very reactive form of MnO. Decomposition is more rapid when oxygen gas is present, due to increased availability of the intermediate Mn at the nucleus surface and this species is abundant in the residual oxide phase. 

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