Basic fluxes

Fig. 7,45 Model of the basic fluxing process (after Goebel and Pettit )...

Naturally, the flux employed will depend upon the nature of the insoluble substance. Thus acidic materials are attacked by basic fluxes (carbonates, hydroxides, metaborates), whilst basic materials are attacked by acidic fluxes (pyroborates, pyrosulphates, and acid fluorides). In some instances an oxidising medium is useful, in which case sodium peroxide or sodium carbonate mixed with sodium peroxide or potassium nitrate may be used. The vessel in which fusion is effected must be carefully chosen platinum crucibles are employed for... 

The basic flux across the membranes may be related to that across a thin film [83]. Pick s First Law of Diffusion indicates that the total flux of diffusant across a homogeneous membrane, /, is proportional to the concentration gradient of the diffusant ... 

Of course, more complicated situations and conditions will require more sophisticated mathematical treatment, especially for the driving force, but the basic flux relationships are similar for any liquid and gas migration through the subsurface. If the hydraulic conductivities and diffusion coefficients are known for the materials and each migrating fluid of interest, then predictive computer models can often handle the difficult calculations associated with multiple fluids, multiple pressures, and multiple types of materials. 

C 550 2300 Practically unmeltable ca. 1,5 Porous Excellent Resists nonoxidizinj acid- and basic fluxing agents... 

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

Although manganese dioxide is a powerful oxidizing agent, it is nevertheless capable of being itself oxidized when it is fused with a basic flux. The trioxide of manganese is acidic in nature and combines with the base to form a salt. Thus it is evident that the presence of a base favors the oxidation. 

Choice of a dissolution method also depends on the range of substances to be analyzed, but in some specific instances the chemical form of the element is of even more importance. For example, particles formed at high temperature usually contain refractory oxides and silicates which may require fusion with either an acidic or basic flux, or hydrofluoric acid treatment to remove silica, or a combination of these techniques. 

Basic flux equation (Nernst-Planck equation) 3.8,3.12... 

Different fluxes can be used. Basic fluxes employed for the attack of acidic materials include carbonates, hydroxides, peroxides, and borates. Oyrosulfates can be used as acidic flux. If an oxidizing flux is required, sodium peroxide can be used. As an alternative, small quantities of alkali nitrates or chlorates can be mixed with sodium carbonate. Basic and acidic fluxes are dissolved in an acid or basic medium, respectively. 

The analysis of oil shale samples for arsenic and selenium present special problems mentioned earlier. Fusion of the shale with sodium peroxide at 600°C in zirconium crucibles has been used with 90-95% recovery. The advantage in this method is that the fusion is eflFective at a temperature below those of most other basic-flux techniques. More importantly, however, the sodium peroxide maintains oxidizing conditions which minimize loss through volatilization. 

Over the last several decades, there have been numerous compilations of the global nitrogen cycle (Table 4). Most cover the basic fluxes—BNF and denitrification in terrestrial and marine... 

Siliceous clays therefore are inherently less refractory than the more aluminous ones and at the same time they are more sensitive to the presence of basic fluxes. 

The reaction of removal of B from Si by using basic fluxes can be expressed ... 

LiBOn 849 Pt, An, glassy carbon Powerful basic flux for silicates most minerals, slags, ceramics... 

NaOH or KOH 318 380 Au, Ag, Ni Powerful basic fluxes for silicates, silicon carbide, and certain minerals (main limitation is purity of reagents)... 

Basic flux A substance with basic characteristics in the molten state used to solubilize refractory samples, principally silicates. Beam The principal moving part of a mechanical analytical balance. 

Basically, flux is calculated as the difference between the amount of nutrient in the flood and ebb tide, but there are several ways of doing this. The amount of nutrient is estimated by combining discharge and water sample concentration. Calculation details are normally glossed over in published papers, and the absolute method is not defined, e.g. The transport was calculated on the basis of the weights of a component imported or exported on each tide (Boorman et al., 1994) and By multiplication with concentration values obtained from the sample, the particulate and dissolved matter could be calculated for each period (Bankers et al., 1984). This does not help readers assess the confidence they may place in the final flux values. 

Some inorganic materials will not dissolve in acids, and fusion with an acidic or basic flux in the molten state must be employed to render them soluble. The sample is mixed with the flux in a sample-to-flux ratio of about 1 to 10 or 20, and the combination is heated in an appropriate crucible until the flux becomes molten. 

When the melt becomes clear, usually in about 30 min, the reaction is complete. The cooled solid is then dissolved in dilute acid or in water. During the fusion process, insoluble materials react with the flux to form a soluble product. Sodium carbonate is one of the most useful basic fluxes, and acid-soluble carbonates are produced. 

A classical example of reactivity of main group metal in thioarsenate melts and how Lewis flux basicity affects the chemistry is that of Sn. Low Lewis basicity fluxes favor the formation of KSnAsSs, and as the basicity increases (e.g. by adding more K2S) the reaction lead to layered K2SnAs2Sg instead.The former compound has infinite chains and the rare pyramidal /3-[AsS4] ligand (Figure 67(a)). The key unit... 

If the main impurity is silicon or phosphorus, a basic flux, usually lime (CaO) or magnesia (MgO), is needed to give reactions such as... 

Resistant to acid and basic fluxes if diese do not oxidize. Some sur ce contamination when metals are fused in vessels made of this material Formation of SIC above 1400 C on contact with silica-containing materials Vessels for silicate melts, sinter processes, production of refractory metals and reduction of metal oxides... 

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