Concentrated acid breakdown

Figure 8.16 The control of amino acid breakdown and protein synthesis in liver. The factors in regulation are as follows (i) the amino acid concentration in the blood regulates the rate of urea production (Chapter 10) (ii) the amino acid leucine, and the anabolic hormones increase the rate of protein synthesis. Mass action is a term used to describe the effect of concentration of substrate on the reaction rate. The control of protein synthesis is discussed in Chapter 20. Control by leucine has been studied primarily in muscle.

The breakdown of an ore with a concentrated acid, on an industrial scale, is only justified if the mineral values are fairly high in concentration and of some economic importance. A high acid wastage upon unwanted minerals, e.g. carbonates, oxides, sulphates, phosphates and silicates, can normally be expected if these are present, and in any case the quantity of acid required merely to wet the ore and form a workable pulp is of necessity high. Corrosion problems are generally more severe than with dilute leaching acids and require more expensive materials of construction. 

Concentrated acids are usually only employed for refractory ores which do not respond to less severe breakdown techniques, and for the same reason might be expected to be used at fairly high temperatures. If cost, availability, and plant corrosion did not already provide sufficient grounds for a choice of sulphuric acid, its higher boiling-point than other acids would usually do so. 

The reaction can be carried out in a stainless-steel vessel, fitted with a slow-speed stirrer, heating jacket and reflux condenser. The concentrated alkali solution is first heated in the breakdown vessel to about 130°C and the monazite, ground to pass a 300-mesh sieve, is added gradually over a period of 30 min. A total reaction time of about 4 hr is then required under reflux at 140-45 °C. Adequate ventilation is needed, as with sulphuric acid breakdown, to remove the radioactive thoron gas under safe conditions. Unlike the acid breakdown process, however, highly corrosive fumes or spray are absent. 

Attempts have also been made to solvent-extract thorium directly from the sulphate liquor obtained by the acid breakdown of monazite. Tributyl phosphate can be used as the solvent provided a large concentration of nitric acid is added to the liquor before extraction. In order to make the process economic, a high proportion of the nitric acid must then be recovered by distillation of the raffinate liquor. Processes are also being developed which are based upon the use of higher alkyl phosphate or amine solvents to extract from sulphate solutions without the addition of nitric acid, as in the case of uranium. For example bis(l-isobutyl 1-3-5 dimethyl-hexyl) amine, di-2-ethylhexyl hydrogen phosphate and Primene JM-T have been used. 

Exposure occurs almost exclusively by vapor inhalation, which is followed by rapid absorption into the bloodstream. At concentrations of 150—186 ppm, 51—70% of the trichloroethylene inhaled is absorbed. MetaboHc breakdown occurs by oxidation to chloral hydrate [302-17-OJ, followed by reduction to trichloroethanol [115-20-8] part of which is further oxidized to trichloroacetic acid [76-03-9] (35—37). Absorbed trichloroethylene that is not metabolized is eventually eliminated through the lungs (38). The OSHA permissible exposure limit (PEL) eight-hour TWA concentration has been set at 50 ppm for eight-hour exposure (33). 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

The formation of deposits on platinised anodes can cause anode degradationThus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl, and O, produced and thus increase the Pt dissolution rate Fe salts may be incorporated into the TiO, oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate . 

Atmospheres polluted by oxidising agents, e.g. ozone, chlorine, peroxide, etc. whose great destructive power is in direct proportion to the temperature, are also encountered. Sulphuric acid, formed by sulphur dioxide pollution, will accelerate the breakdown of paint, particularly oil-based films. Paint media resistant both to acids, depending on concentration and temperature, and oxidation include those containing bitumen, acrylic resins, chlorinated or cyclised rubber, epoxy and polyurethane/coal tar combinations, phenolic resins and p.v.c. 

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