Leaching settling tanks

Here it should be pointed out that inspection programs for production, start-up, and current operation are derived from safety analysis. In the following the individual operational steps will be explained and applied to an example. For this a leaching solution settling tank is selected as subsystem of a petrochemical plant. This system will be described first. 

Leaching solution settling tank B-lOO Tank Heater ... 

Figure 4.15 PI flow chart of the leaching solution settling tank for butene, corrected according to results obtained from the preliminary hazard analysis, with element designation for determination of the logic structure.

Operability Study Application to the Leaching Solution Settling Tank. The explanation of the working method is completed by applying it to the leaching solution settling tank. Thus, for example, there is an examination of the feed lines from the tank storage to the butene tank and of the tank itself. 

To sum up, the following actions are indicated with respect to the leaching solution settling tank ... 

The leached solids must be separated from the extract by settling and decantation or by external filters, centrifuges, or thickeners, all of which are treated elsewhere in Sec. 18. The difficulty of solids-extract separation and the fact that a batch stirred tank provides only a single equilibrium stage are its major disadvantages. 

For the treatment of camotite several methods are available. The method recommended by the United States Bureau of Mines2 is as follows The ore is leached with concentrated nitric acid at 100° C., neutralised with caustic soda, and barium chloride and sulphuric acid added to the solution to precipitate the radium as barium-radium sulphate. The precipitate settles in three or four days, after which time the clear liquid is decanted into tanks and is treated with excess of boiling sodium carbonate solution in order to precipitate any iron, aluminium and chromium present. The solution now contains sodium uranyl carbonate and sodium vanadate. It is nearly neutralised with nitric acid, and caustic soda is added in sufficient quantity to precipitate the uranium as sodium uranate. After filtering, the remaining solution is neutralised with nitric acid and ferrous sulphate added, whereupon iron vanadate is thrown down. By this method it is claimed that 90 per cent, of the radium, all the uranium, and 50 per cent, of the vanadium in the camotite are recovered. 

Impeller-agitated tanks. These can be operated as continuous leaching tanks, singly or in a series. If the solids feed is a mixture of particles of different settling velocities and if it is desirable that all particles reside in the leaching tank the same lengths of time, design of a continuous stirred leach tank is difficult and uncertain. 

In the leaching battery of Figure 14.20(a), the solids are transported between vessels with slurry pumps and are mixed in line with countercurrent solution from the next stage. For the process to be effective, the solids must settle freely. The tanks have sloped... 

Overflow from 1 thickener contains 150 to 200 ppm of solids. This is reduced to 50 to 75 ppm by addition of flocculant and settling in a 60-ft (18-m) diameter by 20-ft (6-m) deep reactor-clarifier. Fine slimes from the clarifier are recycled to leach tank 1. 

Any suitable mixer and settler can be chosen for the individual units in a countercurrent leaching system. In those shown in Fig. 17.3 mixing occurs in the feed troughs (known as launders ) leading to each tank and also in the upper parts of the tanks themselves. Rakes B move settled solids to the discharge, and pumps C move the slurry from tank to tank. 

DISPEI ED-SOLID LEACHING. Solids that form impermeable beds, either before or during leaching, are treated by dispersing them in the solvent by mechanical agitation in a tank or flow mixer. The leached residue is then separated from the strong solution by settling or filtration. 

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