Precipitation in Practice

The simplified overall (total, integral) material balance of the batch precipitation states that the mass increase due to the growth of precipitate crystals of the molecular weight M from the initial size Lo to an arbitrary size L, in an arbitrary time t, is equal to the mass of the solute of volume V delivered by the equimolar doublejet whose molar concentration Cr 

Equation (6.53), whose second term on the left side is now constant, is a second-order ordinary differential equation (ODE). In order to solve this ODE for Q i), two initial conditions are necessary. They come in naturally, by realizing that at time t — 0, the size of crystals is L = Lq (seeds or stable nuclei size). The first initial condition is obtained from Eq. (6.52) as 

The solution to the ODE, Eq. (6.53), subject to the initial conditions (6.54) and (6.55), yields the sought-after growth ramp equation 

Equation (6.56) is parabolic in t. For a nonzero initial size it does not reduce to 0 = 0 at t = 0. This is quite understandable, since the initial crystals whose size is Lq are able to accept the growth material, incoming at the volumetric rate equal to that given by Eq. (6.54). The latter equation determines the magnitude of the allowable initial reactant addition rate for the first moment of the growth stage of batch precipitation. 

The significance of Eq. (6.56) lies in the fact that it prescribes the reactant flow rate at any moment of the growth of precipitate crystals, provided that the number and size of the seeds used are known or alternatively, that the number and size of stable nuclei population at the beginning of the growth stage of precipitation are known from the previous experiments. In the latter case, by knowing the size of the final precipitate and the number of moles 

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Theoretically, if the concentration of ammonia in the feed is sufficiently high, the increase in pH of the SulFerox solution may result in iron precipitation. In practice, this is seldom a... 

Under strongly acidic conditions, trithiane or trithioformaldehyde, (CHsSla, is the principal reaction product of foimaldehyde and hydrogen fulfide and is precipitated in practically quantitative yield . Sulfuretted... 

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

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. 

Recovery of Uranium from Leach Solutions. The uranium can be recovered from leach solutions using a variety of approaches including ion exchange (qv), solvent extraction, and chemical precipitation. The most common methods in practice are ion exchange and solvent extraction to purify and concentrate the uranium prior to final product precipitation. 

The actual yield may be obtained from algebraic calculations or trial-and-error calculations when the heat effects in the process and any resultant evaporation are used to correc t the initial assumptions on calculated yield. When calculations are made by hand, it is generally preferable to use the trial-and-error system, since it permits easy adjustments for relatively small deviations found in practice, such as the addition of wash water, or instrument and purge water additions. The following calculations are typical of an evaporative ciy/staUizer precipitating a hydrated salt, if SI units are desired, kilograms = pounds X 0.454 K = (°F 459.7)/I.8. 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

Fe(H20)6] (and, indeed, all other Fe" species in Table A) unstable wrt atmospheric oxidation. In practice the oxidation in acidic solutions is slow and, if the pH is increased, the potential for the Fe "/Fe" couple remains fairly constant until the solution becomes alkaline and hydrous Fe203 (considered here for convenience to be Fe(OH)3) is precipitated. But here the change is dramatic, as explained below. 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

The precipitate must be so insoluble that no appreciable loss occurs when it is collected by filtration. In practice this usually means that the quantity remaining in solution does not exceed the minimum detectable by the ordinary analytical balance, viz. 0.1 mg. 

In practice, aeration towers using coke or volcanic lava tend not to be as efficient as spray ponds in facilitating the precipitation of ferrous hydroxide consequently, there is usually a requirement for a cationic flocculant to aid the precipitation of the insoluble materials into a larger floe or denser sludge that can be removed by clarification or sand filtration. 

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