Flow rate In m hr

Figure 9.6 Z-nu, the layer not used is plotted versus the flow rate. Z-nu is independent of the height of the column, but as shown on the figure dependent upon the flow rate.

Clinoptilolite is available in different purities, dependent on the geological formation of this clay mineral. The clinoptilolite from California has a purity of 8595%, while a Hungarian type from Tokaj has a purity of only 60-70%. The capacity is roughly proportional to the purity.

Investigations of the latter type of clinoptilolite, see Jorgensen et al (1975) and (1978), have demonstrated that a treatment of the clay mineral by sodium hydroxide or sodium carbonate, before use, will improve the uptake of ammonium. The results are expressed by use of the following equation:

where C is the equilibrium concentration of ammonium ions in solution, meqv /I, C0 is the total initial concentration of ammonium ions in solution, meqv /I, Q is the ammonium ions taken up by clinoptilolite meqv /g, Q0 is the total ion exchange capacity of the sorbent, meqv/g, while K and n are characteristic constants, which can be found by use of a logarithmic plot of equilibria data. The equation can be used for all types of waste waters and clinoptilolites. In each case equilibria data must be used to find K, n and Q0.

Figure 9.7. The selectivity coefficient a-50% of ammonium competing with potassium, sodium, calcium or magnesium versus concentration ratio of competing ion / ammonium. Note that the graph is double logarithmic.

Concentration ratio competing ion: ammonium

Figure 9.7. The selectivity coefficient a-50% of ammonium competing with potassium, sodium, calcium or magnesium versus concentration ratio of competing ion / ammonium. Note that the graph is double logarithmic.

Table 9.2 gives the results of investigations of treated (with sodium hydroxide or sodium carbonate) and untreated Hungarian clinoptilolite, using ammonium solutions in distilled water to find the equilibrium data.

The equilibrium curve, resulting from equation (9.7) can be used directly in the design of ion exchange columns as presented in Section 9.4. Note that the untreated clinoptilolite gives an equilibrium curve quite different from the treated one and that the treated clinoptilolite will give a far better uptake of ammonium; see Table 9.2.

The regeneration of the ion exchange material is carried out either by sodium or calcium ions by passing the regenerant through the clinoptilolite in the opposite flow direction of the normal service cycle. Lime-slurry was used for the first studies of this process. It was, however, found that elution with lime could be speeded up by the addition of sufficient sodium chloride (0.1 M). Ammonium ions are converted to ammonia, so it can readily be removed from the regenerant, and the volume of regenerant required for complete regeneration decreases, with increasing pH of the regeneration liquid. Precipitation of calcium carbonate and magnesium hydroxide occurs, however, at high pH, which leads to clogging of the exchanger inlets and outlets.

0 0

Post a comment