Strong base resin

Weak and strong acid-type resins are for removal of cations and are called cation exchangers. Weak and strong base resins remove anions and are called anion exchangers. In addition to these four resin types, there are specialty resins used in appHcations where higher specificity for certain ions under challenging conditions is a critical factor. 

Weak Base. Weak base anion-exchange resins may have primary, secondary, or tertiary amines as the functional group. The tertiary amine -N(CH2)2 is most common. Weak base resins are frequentiy preferred over strong base resins for removal of strong acids in order to take advantage of the greater ease in regeneration. 

This interchangeof ions is similar to that of the strong base resins. 

Like strong acid resins, strong base resins are highly ionized and can be used over the entire pH range. These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water ... 

In an ion exchange wastewater deionization unit, the wastewater would pass first through a bed of strong acid resin. Replacement of the metal cations (Ni. Cu ) With hydrogen ions would lower the solution pH. The anions (S04. Cl ) can then be removed with a weak base resin because the entering wastewater will normally be acidic and weak base resins sorb acids. Weak base resins are preferred over strong base resins because they require less regenerant chemical. A reaction between the resin in the free base form and HCl would proceed as follows ... 

Variations on this procedure are set out in Table IV. It can be seen that adding excess reagents does not Improve the capacity obtained, but that sequential addition is of some benefit (M3, M4). Strong base resins cannot be prepared by this route since tertiary amines do not react (M5), and a very weak secondary amine also gives very little reaction (M6). 

Strong- and weak-base resins exchange anions with their aqueous environment and therefore extract only metals that form anionic complexes in solution. Strong- and weak-base resins display a similar affinity for anionic species, which increases with the charge and the polarizability of the anion. Strong-base resins have quaternary amine functional groups that possess a permanent... 

Anion-exchange resins are eluted either by reversal of the equilibria shown in equations (94) to (96) or, in some cases, by chemical destruction of the anionic complex ML/-. The elution of strong-base resins requires a large excess of the co-ion, X-, whereas the elution of weak-base resins can be achieved most effectively by treatment of the resin with a stoichiometric amount of hydroxide ions, which restores the resin to the free-base form ... 

Figure 13 The distribution of various ions between aqueous hydrochloric acid solution and a strong-base resin, Dowcx 1 X-10, as a function of hydrochloric acid concentration (after Marcus and Kertes, ref. 132)...

Far less selectivity can be achieved in cyanide solution because many metals, both precious and base, form anionic complexes in even very dilute cyanide solution. This is illustrated by the equilibrium absorption isotherms for various metal ions in a leach liquor from a gold cyanidation plant on a strong-base resin (Figure 14).358... 

Figure 14 The equilibrium loading of various metal cyanide complexes from a gold plant pregnant solution onto a strong-base resin (A101DU) (after Fleming and Cromberge, ref. 358)...

The affinity of a strong-base resin for various anions commonly present in uranium process solutions can be ranked as follows U07(S04)34- > U02(S04)22- > NO, > Cl- > HS04 > Fe(S04)2- > SO4-. 

In the 1950s, uranyl sulfate was stripped from the strong-base resins with a concentrated solution of chloride or nitrate, these being the two most efficient anions in the above sequence. Uranium was recovered from the strip solution by precipitation of the diuranate species with ammonia ... 

Weak-base resins have been tested from time to time but have not found wide acceptance in the uranium industry, The main reason for this is that the major advantage of weak- over strong-base resins, viz. elution by neutralization, cannot be utilized in uranium processing since it is not possible for the weak-base resin to be converted to the free-base form without diuranate precipitating within the pores of the resin (unless a complexing agent such as carbonate is added to the eluate). In the presence of carbonate, uranium remains in solution as the uranyl carbonate anion, even in very alkaline solution, so is readily eluted from a weak-base resin in the free-base form. This eluate would then be treated as depicted in equations (105) and (106) for the recovery of uranium. Alternatively, weak-base resins can be eluted by ion-exchange mass action. 

The removal of thiocyanate from a strong-base resin can be achieved effectively by treatment with an iron(III) salt.366 In the presence of a slight excess of Fe3+ ions, thiocyanate forms the cationic complexes FeSCN2+ and Fe(SCN)2+ ... 

Thiocyanate can be recovered and recycled from this solution by the precipitation of iron as iron(III) hydroxide.366 Aurocyanide can also be eluted from a strong-base resin by chemical conversion of the gold to a cationic complex with the thiourea ligand, as shown above (equation 98). This method of gold elution is favoured in the Soviet Union, but suffers from the drawback that elution of the other metal cyanide complexes is generally poor, and multi-elution procedures are necessary. 

Strong Base Anion Exchangers. As in the synthesis of weak base anion exchangers, strong base resins are manufactured from styrenic as well as acrylic copolymers. Those based on copolymers of styrene and divinylbenzene are chloromethylated and then aminated. These reactions are the same as for the styrenic weakbase resins. The essential difference is the amine used for amination. Trimethylamine [75-50-3], N(CH3)3, and A/,A/-dimethylethanolamine [108-01-0], (CH3)2NCH2CH2OH, are most commonly used. Both form quaternary ammonium functional groups similar to (8). 

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