Hypochlorite recycling

Mr M Beekman Akzo Nobel Chemicals, Oosterhorn 4, PO Box 124, 9930 AC Delfzijl, Netherlands. Hypochlorite Recycle to Diaphragm Cells. E-mail M.Beekman AkzoNobel.com... 

Start-up of the hypochlorite recycling process was successful in July 1999 and has been working very well since. The procedure has proved that hypochlorite recycling is indeed an excellent way to eliminate completely chlorate and bromate emissions from the hypochlorite destruction unit. 

An additional advantage of the hypochlorite recycling process is the chlorination of the feed brine in the brine-degassing unit. Organic and nitrogen-containing components are oxidised. The reaction products are removed via the vent-gas to the chlorine destruction unit. Less NCI3 is formed in the electrolysis cells because part of the... 

Recycling the hypochlorite to the feed brine has provided an excellent possibility of eliminating completely the chlorate and bromate emissions of the chlorine destruction unit of a diaphragm electrolysis plant. The main advantage of the hypochlorite recycling and cathodic reduction procedure is the reduction of bromate to bromide. 

To maintain the acidity of the brine at pH5, more hydrochloric acid is required during hypochlorite recycling to the feed brine. This extra acid demand is the cause of the largest increase of variable production costs - approximately 100 000 Dutch guilders per year. Alternative solutions showed variable costs up to one million Dutch guilders per year. The investment for this project proved to be the best economical alternative to solve the chlorate and bromate emissions problem. 

The disadvantage of the hypochlorite recycling process is the small increase of chlorate and bromide concentrations in the cell-liquor. However, this is offset by higher chlorine production (0.1% more), resulting in a higher current efficiency. 

The reactor effluent, containing 1—2% hydrazine, ammonia, sodium chloride, and water, is preheated and sent to the ammonia recovery system, which consists of two columns. In the first column, ammonia goes overhead under pressure and recycles to the anhydrous ammonia storage tank. In the second column, some water and final traces of ammonia are removed overhead. The bottoms from this column, consisting of water, sodium chloride, and hydrazine, are sent to an evaporating crystallizer where sodium chloride (and the slight excess of sodium hydroxide) is removed from the system as a soHd. Vapors from the crystallizer flow to the hydrate column where water is removed overhead. The bottom stream from this column is close to the hydrazine—water azeotrope composition. Standard materials of constmction may be used for handling chlorine, caustic, and sodium hypochlorite. For all surfaces in contact with hydrazine, however, the preferred material of constmction is 304 L stainless steel. 

In the second proposed alternative process, tert-huty hypochlorite, formed from the reaction of chlorine and tert-huty alcohol, reacts with propylene and water to produce the chlorohydrin. The alcohol is a coproduct and is recycled to generate the hypochlorite (114—116). No commercialisation of the hypochlorous acid and tert-huty hypochlorite processes for chlorohydrin production is known. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

In another cocrystalHzation process, lime is mixed with 50% caustic and recycled filtrate and chlorinated to yield a slurry of calcium hypochlorite dihydrate and NaCl crystals that are separated in a hydrauHc classifier. The underflow is mixed with centrate mother Hquor and sent to a wet screen classifier the overflow is recycled to the hydroclone and the salt-rich bottoms are centrifuged. The centrate is recycled to the chlorinator and the salt used as feed to chloralkaH ceUs. The Ca(OCl)2-rich overheads from the hydroclone are centrifuged, the cake going to a dryer and the filtrate sent to the wet screen classifier (207). 

Thann A process for making crystalline calcium hypochlorite by passing chlorine into an aqueous slurry of calcium hydroxide. There are several such processes in this one, some of the filtrate is recycled in order to produce larger crystals. Invented by J. Ourisson in France in 1936. 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

There are several crucial steps in the process of recycling hypochlorite solution. First, hypochlorite is formed in the chlorine destruction where chlorine reacts with the sodium hydroxide solution. This solution is added to the brine-degassing unit. Partial conversion to chlorate and bromate takes place, which continues in the anolyte... 

After extensive research and several tests, the option selected was recycling hypochlorite to the feed brine of the electrolysis cells. For this purpose, hypochlorite feed pipes were manufactured and the hydrochloric acid feed capacity to the brine degassing tanks was enlarged. 

There are no longer any chlorate and bromate emissions from the chlorine and hypochlorite destruction units. All hypochlorite is recycled into the feed brine. This process has been operating efficiently since July 1999. 

Fig. 14.4 Sodium chlorate concentration in cell-liquor before and after recycling of hypochlorite.

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

A scrubber has a closed-loop recycle of caustic. The make-up caustic solution is added continuously on an ORP (oxidation-reduction potential) controller. The caustic concentration will typically be 20 wt% and thus the blow-down stream will contain approximately 15 wt% hypochlorite. Such a system will typically be provided with a large reservoir of caustic that is released on a once-through basis for containment of emergency relief streams or operated with a permanent excess of caustic, resulting in a hypochlorite concentration in the blow-down stream in the range of 6-12 wt%. 

As noted above, batch and semi-batch-based operations result in periodic high loads and subsequent over-design and increased capital cost. By destroying the hypochlorite in situ, within the scrubber recycle loop, the end of cycle concentration can be reduced and the load on the end-of-pipe hypochlorite destruction system lowered allowing an overall cost reduction. The reduced free chlorine concentration also leads to improved process safety, although increased heat removal is required. 

The process concept is shown in Fig. 26.7 where the recycle loop of the caustic scrubbing liquor passes through a fixed-bed reactor and then through the normal cooler. The blow-down of spent caustic and make-up with fresh caustic can be carried on in the same fashion as without the in-loop hypochlorite decomposition. Consideration of the optimum locations for removal and addition may, however, be slightly different. 

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