IES960071A2 - Improvements in cathodes - Google Patents

Abstract

The invention relates to an electrolytic cell for use in for use in the chloralkali industry having a cathode comprising stainless steel having a low carbon content of between 0.028 and 0.032% by weight <Fig.1>.

Description

The invention relates to an electrolytic cell for use in for use in the chloralkali industry having a cathode comprising stainless steel having a low carbon content of between 0.028 and 0.032% by weight .

The present invention relates to an electrolysis process for the manufacture of hydrochloric acid, caustic soda liquor and sodium hypochlorite. In particular the invention relates to the use of an improved cathode, having a low carbon content, in the electrolysis process, in order to reduce erosion of the cathode.

The electrolysis process uses salt and water as starting materials to produce hydrochloric acid, caustic soda liquor and sodium hypochlorite. Salt is stored on a storage pad. From the pad the salt is transferred by loader to a pit saturator. Softened water and recycled spent brine are also introduced into the saturator and by drawing this liquid through the bed of salt, a saturated brine solution is obtained. The solution is then filtered and the total hardness is reduced from approximately 5 ppm to less than 20 ppb by ion exchange. The resin used in the ion exchange columns is a specialised chelating resin and requires regenerating every 10 days approximately. The softened brine enters storage from where it can then be fed to the process. k) CM The electrolysis cells require DC current for operation and the 10 KV supply from the grid is passed through two sets of rectifiertransformers to be converted from AC to DC output.

The electrolysis cells used in the process are monopolar with two cells per electrolyser. An electrolyser consists of a titanium anode chamber with two mesh screens, two cathode chambers with stainless steel plates, two membranes separating anode screen from cathode plates and two bulkheads to sandwich the electrolyser components together.

The electrolysers are arranged in modules of five and connected in series electrically so that the same load flows through each.

Saturated brine is pumped into the anode compartment where the sodium chloride is depleted in chloride ions by the anode reaction and in sodium ions by transport through the membrane. Chlorine gas is generated and flows from the anode with the depleted brine. To the cathode a diluted sodium hydroxide solution is circulated where it is enriched both in sodium ions which pass through the membrane from the 96007 1 - 2 anode and in hydroxide ions by the cathode reaction. Hydrogen gas is generated and leaves the cathode chamber with the enriched sodium hydroxide solution.

The reactions which take place are represented by the following equations:Anode Reactions: NaCl Electrical Energy Na++Cl~ (1) --------> cr+cr -......> Cl2(g)+2e (2) Cathode Reactions: H20 Electrical Energy H++0H (3) --------> 2e+H++H+ > H2(g) (4) Na++0H > NaOH (5) Overall Reaction: 2NaCl + 2H20 > Cl2 + H£ (g) + 2 NaOH (6) The enriched sodium hydroxide solution is collected in a caustic circulation tank from where part is pumped directly to storage and part is diluted and recirculated back to the electrolysers. The spent brine which contains approximately 200 gpl sodium chloride is saturated with dissolved chlorine gas and undergoes dechlorination in a three stage process. The first stage involves pH reduction using hydrochloric acid. The solubility of the chlorine gas is greatly reduced at low pH values and most of the chlorine 'gasses off' from the solution. The second stage involves scrubbing the low pH solution with air to further reduce the levels of chlorine remaining down to less than 10 ppm.

Final dechlorination is achieved through chemical destruction using sodium sulphite. This stage is performed at pH values in the range 10 - 12 and sodium hydroxide solution is used to increase the pH. The L960071 chlorine-free solution is returned to the pit saturator for re-saturation.

It can be seen from Fig. 2 that a certain amount of hydroxide passes through the membrane from the cathode chamber to the anode.

This results in the formation of an impurity, sodium chlorate, which builds up in the brine loop. To prevent this, a small quantity of spent brine must be purged to drain, the purge rate being dependent on the rate of increase of sodium chlorate in the system. Factors affecting this include membrane condition (rips, blisters and holes), operating load and initial feed brine concentration. This purge also acts as a sulphate purge which builds up in the system through the addition of sodium sulphite.

The hydrogen gas produced in electrolysis is cooled and piped directly to the HC1 synthesis furnace. The chlorine gas is piped to both the HC1 synthesis furnace and the sodium hypochlorite system.

In the manufacture of hydrochloric acid, chlorine gas is burned in hydrogen gas to form hydrogen chloride gas according to the reaction > 2 HC1 (6) The gas is then scrubbed out in de-ionised water to form hydrochloric acid. The process takes place in a graphite lined furnace which is jacketed and cooled to remove the heat of reaction. The system, for safety reasons and product quality, must be operated with excess hydrogen. As both gases are produced in the same molecular ratio, chlorine is fed continuously to the sodium hypochlorite reactor where it is reacted with sodium hydroxide according to equation (7): Cl2 + 2 NaOH NaOCl + NaCl + Η£0 (7) Sodium hypochlorite is produced batch-wise, initially with a charge of approximately 20% NaOH solution and reacting this with chlorine down to a residual sodium hydroxide content of between 0.3 and 1.0% and available chlorine content of between 14% and 15%. It is of utmost importance that the residual sodium hydroxide is kept above 0.3% ··0071 - 4 NaOH as the solution becomes highly unstable at levels below this and may even result in the release of chlorine gas.

By taking chlorine gas continuously to sodium hypochlorite manufacture, an excess of hydrogen is obtained at HC1 synthesis. The amount of chlorine to sodium hypochlorite can be varied depending on requirements for either sodium hypochlorite or hydrochloric acid. Each of the three products is pumped to dedicated storage tanks from where they can then be filled into bulk road tankers, IBC's, barrels or carboys.

A significant problem with the process was that the cathodes of the electrolysis cells became eroded rapidly. Initially the discharge nozzles for output of caustic and hydrogen gas started to leak and new nozzles were welded in place. Then the corner welds of the cathode started to leak and had to be rewelded. Eventually, it became apparent that the box frame had been eroded from 4 mm thick to paper thin and these box sections had to be replaced. Since the cathode is of a louvered design in order to promote circulation of the electrolytic solution in the cell, as the carbon coating on the cathode became worn off, the carbon blocked the louvers and reduced circulation. The erosion became so bad that leaks appeared in the cells which resulted in caustic solution at a temperature of 90°C spraying from the cell. Such leaks had to be repaired by welding, necessitating downtime in the cel 1.

The installation of a nickel sacrifical bushing to prevent destruction of the nipple and cathode frame, as recommended by the equipment supplier, proved unsuccessful in preventing erosion.

Accordingly, it is an object of the present invention to provide an electrolytic cell having a cathode which would not erode as easily as the prior art cathodes. Prior art cathodes used in the chloralkali industry are either mild steel cathodes, which have a high carbon content or nickel cathodes, nickel being very expensive. It is therefore a further object of the present invention to provide a replacement cathode having a low cost. It was a further object of the invention to provide an electrolytic cell in which no part in contact with caustic solution would erode easily. 990071 - 5 According to the present invention there is provided an electrolytic cell for the production of hydrochloric acid, caustic soda and sodium hypochlorite, having a cathode comprising stainless steel with a low carbon content of 0.028% to 0.032% by weight, preferably 0.03% by weight. Preferably this stainless steel has the following composition: Composition (maxima) % by weight Carbon Si 1 icon Manganese Phosporous Sulpher Chromium Nickel 0.03 .00 .00 0.045 0.030 17.00-19.00 9.00-12.00 The stainless steel may suitably be grade 304L available from Amari Ireland Ltd. or The Steel Company of Ireland.

The electrolytic cell may further comprise a faceplate on the bulkhead comprising stainless steel with a low carbon content as defined above. Preferably the faceplate is provided on all surfaces of the bulkhead which come in contact with the caustic solution.

The invention will now be described in greater detail with reference to the accompanying drawings in which:Figure 1 is a schematic diagram of an electrolyser, Figure 2 is a diagramatic representation of the principles of operation of the electrolytic cell, and Figure 3 is a drawing of a cathode frame assembly in accordance with the present invention.

Figure 1 shows a schematic diagram of an electrolyser (1). The electrolyser (1) consists of an anode chamber (2) with two mesh screens (3) and two cathode chambers (4). Two membranes (not shown) separate the anodes (2) from the cathodes (4). Two bulkheads (6) sandwich the electrolyser components together. The anode (2) of one electrolyser is - 6 connected to the cathode (4) of the next electrolyser in the cell room via a busbar (5). As shown in Figure 3, the cathode (4) comprises a plurality of louvers (7). The cathode chamber (4) is also provided with a feed inlet (8) and a drain (9) and two discharge ports (10) for the discharge of hydrogen gas and enriched sodium hydroxide solution.

Example The original electrolytic cells comprising mild steel had an average life-span of three years with substantial erosion being apparent at the end of that time.

Prototype cells, using 304L stainless steel as the cathode and as a faceplate on the mild steel bulkhead were introduced. The faceplate served to convert the mild steel bulkhead into a low carbon content steel bulkhead i.e. all parts of the cell in contact with the caustic solution were now made of low carbon content steel. No evidence of erosion was detectable in these cells after 2-3 years in operation.

Claims (4)

1. An electrolytic cell for use in the chloralkali industry having a cathode comprising stainless steel having a low carbon content of between 0.028 and 0.032% by weight.

2. An electrolytic cell as claimed in Claim 1 further comprising a faceplate on the bulkhead comprising stainless steel having a low carbon content of 0.028 to 0.032% by weight.

3. An electrolytic cell as claimed in Claim 1 or Claim 2 wherein the stainless steel has a carbon content of 0.03% by weight.

4. An electrolytic cell as claimed in any preceding claim wherein the stainless steel has the composition