IES69659B2 - Improvements in busbars for electrolytic cells - Google Patents

Abstract

In order to reduce the occurence of hotspots in the contact area between the cathode and busbar shunt in an electrolytic cellroom, there is provided an electrolytic cell for use in the chloralkali industry having a contact between the busbar and the cathode comprising a copper connector welded onto the cathode .

Description

Improvements in Busbars for Electrolytic Cells The present invention relates to an improved contact between busbars and cathodes in electrolytic cells for use in the chioral kali industry, which eliminates the formation of hotspots in the contact area.

In the chloralkali industry, 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.

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 anode and in hydroxide ions by the cathode reaction. Hydrogen gas is * S«9 «5 0 - 2 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) --------> Cl“+cr ........> Cl2(g)+2e (2) Cathode Reactions: HgO Electrical Energy H++OH (3) --------> 2e+H++H+ > H2(g) (4) Na++0H —-—> NaOH (5) Overall Reaction: 2NaCl+2H20 ........> Cl2+H2(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 - 12 and sodium hydroxide solution is used to increase the pH. The 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 (6): H2+C12........> 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): Cl 2 + 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% 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 occurs in the area of contact between the stainless steel cathode and the copper busbar which serves to distribute current from cell to cell throughout the cell room. The mild steel cathode is linked to the copper busbar by a stainless steel lug, but the electrical conductivity of stainless steel is not as high as that of copper or mild steel which results in heat generation at the lug. These hotspots had to be sprayed with water to cool them down. They increased resistance throughout the cellroom, which in turn increased the voltage in the cell resulting in reduced electrical efficiency, higher electrical operating costs and production down time. This amounted to a significant loss in profit to the process.

The copper busbar was bolted to the stainless steel lug and the stainless steel cathode was welded to the lug. The busbar and the cathode are made of materials which are not compatible for bolt contact joints, so the problem could not be solved in this way. Use of a heavier copper at the contact points also proved unsuccessful, as did the use of copper compounds as a paste or glue at the contact points.

It was thus an object of the present invention to provide a contact between the busbar and the cathode which eliminated the creation of hotspots.

According to the present invention there is provided having an electrolytic cell for use in the chloralkali industry having a contact between the busbar and the cathode comprising a copper connector welded onto the cathode. The connector suitably takes the form of a plate.

The copper plate is preferably welded onto a lug on the cathode and bolted onto the copper busbar.

The cell is preferably further provided with a laminated copper busbar shunt. The ends of the busbar shunt are suitably press-welded for connection at one end to the anode and at the other end to the copper plate.

The invention also provides a method of welding a copper connector onto a stainless steel cathode comprising forming a mould about the copper connector and the cathode, filling the mould with powdered metal, igniting the powdered metal in the mould and heating to about 1000°C for about 10 minutes to fuse the metal to the cathode and copper connector. Preferably the powdered metal is copper, and the connector is a copper plate.

The invention will now be described in greater detail with reference to the accompanying drawings in which:20 Figure 1 is a schematic diagram of an electrolyser showing the prior art solid busbar connecting the anode of one cell to the cathode of the next cell, Figure 2 is a diagramatic representation of the principles of operation of the electrolytic cell, and Figure 3 is a drawing of a contact between a cathode and a busbar 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 chambers (2) from the cathode chambers (4). Two bulkheads (6) sandwich the electrolyser components together. The anode (2) of one electrolyser is connected to the cathode (4) of the next electrolyser in the cellroom via a busbar (5). As shown in Figure 3 a busbar shunt (7) connects individual electrolyser cells (1) and transfers current between them. The anode (2) is connected to the cathode (8) by means of a flexible, laminated copper busbar shunt (7). The ends (11) of the busbar shunt (7) are press-welded together to form a solid copper element for connection to the anode and cathode. The busbar shunt (7) is bolted at one end onto a copper plate (12) which in turn is welded onto the cathode (8). At the other end the shunt (7) is bolted directly onto the anode (2).

Example In order to connect the cathode (8) to the copper plate (12) a mould is formed about a lug (9) on the end of the cathode, which is external to the electrolyser, and one end of the copper plate (12).

The mould is then filled with powdered copper which is ignited and heated to about 1000°C for about 10 minutes. This fuses the powdered copper in the mould to the cathode and copper plate (12), forming a weld (13) between the two. The free end of the copper plate (12) is then bolted onto the busbar shunt (7).

Introduction of the copper plate (12) as contact between the cathode and busbar has eliminated hotspots in this area. The electrical conductivity is increased giving a lower voltage drop across the cellroom, current efficiency is increased and hotspots are not formed. It is possible to run 50 electrolysers in sequence without any problem from hotspots.

The copper plate (12) has the further advantage that it allows greater freedom in dismantling an electrolytic cell since the busbar shunt (7) can simply be unbolted from the copper plate (12) if one cell is to be taken out of the series of cells in the cellroom.

The provision of a laminated busbar shunt (7) allows air circulation which further reduces the liklihood of hotspots arising.

Claims (5)

1. CLAIMS > * 5

1. An electrolytic cell for use in the chioral kali industry having a contact between the busbar and the cathode comprising a copper connector welded onto the cathode.

2. A cell as claimed in Claim 1 having a laminated copper busbar shunt.

3. A method of welding a copper connector onto a stainless steel cathode comprising forming a mould about the copper connector and cathode, filling the mould with powdered metal, igniting the powdered metal in the mould and heating to about 1000°C for about 10 minutes 15 to fuse the metal to the cathode and connector.

4. A method as claimed in claim 3 wherein the powdered metal is copper. 20

5. An electrolytic cell substantially as described herein with reference to the Examples and/or the accompanying drawings.