The amateur will be using (probably) a non pH controlled cell, this is OK. All industrial setup's use pH control.
The amateur will probably use what is to hand which may be a cheap power supply, cheap Gouging rods etc. The cell container can come in the form of a bucket with a lid.
The plastic that the bucket is made from must be able to withstand the cell electrolytes and fumes. A PVC bucket is ideal. Some holes are drilled in the lid to accommodate the electrodes and a vent tube.
The vent tube is useful as it allows gasses to be vented out of the building in a controlled manner. A hole about 5cm in diameter is also made in the lid for looking into the cell and also for adding water or NaCl solution as required. A piece of plastic is needed for to cover this hole and stop fumes and mist from getting out of the cell onto connections etc. A stainless steel or even mild steel bucket can be used. This can then be used as the Cathode thus doing two jobs at the same time. Remove solution from container when current is stopped or it will corrode.
The electrodes should be sealed into the lid because if fumes and mist are getting on connections you will have problems trying to maintain them. Also gases coming to the surface of the solution generate
a mist of the solution which
will drift out of the cell. This mist will rust everything in the vicinity of the cell and beyond!.
The design parameter that always raises it's head is the Anode current density. This parameter is important because you must keep below a certain Anode current density to avoid eroding the Anode. This is particularly true with Graphite. Anode current density is the Anode current (cell current) divided by the total surface area of the Anode which is in the electrolyte. It is usually quoted in Amps per square cm. When you have decided what the Anode surface area will be and what the current density on the Anode will be, you will know what the maximum current through your cell can be. You do not have to run this maximum, you can run less current into the cell if you so wish.
Another design parameter that gets discussed is the ratio of the size of the cell (the liquid volume) to the current going into the cell. This in not a critical parameter. It has little effect on current efficiency. When the current going into the cell is raised (if you add more Anodes say and adjust power supply) and the cell volume is kept constant in order to shorten the run time of the cell, there will come a point where the heat generated by the process will not be able to escape fast enough from the cell and the temperature of the cell will be too high. If you must run a large current into a small volume of electrolyte you can keep the cell cool by putting it into a large container of water. Industrial setup's uses cells that are small in relation to the current going into them. They use cooling coils in the cells to keep the cell cool and transport heat away. This keeps the real estate used by the cells small. The amateurs cells can be large in comparison to the current going into them. The only disadvantage in doing this is that the run times of the cells can be long. This can leave the impatient amateur feeling as if nothing is happening at all as he is forced to wait perhaps two weeks before he can take out a crop of Chlorate. The cells WILL still be making Chlorate at the same rate as a smaller cell with the same current going into it.
The amateur very often comes up with some weird and wonderful arrangement of electrodes for to stop Chlorine gas escaping from the cell. It should be noted in industry that most cells use a very simple arrangement of parallel plates in a vertical arrangement. With the amateur cell the pH is not controlled. It is impossible for Chlorine gas to escape out of a cell which has a high pH (amateur cell). There will be a smell of Chlorine at the start but once the pH rises no Chlorine of any significance will come out of the cell. Add NaOH or KOH at start if there is a problem regarding smell of Cl2.
The temperature of the cell (IMO) is of little importance in non pH controlled set up. All Chlorate is made by electricity (not by species meeting in the bulk of the electrolyte as you have in pH controlled setup's) with little or no bulk chemical reactions going on. Temperature should be kept at a level so that Anodes are not damaged/eroded. Very low temperatures may cause Na or K Chloride to come out of solution.
The power supply for to supply the cell with this current will ideally be a controlled
current source. With a controlled current source you set the power supply to put a certain current
into the cell and the voltage across the cell will vary as the cell resistance varies.
Most power supplies for sale or to hand are NOT controlled current sources. They are
controlled voltage sources (either fixed or variable).
This means that when you connect the supply to the cell the voltage
across the cell in rock steady at whatever the voltage of the supply is set at (assuming the supply is not being abused) and the current put into the
cell is dictated by that (rock steady) voltage and the resistance of the cell.
If the supply has a variable voltage
output then the current can be varied by lowering or raising the voltage of the supply.
When the power supply is a fixed voltage
output (you cannot vary the Voltage with a knob. An example is a computer power supply) then the only way you can vary the current is by
manipulating the electrodes in the cell or you could add a resistor or a number of diodes in series with the line going into the
cell to lower the current. With a computer supply (5 Volts output) the current going into the cell will
probably be within acceptable limits. If you have a fixed 10 or 12 volt supply then the current will
probably be excessive and you will need a resistor or diodes to limit current. Diodes work by dropping 0.9 Volts across themselves, they effectively lower the power supply voltage seen by the cell by 0.9 Volts. Another trick with a 12 volt supply, is to put two cells in
series so that less current will flow into each cell.
12 Volt battery chargers can make an acceptable supply as they tend to have controlled current characteristics.
They are a half way house between a controlled current source and a controlled voltage source.
You will need a current meter in the line going into your cell (or in the power supply itself) in order
to measure current. Voltage across cell can also be noted with a meter if disired.
Do not get obsessed with the voltage across the cell. The voltage across the cell will have a value no doubt. Some folks are inclined to latch onto the voltage appearing across the cell as if it were some very important, almost magical, parameter. Some times you will hear a statement like: "I ran my cell at 3.6 volts, therefore I was making Chlorate. I increased the voltage to 7.4 volts and I am now making Perchlorate". It is NOT the Voltage across the cell that decides if you are making Chlorate or Perchlorate. If the Voltage is too low not enough current will get pumped into the cell. This will give you slow production. If you have a bad connection which causes a large Voltage drop this will lower current going into the cell too. Think in terms of Anode current density.
![[pH controlled cell and crystallizer]](phcell.gif)
A one compartment system can be used also. It should be fairly large in relation to Anode size so that bulk reactions can take place in order to obtain maximum benefit from pH control.
Example 1
The designer has decided that he wants to make 1Kg of Sodium Chlorate per week. he is not too strapped for cash.
He has no Anode(s) or no power supply yet. By looking at the run times section for Sodium Chlorate
on this page the following fact is noted.
That’s the same as 60.0768 grams formed per week per amp (There are 10,080 minutes in a week)
Therefore to make 1000 Grams per week you will need to run 1000/60.0768 = 16.64 Amps (say 17).
Looking at the max. allowable current densities on the different Anode materials will give figures
similar(ish) to theses: Platinum 300mA/square cm, Graphite 30mA/square cm, Lead Dioxide 300mA/square cm,
and Magnetite 30mA/square cm. He may like to run his Pt, LD, or Magnetite Anodes at a higher current density as the figures above are fairly conservative.
He will not want to run his Graphite Anode at a higher current density so as to keep erosion at bay.
Having decided what Anode material he is going to use he then sets about deciding how much surface area
he needs on the Anode. Assuming Graphite, he will need 17/.03 = 566 square cm Min. That’s the Anode surface
area that is immersed in the electrolyte remember. He then decides if he is going to use Gouging rods,
EDM Graphite or some other Graphite. He may wish to use the Graphite at a much lower Anode current density to keep Anode erosion ever lower. If he wishes to halve current density, he will do so my simply doubling the Anode surface area. If using rod shaped electrodes be advised that as the rod wears it will have a smaller surface area, the current density will rise (assuming current into cell remains the same) and wear rate will increase. It may be advisable to start with a low current density so that as the Anode wears the current density will not go above 30mA per square cm. If using flat pieces of Graphite the Anode wear will not have much effect on the surface area of Anode.
In order to make 1Kg of Chlorate you need exactly (1000 X 0.55) grams of NaCl. The 0.55 comes from the division of the molecular weights. The electrolytic cell and the
Anode will not operate under such clear cut conditions. You will need at least 100 grams per litre NaCl
to be left in the cell after your run so that the conditions for Graphite Anode erosion are met. If you try
to use up all the Chloride the Anode will get eaten away rapidly coming toward the end of the run.
So: We need to use at least 550 grams of salt for to be converted to Chlorate + 100 grams per litre to be left in the cell at the end of the run. He will need a container that will hold approx. 3 litres of solution. This is a minimum size. If you run 17 amps into this 3 litre container you will have problems keeping it cool. Better to just use a container that is much much bigger than this. About 25 litres would be great filled with saturated NaCl solution. This will stay cool. About 330 grams NaCl will dissolve in 1 litre of water. At 17 amps you will still get the
same amount of product forming (assuming similar current efficiency which is probable). It may not suit the impatient amateur as he will not be able to draw product from this larger container until a few weeks have passed. He may add Chromate, Persulphate or NaF to increase current efficiency. He will add water to the cell to keep volume steady. He may add salt solution. If he adds salt solution he needs to factor this into the run time.
Firstly test supply with a resistor to see if it is working. We have a power supply that puts out a fixed voltage of 5 volts DC and a few other voltage outputs (12V and others). We use the 5 volt output which is the main output. The supply also has a maximum current output on this 5 volt output which is hopefully stated on the supply. The maximum current that this line can supply is the maximum current you can put into you cell without risking damage to the supply. Calculate the Anode surface area from this maximum available current. Say the maximum current is 10 amps this means that you need at least an Anode surface area in the electrolyte of 10/.03 = 334 square cm. (This comes from the fact that you want to keep the Anode current density below 30mA per square cm to keep Graphite erosion at bay). How many Gouging rods do you need to give you this surface area? The area of a rod is (ignoring the small circle at the bottom of the rod) Pi X Dia. X h (remember h is the length of rod actually in the electrolyte). Use at least this amount of rods. It would be great to use twice as many to keep Anode current density real low and therefore keep erosion low. This is especially true as the Gouging rods wear, their surface area will decrease, current density will increase and erosion will increase.
Use a container that is about 10 litres or larger in size filled with saturated NaCl solution. About 330 grams NaCl will dissolve in 1 litre of water. Use a 20 (or a 30!!) litre container if you like. Calculate the run time and keep in mind that you must stop running the cell when you have got to the stage where you have 100 grams per litre Chloride so as to keep erosion on the Graphite to a sensible limit. He may add Chromate, Persulphate or NaF to increase current efficiency>
The amount of the Anode that will actually be in the electrolyte is used to calculate the surface area of the Anode. A current density of 300mA per square cm is easily accommodated with a Lead Dioxide Anode. He may wish to run his Anode at a smaller current if he so wishes but since he is impatient for product he may decide to run the Anode at a higher current density. Assuming the area of the Anode is 70 square cm, this will allow a current of 70 X 0.300 = 21 Amps to be run though cell.
The cell volume will be kept small so that Chlorate can be extracted as soon as possible. Looking at run times described elsewhere on this page he sees that with 21 amps running into the cell will give him about 21 X 0.00596 = 0.12516 grams Chlorate formed per minute, which is 180 grams per day. He will set up his cell so that he can extract Chlorate after two days, ie. 360 grams theoretical. In order to make 360 grams Chlorate, 360 X 0.55 = 198 grams Chloride are needed. Since Lead Dioxide does not get eroded by low Chloride concentration there is no need to have a quantity of 'buffer' Chloride in the cell as far as Anode erosion is concerned. It will be impossible to convert all of the 198 grams of salt into 360 grams Chlorate as run times are (as described elsewhere) based on having a decent amount of Chloride in the cell at the end of each run. He will use 270 grams NaCl dissolved in somewhat less than one litre of water. Also if he uses too little Chloride he may get Perchlorate forming if the concentration of Chloride falls below (approx.) 10 grams per litre. He may add NaF or Persulphate but not Chromate to increase current efficiency.
This is entirely up to himself. He will look at the price of the wire and weep. If he wants to run the set-up at the max. current that his supply can give (looking at typical current densities used for Pt. of 300mA per square cm) he will need 3 amps/0.300 amps = 10 square cm of Platinum in the electrolyte. He will purchase Pt wire with a sensible diameter which will carry 3 amps. If he purchases Pt. of (say) A.W.G.(B & S) = 42 (that’s S.W.G. between 45 & 46) this will have a diameter of 2.5 thousands of an inch. This measly wire will not carry 3 amps. It will melt. He will purchase wire of approx. 25 A.W.G (26-27 S.W.G) which will carry
3 amps. This wire is still thin, around 19 A.W.G. would be nice so that the Anode will be self supporting.
The surface area of a cylinder is D X Pie X L. Assuming we use 19 A.W.G. (D = 0.9119mm) we need a length of wire which is
10/(3.142 X 0.09119) = 35 cm. He will need another inch to come up out of the cell, say 38 cm total length. He will bend the wire into a shape for to use as the Anode. Some like to wind the wire onto a former (say a glass tube). Winding the wire onto a tube will have some consequences for current distribution on the Anode. The Anode will have a very small current density on the wire next to the tube, it will have a larger current density on the side of the wire away from the tube. Keep this in mind. It would be good to wind the wire in a spiral of about 2 inches Dia. that is self supporting.
The price of this Anode will not be small. The smaller the Dia. of the wire you use the less Dollars you need to spend to get a certain surface area. See the section on the Pt. Anode. You could buy half the length suggested and squeeze (run through a mandrill) or hammer the wire so that it's surface area is doubled (or more) thus saving Dollars.
If he uses a Platinum clad Anode all his current carrying problems will end as the substrate of the Anode will carry 3 amps easily. He still needs 10 cm squared of Anode area in the electrolyte.
Since the Anode is so expensive he may decide to purchase less surface area and run the Anode at a higher current density, still running 3 amps into cell. Pt. will take quite a high current density without eroding. He could also purchase light wire that will not carry 3 amps but will carry (say) 0.5 amps. He will cut this wire into 6 equal lengths and have 6 Anodes coming out of the cell connected to a bus bar (like a comb arrangement). Anode surface area in the electrolyte must still be 10 square cm to give similar erosion conditions to the single Anode. The 6 Anodes will save cash and give him more work.
Looking at the run time section elsewhere on this page it is noted that 3 amps will give you (0.00596 X 3) = 0.01788 grams Chlorate formed per minute = 180.25 grams per week (current efficiency about 50%). This will use 180.25 X 0.55 = 100 grams Chloride per week. The 0.55 comes from division of the molecular weights of NaCl and Na Chlorate. Assuming that he wants to harvest Chlorate once per week he will need 100 grams Chloride in solution + (about) 100 grams Chloride per litre left at the end of the run so that erosion of the Anode is kept at bay and to keep current efficiency at a sensible level. Platinum will erode somewhat if used in a cell that has a low Chloride concentration in it. About 330 grams NaCl will dissolve in 1 litre of water. He will dissolve approx. 145 grams NaCl in about 0.45 litres water. After a week approx. 40 grams of Chloride will be left per 100ml solution. He will stop his cell at that point so that erosion of the Anode is kept at bay. He will add NaF, Chromate or Persulphate to increase current efficiency. He may decide to top up the cell with water or salt solution. If he used salt solution he must factor this into the run time.
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