Aluminum smelter startup

How is a Hall-Herault aluminum smelting cell started up and shut down?
All the descriptions and diagrams I've seen show it in
steady state operation, with no hint how the electrolyte is melted initially, nor how it's tapped off before cooling so the structure of the cell doesn't turn into a big rock on cooldown.
Thanks for reading, and any hints!
bob prohaska
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Link 1 below has an abstract and references for a chapter called "Cell Preheat/Start-up and Early Operationon", in a book on light metals production. Springer wants $29.95 for a pdf of the chapter, or $319 for the book but the chapter's references are in publications engineering libraries might have on hand. Gas preheating using heater assemblies on wheels seems to be common. About getting the aluminum out, per wikipedia (link 2) "The liquid aluminium is removed from the cell via a siphon every 1 to 3 days in order to avoid having to use extremely high temperature valves and pumps. Alumina is added to the cells as the aluminum is removed." Of course, then aluminium is being siphoned out, they usually stop short of getting it all because the molten cryolite that floats on the molten aluminium needs to be left in the cell. But if the cell is being shut down for cathode changing, they probably need to siphon everything out.
series, with about 5 volts DC (at 100-300 kA) across each cell, and each cell operates full time except when the cathode is being changed out. Per wikipedia (link 2) "Cathodes are typically replaced every 2-6 years. This requires the whole cell to be shut down." (Anodes are much smaller, wear much faster, and are replaced far more frequently, without stopping the cell.) For cathode replacement, I imagine that cells are shut down in rotation, rather than shutting down the whole line, but don't know for sure. Just attach a foot-thick bussbar across the cell's connections, while avoiding ground, and start siphoning out?
1. <https://link.springer.com/chapter/10.1007/978-3-319-48156-2_107
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A bit spendy for a matter of curiosity. I'm surprised there's not more online.

That's very much a surprise. Given that aluminum smelters run on electricity I expected an electric preheat, maybe using SiC resistance elements in the cell walls. Gas seems alien.

Perhaps that's the answer to my shutdown question. Just tap off all the aluminum, and then keep going. I guess it depends on how the siphon is arranged.

The link worked, but the pdf preview wouldn't download, reporting one page requested and three as minimum.
Thanks for posting!
bob prohaska
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Big gas ovens are used to pre-bake some anodes, vs either gas or electric for preheating the cell and cathode; don't know which is more common. Link 3 has some Egyptalum Co. electric preheat startup details, such as time/temperature for the 70-hour cell startup process; voltages, currents, power used (eg 18 MWh at Egyptalum, vs 30 MWh for twice-as-big cells at Hydro Rheinwerk, link 4); resistors, eg nichrome, stainless steel, graphite.
Both gas and electric preheaters are computer controlled because a
"pot instability" leading to shorter pot life. Note, the heatup phase is to bake (cook out the tar or pitch resin binder) the new pressed graphite cathode and bring it up to operating temperature, after which some tons of molten cryolite, alumina, etc are added to the cell and its electrolysis process starts up.
3. <https://jpme.journals.ekb.eg/article_96413_1fe96828620111dc4c49dc8e80a67568.pdf 4. <https://www.researchgate.net/publication/268293355_Improvements_in_the_electrical_preheating_of_hall_-_Heroult_pots abstract

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On Sat, 2 Oct 2021 01:50:34 -0000 (UTC), bob prohaska

"Presently there are two main preheating methods being used, i.e. gas preheating/baking and resistor bed heating/baking."
The resistor bed method is based on using a layer of coke or graphite particles between the anodes and cathode block surface to provide ohmic voltage drop and act as a heating element . Some plants use shunts to deflect a part of the electric current directly to the next pol without passing through the resistor bed. The shunts will enable a more gentle start of the preheating period and by gradually increasing the load passing through the resistor bed the preheating time is extended A typical gas or fuel bake equipment consists of either two large propane, LNG or oil burners or multiple small gas nozzle/bumers . Preferably, steel sheets are used to protect the cathode surface from direct flame exposure
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The mental picture is coming together. Preheating with fuel-air flames, granulated coke resistors and pre-melting the cryolite before pouring it into the preheated electrolytic cell were all things I didn't think of.
Thanks for posting!
bob prohaska
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"bob prohaska" wrote in message
The mental picture is coming together. Preheating with fuel-air flames, granulated coke resistors and pre-melting the cryolite before pouring it into the preheated electrolytic cell were all things I didn't think of.
Thanks for posting!
bob prohaska
----------------
How will you power this project?
I've been looking into loads that utilize alternate energy efficiently as it's generated and so far the only practical one for me is DC refrigeration, with an Alpicool freezer. Heating water is out because the elements in an electric water heater can fail by shorting to the water and thus the tank, which I've seen happen without tripping the breaker, and could possibly put 120VAC on my otherwise Low Voltage (<50V) solar panel wiring.
Carbon fiber mat has low electrical resistance and excellent high temperature properties and might make a good distributed heating element. The sample I got from a vacuum oven maker's scrap heap measures 3 - 5 Ohms through its 1" thickness.
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No project involved. I'm not contemplating making an aluminum smelter, just curious about how they're operated.

Not quite following you. An isolation transformer on the 120VAC side will protect the DC side from electrical mischief.
If I gather right, you're looking for useful applications for intermittently available electric power. There aren't very many. Pumped water stores well and keeps well, geography permitting. Purifying water via reverse osmosis works well and keeps well, if you need purified water. Charging batteries, of course, but they're expensive per unit of energy stored.
I don't think any high-temp process will be practical on a small scale. Too hard to insulate, and too much mechanical stress from thermal cycling. Even reverse osmosis has some limits; the membranes don't like pressure changes. They're more tolerant than most things, but far from perfect.

The carbon fibers I've handled (25 years ago) were all pretty fragile outside of a resin matrix. It didn't take much flexing to break them.
How much energy are you trying to utilize and over what timescale?
bob prohaska
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"bob prohaska" wrote in message

No project involved. I'm not contemplating making an aluminum smelter, just curious about how they're operated.

Not quite following you. An isolation transformer on the 120VAC side will protect the DC side from electrical mischief.
If I gather right, you're looking for useful applications for intermittently available electric power. There aren't very many. Pumped water stores well and keeps well, geography permitting. Purifying water via reverse osmosis works well and keeps well, if you need purified water. Charging batteries, of course, but they're expensive per unit of energy stored.
I don't think any high-temp process will be practical on a small scale. Too hard to insulate, and too much mechanical stress from thermal cycling. Even reverse osmosis has some limits; the membranes don't like pressure changes. They're more tolerant than most things, but far from perfect.

The carbon fibers I've handled (25 years ago) were all pretty fragile outside of a resin matrix. It didn't take much flexing to break them.
How much energy are you trying to utilize and over what timescale?
bob prohaska ------------------------------
Due to surrounding trees and the orientation of my roof I've found semi-permanent homes for only about 150W of panels. If needed I can quickly set up another 400W in the yard or driveway and manually re-aim them every few hours. If the sky is clear that appears to be enough to support my low winter electricity demand, since my wood stove provides heat, cooking and hot water, even for laundry and showers.
Using the solar power as DC instead of AC would eliminate the ~50W constant loss in the inverter, reducing battery cost by decreasing the nighttime depth of discharge and the number of batteries required. My compact refrigerator consumes about 100W half the time so the inverter doubles the overnight drain. Panels have fallen below $1/Watt and it's the cost of batteries that makes my solar more expensive than grid power.
If the inverter could power up when it senses load demand its loss would be halved, but my (free) APC 1400 UPS doesn't support turn-on from idle in its command set and would require hacking the circuit board to accomplish it. Tripp Lite inverters can sense load if you have to buy. Apparently all true-sine inverters have significant no-load power demand and the cheaper, more efficient square wave ones aren't suited to refrigerator compressors, other AC motors or capacitive-input (light weight) power converters. A scope shows that "modified sine" means a pulse-width-modified square wave.
I'd like to create a low cost home solar system that someone else could assemble from purchased or salvaged components without needing electrical engineering and technician skills. So far mine is cheap and effective, but not simple or easy and I don't operate some parts of it unattended or overnight.
I've been testing how second-hand AGMs hold up. My experience so far agrees with Internet advice that their storage life averages about half what a well-maintained flooded marine battery can give. A marine battery I bought circa 2008 still runs my DC freezer for 24 hours. https://www.mastervolt.com/determining-the-lifespan-of-a-battery/ "If kept in a charged state when unused, the common lifespan of a 12-volt Gel or AGM battery is up to six years." AGMs appear to fail by irreversible sulfation (or contact corrosion?) that increases internal resistance although the voltage seems normal, whereas I've had reasonably good results from desulfating an old wet battery with a current-limited higher charging voltage, a good side job for solar power. If you can't design and build a DC-powered metered adjustable power supply this looks good so far: https://www.aliexpress.com/item/4001230106243.html I haven't gotten more than 2.7A continuously from it but that's enough. 14 AWG silicone wire was slightly too big for the terminals.
The only advantage I see for AGMs is that they can be fully recharged quickly without releasing hydrogen. I charge my wet batteries at the float voltage where they don't gas, or fully recharge during winter daylight. Most of my industrial experience is with Lithiums and I'd use them if they weren't so expensive.
jsw
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"Jim Wilkins" wrote in message

Not quite following you. An isolation transformer on the 120VAC side will protect the DC side from electrical mischief.
---------------------------------
That's a 5 KVA transformer whose cost I'd never come close to recovering, plus the tank is grounded in uncertain ways through the plumbing. I found that out with a clamp-on ammeter when the Neutral connection at the weatherhead corroded.
I don't leave second-hand equipment of unknown history or condition permanently powered up, for instance when not in active use the air compressor's breaker is Off. I can test for HiPot leakage but not burnt insulation etc.
The solar controllers I have now switch the negative side and can't regulate current if both the source and load are grounded. My solar power system comes under the exemptions for Low Voltage and Separately Derived, isolated from the grid, and I intend to keep it so.
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On that size scale battery charging is probably the best possible use for excess solar power.

Solar power is far from cheap. People get seduced by the "sun is free" and forget the cost of infrastructure, intermittency and upkeep. That's why we're so fond of fossil fuels 8-)

Sounds like the "free" UPS is the wrong tool for the job. A DC thermostat circuit controlling an individual inverter for the fridge alone is closer to an appropriate setup. Hacking still required, but on the fridge, not the power supply. And, the inverter can be tuned to the load.

The problem you're solving is a close cousin to that faced by long-distance sailboaters. You don't need to worry so much about weight, but in most other respects the solutions will be similar, and similarly expensive. Either in know-how or money paid for someone else's know-how.
Wish I had some better ideas!
bob prohaska
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"bob prohaska" wrote in message wrote:

Sounds like the "free" UPS is the wrong tool for the job. A DC thermostat circuit controlling an individual inverter for the fridge alone is closer to an appropriate setup. Hacking still required, but on the fridge, not the power supply. And, the inverter can be tuned to the load.
---------------------
Unlike the boater I can risk inadequate or unreliable solutions because I still have the house's standard appliances and electric heat. My electronic know-how is enough to design computerized devices and DC power supplies but not switchers, though some very nice digital regulators are now available at a reasonable price, such as the DPS5020, which enable building powerful and efficient lab supplies with flea-market variacs and transformers or solar input. I used to order less capable Agilent lab supplies that cost $1000. The high-end automated industrial test equipment I used to build consisted mostly of digitally controlled 4-quadrant (+/-V, +/-I) power supplies. The Keithley Sourcemeter is a good example though these were designed in-house.
I knew from the start that I didn't have a good location for enough roof panels to support daily cycling, and designed a small system mainly to experiment with and measure performance. Though it isn't what I would have bought new (Tripplite?) the free 900W APC UPS is quite adequate for experimenting and large enough to start and run my compact refrigerator. I have a 2200W HF inverter generator which I think is about the minimum practical size to support a kitchen, as long as the fridge is unplugged when running the microwave or coffee pot. The APC tends to reject non-inverter generator AC and revert to battery even at its lowest sensitivity setting.
As it presently exists, 900W of AC with 2 KWH of battery energy, the system is able to run my refrigeration and laptop off-grid during daytime and overnight cold front thunderstorms. Other loads have their outlet strips shut off. I consider the laptop essential for Internet weather radar which is far more useful than radio reports. It appears able to support a long term winter grid outage if the sun is out, with generator support at meal times, but it needs attention and manual panel tracking. It was relatively inexpensive and enough to keep me functioning fairly normally when everything is shut down and I have nothing better to do. Part of the experiment is determining an acceptable level of alternate energy inconvenience.
The APC can be started remotely with a FET in parallel with the On button and a logic input from the beeper driver since a cold-start-on-battery requires pressing the button until the beeper sounds. These might need optical isolation to avoid ground loops.
I could add a thermostat or thermocouple to the fridge without drilling by slipping the wires through a caulked slit in the door gasket.
A relay at the fridge's AC plug would permit either sensing resistance or connecting to power. I put a $15 current-sensing relay on the fridge cord to signal visually and electrically whether its compressor is on or off. The relay LED shows when to wait a few minutes before moving the plug from grid to inverter, to reduce the starting surge.
I could set up an Arduino to sense fridge demand, start the APC and switch the fridge plug from sensing to power, then put the APC back to sleep by serial command when the fridge motor stops. Unfortunately there isn't a wake-up command that works when it's on battery. At least this older APC model doesn't have a shutdown timer and can operate indefinitely at ~75% power.
You are correct that it isn't the right tool, for someone who wants a simple and painless substitute for grid power. The serial connector does NOT take a standard cable, it has battery voltage coming out (or going in, at test?) on it. I wrote a program to talk to it.
For me testing, measuring and fine-tuning is what I do.
jsw
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"Jim Wilkins" wrote in message ... You are correct that it isn't the right tool, for someone who wants a simple and painless substitute for grid power. The serial connector does NOT take a standard cable, it has battery voltage coming out (or going in, at test?) on it. I wrote a program to talk to it.
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I forgot to mention that as received it needed its battery constant and float voltage reprogrammed, with a user-wired serial cable.
jsw
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