# Machine that produces LS from used cooking oil



## BintuluHawks (Mar 30, 2016)

Hi! We're a robotics team from Malaysia. We've been diligently reading from this forum and it's been a HUGE help in our project! We plan to make a robot that automatically converts used cooking oil into LS for our school cafeteria. We aren't using the glycerin method because glycerin and distilled water are hard to come by and inconvenient for the cafeteria workers, so we use tap water. In recent experiments, we've come across plenty inquiries. 

Does the oil need to reach a certain temperature before pouring in KOH, and if yes, what and why? 

Anyway, it's very exciting to be part of this forum of experienced soapmakers and we hope that this is will be giant leap forward for our robot. Any advice is greatly appreciated. Many thanks in advance!


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## Susie (Mar 30, 2016)

The oil does not need to be any specific temperature.  The KOH does need to be mixed with the appropriate amount of water before adding it to the oils, though.  If mixed with water immediately before adding it to the oils, the resulting high temperature from that mixture should be sufficient to melt any solid oils.  Does the cafeteria only use one type of oil to cook with so you can figure the correct amount of KOH?

I think this is an AWESOME idea!


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## cmzaha (Mar 30, 2016)

I think that is a fantastic idea, but is the used cooking oil clean? I assume your straining it well to remove any food bits, but also wonder if it needs to be washed, similar to rendering fats. It might only be rancid fats that need the actually cleaning other than just straining it well. I think DeeAnna has a post somewhere about using used oil. With all the markets I do I have thought about talking to the vendors and collecting their used oil and making "Farmer Market Soap." just have not decided if I want to go through the hassle of cleaning it.


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## dixiedragon (Mar 30, 2016)

Do you have a process for cleaning the oil first?


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## BintuluHawks (Apr 4, 2016)

Why the oil does not need a specific temperature?
And yes, our school's cafeteria only uses one kind of oil which is palm oil. The ratio of KOH: Used Cooking Oil is 1: 4.2.
And thank you for your reply tho!
The cooking oil that we used is not clean 'cause we're not filtering our cooking oil. Btw, do we need to filter the oil and what's the advantages of filtering the cooking oil if it's necessary.


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## The Efficacious Gentleman (Apr 4, 2016)

Filtering would remove any lumps of old food from the oil. Things like that can go rancid even in the soap, which no one wants.


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## BintuluHawks (Apr 4, 2016)

*Liquid Soap and Suitable Materials*

Hi again! We hereby intend to ask the reasons why we need to stir the soap paste continuously. If we want to design our own stirrer to stir the mixture, what kind of material should we use so that there's no reactions will occur? Is a rice cooker made of aluminium suitable to be used?

Thanks!


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## Obsidian (Apr 4, 2016)

No, you absolutely can not use aluminum to make soap, it will react with the KOH. Stainless steel is the only safe metal for soap making.


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## Susie (Apr 4, 2016)

You only need to stir to emulsion for liquid soap.  No need to continue to stir.  Just put a lid on it, and walk away.  Come back several hours later to find paste.  Then begin to dilute.  Stirring and heating will speed getting to paste stage, but it is not necessary.  

It is quite necessary to filter that oil of food particles, though.  You will get rotting and rancidity if you do not.


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## topofmurrayhill (Apr 4, 2016)

BintuluHawks said:


> Hi again! We hereby intend to ask the reasons why we need to stir the soap paste continuously. If we want to design our own stirrer to stir the mixture, what kind of material should we use so that there's no reactions will occur? Is a rice cooker made of aluminium suitable to be used?
> 
> Thanks!



I would like to hear that you understand the danger of the materials that your team and the cafeteria workers could be exposed to. Do you know what an MSDS is and did you read one for KOH? If you didn't think of doing that, it's possible that you are in over your head with this. So here is an excerpt from an MSDS and I would like YOU to tell ME if you should use the aluminum rice cooker.

*Special Remarks on Fire Hazards:*
Violent reaction or ignition under appropriate conditions with acids, alcohols, p-bis(1,3-dibromoethyl) benzene,
cyclopentadiene, germanium, hyponitrous acid, maleic anhydride, nitroalkanes, 2-nitrophenol, potassium peroxodisulfate,
sugars, 2,2,3,3-tetrafluoropropanol, thorium dicarbide. Molten ortho -nitrophenol reacts violently with potassium hydroxide.
When potassium hydroxide and tetrachloroethane are heated, a spontaneously flammable gas, chloroacetylene, is formed.
When phosphorus is boiled in a solution of potassium hydroxide, phosphine gas is evolved which is spontaneously flammable.
1,2-Dichloroethylene and Potassium hydroxide reaction produces chloroacetylene which is spontaneously flammable in air.
Potassium Persulfate and a little Potassium hydroxide and water will ignite. When wet, attacks metals such as aluminum, tin,
lead, and zinc, producing flammable hydrogen gas.


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## dixiedragon (Apr 4, 2016)

Regarding cleaning the oil, you must filter it. There are  bits of food in there. But I also think you need to wash it. The oil is full of salt, seasonings, small particles of food that filter may not catch. The salt in particular is a problem - it will inhibit lather. I think what you need to do is first, filter the oil. The melt the oil with an equal amount of water. This doesn't have to be exact. Let the melted oil and water reach room temperature. During this process, the oil will float to the top and the water - along with impurities such as salt, bits of food, etc - will sink to the bottom. Then put it in the refrigerator. This will cause the oil to harden enough that you can simply lift the hard cake of fat off the top of the water. 

Taste the water. You want the water to have no taste. If the water has a taste - salty, meaty, whatever - clean the fat again.

The melting point of palm is 35C. So if it's a cool night, you may not need the refrigerator at all - you could just put the pot of palm oil and water outside to cool and solidify.


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## DeeAnna (Apr 4, 2016)

Know the process of soap making first before you get into designing your machinery. 

1. Certainly do your literature search here and elsewhere....
2. But you also need to actually make the soap on a small scale to prove in the method you think will work and to evaluate the end product. If the soap or the process doesn't turn out well on a pilot scale, you have the option to re-evaluate and refine what you're doing until things do work properly. This will also give you the opportunity to get feedback from your customers -- the people who will be using your soap. Do they like it; does it work well? If not, how does it need to be changed so the soap performs better for them?
3. Once you're familiar with the process and know you can make good soap from the materials you must work with, then move on to scaling up the process and developing the machinery. 

If you skip step 2, you're going to raise the risk of failure pretty high, especially since you obviously know very little about how to make soap and you're using kitchen fats, which are going to be more variable in quality and composition and not as clean as needed. Used fats for soap is an issue that most soap makers here will have little or no experience with, so you may need to do a scholarly literature search on the matter. It will be critical to know how to clean up the used fats so you can make a quality soap.


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## BintuluHawks (May 21, 2016)

Hi again!
We would like to ask if the soap that we made is biodegradable or not?If Yes,why is it?
Thank you!


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## Susie (May 21, 2016)

True soap should be fully biodegradable if it is made with fats and KOH or NaOH and EOs.  I am not saying that FOs are not biodegradable, I simply have no knowledge whatsoever, how they are made, so I will not say they are biodegradable without more information.  

https://en.wikipedia.org/wiki/Biodegradation


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## BintuluHawks (Jun 16, 2016)

*A big thank you*

We heartily appreciate all the replies we got. We are using stainless steel for our device, and polypropylene for storage of KOH aq. We visited a soap making factory to gain more insight from the chemist there, and we would like to ask is it necessary to let the soap for a year after the soap making process? 
Previously, the chemist told us that the LS needs to be left for a period of time after the soap making process for it to be fully oxidized and eliminate the excess Potassium Hydroxide in it. We've been told that bar soap needs to be left for 3 months while 1 year for liquid soap. Is this true? If so, why? And what is the effect of using the soap which has not been left for a period of time? Besides, is the Potassium Hydroxide fully eliminated after the soap-making process?


Thank you!!


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## DeeAnna (Jun 16, 2016)

The general consensus for many soap makers on this forum is to cure bar soap for 4 to 6 weeks before use by the consumer. Some types of bar soap lather better and last longer if they cure for several more months to a year (or even more) -- 100% olive oil (castile) soap and aleppo soap are examples -- but 4 to 6 weeks is common for soaps made with a mixed blend of fats.

The general thinking I have heard about liquid soap is that it is ready for use much faster -- perhaps a few days to a week or two after it is made. Some people will let liquid soap sit (sequester) for several weeks to a month. This has two purposes. It allows the liquid soap to reach its final texture and thickness. It also allows any solid materials in the soap to settle out to improve the clarity of the liquid soap. If there are solids that do settle out, the liquid soap is carefully poured off leaving the solids behind. 

The disadvantages of using a soap too soon is that the soap may not be as mild to the skin and the lather will not be as nice. Bar soap that is not cured long enough will also be physically softer and will not last as long when used.

If the recipe for the soap you make is properly designed, it is true the alkali (potassium hydroxide or sodium hydroxide) will be completely used up in the saponification reaction. But a chemical reaction is like a fire -- the fire takes a short while to get started, it burns hot for some time, and it takes a long time for the last embers to die down to cold ashes. While it is true that almost all of the saponification reaction is done within a few hours to a day or two, the last few percent of the reaction (the time for the embers to die) takes several more days to as much as a week or two to finish up.

Edited to add: The method of making the soap will also affect the cure time. My comments (above) are based on how most of us on this forum usually make soap -- with a stick blender or other high-intensity mixer and a cold-process or hot-process recipe that has a small to moderate superfat (also called lye discount). My comments may not apply to other types of soap making methods and other types of recipes.

If the soap is made by hand stirring or other low-intensity mixing, for example, then the time needed for complete saponification will take longer. A bar soap might need another month of cure time perhaps. That said, I'm not at all sure why liquid soap should need a whole year of curing before use -- even with hand stirring, I would expect a liquid soap made with modern hand-crafted methods would be ready for use within a few weeks.


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