Sciencemadness Discussion Board

Reconditioning old hydraulic oil.

bolek - 17-5-2023 at 04:47

This is my first post on the forum so hello everyone :-)

This post is about reconditioning used hydraulic (and other machine) oils.

First a little introduction.

As a result of another hobby of mine I have various machines that take different industrial oils. One of them is a hydraulic surface grinder. The machine has 80L + of hydraulic oil and when I recently checked the prices I was surprised quite a bit.. Let's say changing oil in that thing would cost me a quarter of what I paid for it a couple of years ago.

So to the point, I need to recondition slightly over 80L of an old hydraulic oil using my "multipurpose lab" that includes a little chemistry kit.

So what's wrong with the old oil? For one it contains particulates (this being a grinder) that need to be filtered out, possibly water that needs to be dried out, potentially varnishes that are heavier and thicker that I'll try to avoid picking up from the bottom, but it would be nice to get rid of somehow without having to wait months for them to drop to the bottom. (I have a centrifuge, but there is no way to centrifuge 80L in it in any reasonable amount of time).

I'm starting this thread in hope there is someone here that may have done such a thing and has some tips. Or anyone else has ideas that may help.

I'm planning to filter the oil with one of: a 10 micron water bag filter, a couple of sintered glass filters in glass funnels (g2, g4) using vacuum.

Dehydrating with dry cacl, mgso4 or silica.

I don't know about varnishes etc.

Antioxidants etc will be replenished by adding a little new oil.

I have to plan this very well, because once I disturb the sediment in that tank I can't just put it back in as is. Grit and other stuff that is now settled on the bottom will damage the machine. My plan shouldn't take more than a week ideally.

Anyone has any ideas?

Ubya - 17-5-2023 at 10:46

As a first quality test of your oil, you can measure its total acid number (mg of KOH needed to neutralize 1g of oil). Oxidation produces acidic species, so knowing how acid is your oil, you can estimate how degraded it is.

If it is too acidic, you can filter and add additives, but the oil is bad to begin with.

If your oil passes this test (I guess you can check the acid number for the original oil) you can filter and dewater it.

10 microns is still a bit too big, i'd go down as low as possible, 5 microns or even 1. Gravity or vacuum won't do much, you'll need a pump to push all the oil throught the filter (the sell oil filters of appropriate porosity).
CaCl2 or MgSO4 would work (I think), but maybe zeolites would be better? Basic zeolites could in theory help by also removing acidic oxidation byproducts from the oil

unionised - 17-5-2023 at 12:57

The deeply unhelpful answer is that, if you want to really get the oil back to its original condition, you probably need to vacuum distill it.

Good luck.

Rainwater - 17-5-2023 at 17:04

Prevention is key, filters, cooler, ph buffer.

Setting up a 1 micron filter, is just going to go through a lot of filters. And at 200 bucks a pop. Ya. No.

At work we cycle the fluids of each machine once a month from 40 different factories. About 10000 gallons total. Logging of what machine each drum came from, what we had to fix (ph, chloride etc) is done, and the oil goes back to its machine. We try to process only one viscosity at a time, but it with 300 machines and 20 different types of oil, it doesnt always happen like that.

This setup has a lot of parts and can process 500 gallons an hour, so please bear with me.

Water and acidified oil are the primary contaminates we deal with chemically.

The oil is transfered into in a new drum, desicant is added to remove water.
Silica gel cat litter.

After that we use test strips to check for chloride contamination.
Im not sure what they pour in to get that out buts it stinks up the entire shop

The drum is then circulated with a pump for about 30 minutes and tested again for chlorine. After that passes the ph is adjusted, then it is sent to the filter station.

Each section has a differential pressure gauge attached so we can see what needs cleaning.

First up is a simple strainer feeding a 1in gear pump, 200 psig.

Then another custom strainer made out of this stuff
When the differential is > 50 psig I swap out the sock.

Its called a tube strainer, its a 4in wide pipe, 8ft long. housing that accepts a stainless steel filter, oil enters the top center, junk gets trapped on the inside of the sock and clean oil flows out the bottom.when they get dirty the mesh comes off, gets washed and goes into the kiln at 800c under nitrogen for a few hours.

When I have to replace the mesh I get a welder over there and he makes a new filter by rolling the fabric around a jig and sealing the side and 1 end. Basicly its a long sock.

After the strainers come a 1 micron
Then its back into the shipping barrel. A sample is sent to the in house lab and with their blessing the oil is back in production. The strainers get changed every day or so, the filter, maybe once every other month.

bolek - 17-5-2023 at 22:36

Quote:
Quote:
Quote: Originally posted by Ubya  
As a first quality test of your oil, you can measure its total acid number (mg of KOH needed to neutralize 1g of oil). Oxidation produces acidic species, so knowing how acid is your oil, you can estimate how degraded it is.

If it is too acidic, you can filter and add additives, but the oil is bad to begin with.

If your oil passes this test (I guess you can check the acid number for the original oil) you can filter and dewater it.


Great idea, thank you :-) I can test a small sample to get an idea how old this oil potentially is (I hope it isn't what the machine was filled with when it was first bought in 1971). I found an article with a helpful chart that shows increase of the acid number from 0 to 2500 hours of use. My oil should be 0.5~0.6 mg KOH/g
Quote: Originally posted by Ubya  

10 microns is still a bit too big, i'd go down as low as possible, 5 microns or even 1. Gravity or vacuum won't do much, you'll need a pump to push all the oil throught the filter (the sell oil filters of appropriate porosity).
CaCl2 or MgSO4 would work (I think), but maybe zeolites would be better? Basic zeolites could in theory help by also removing acidic oxidation byproducts from the oil


It seems the biggest difficulty will be pushing that oil through the filter(s).
Quote: Originally posted by unionised  
The deeply unhelpful answer is that, if you want to really get the oil back to its original condition, you probably need to vacuum distill it.

Good luck.


I'd get quite an electricity bill afterwards.

But talking seriously. How to prevent oxidation while boiling? Probably sending nitrogen through the oil during distillation is needed. The boiling temperature seems in the region of 360C. Although I did boil sulphuric acid once I don't think I'd trust my glassware enough to heat potentially combustible substance in it like that.

I wonder if the still for this could be built from hot rolled mild steel? I have plenty lying around and I can tig weld enough to trust pressure vessels made like this (after testing with liquid). Of course stainless would be best, but I only have a little of it.

Probably the time and effort required to boil all this oil is one thing that makes this unfeasible. Let's say I used 2200W heater (which I have) and I use rock wool like product for insulation. Rough calculation tells me it'll take 60MJ of energy to boil 80l of this oil (double that for all the escaped heat). That would be 15 hours of my heater... It all depends on how much heat escapes, probably my estimate of double is extremely optimistic.


Quote: Originally posted by Rainwater  
Prevention is key, filters, cooler, ph buffer.

Setting up a 1 micron filter, is just going to go through a lot of filters. And at 200 bucks a pop. Ya. No.

At work we cycle the fluids of each machine once a month from 40 different factories. About 10000 gallons total. Logging of what machine each drum came from, what we had to fix (ph, chloride etc) is done, and the oil goes back to its machine. We try to process only one viscosity at a time, but it with 300 machines and 20 different types of oil, it doesnt always happen like that.

This setup has a lot of parts and can process 500 gallons an hour, so please bear with me.

Water and acidified oil are the primary contaminates we deal with chemically.

The oil is transfered into in a new drum, desicant is added to remove water.
Silica gel cat litter.


I was hoping someone like you will show up and say something like that :-)

I happen to have plenty of cat litter silica gel
Quote: Originally posted by Rainwater  


After that we use test strips to check for chloride contamination.
Im not sure what they pour in to get that out buts it stinks up the entire shop

The drum is then circulated with a pump for about 30 minutes and tested again for chlorine. After that passes the ph is adjusted, then it is sent to the filter station.

Each section has a differential pressure gauge attached so we can see what needs cleaning.

First up is a simple strainer feeding a 1in gear pump, 200 psig.

Then another custom strainer made out of this stuff
When the differential is > 50 psig I swap out the sock.

Its called a tube strainer, its a 4in wide pipe, 8ft long. housing that accepts a stainless steel filter, oil enters the top center, junk gets trapped on the inside of the sock and clean oil flows out the bottom.when they get dirty the mesh comes off, gets washed and goes into the kiln at 800c under nitrogen for a few hours.

When I have to replace the mesh I get a welder over there and he makes a new filter by rolling the fabric around a jig and sealing the side and 1 end. Basicly its a long sock.


Interesting. It seems having a pump push that oil is inevitable. I have an old (but working) hydraulic pump from a tractor. The problem is destroying it with particulates.

I do have some fine steel mesh (not that fine). I will have to make such tube filter to at least pre-filter the biggest particles(after letting it stand for a day hoping the worst falls to the bottom). Then I'd probably have to sacrifice this gear pump and push it through some cheap oil filters. I'd have to rig up a way to run this pump as well as fittings and tubing for oil pickup and drop off. At least I have a couple of clean plastic barrels I can use as containers.

Regarding the gear pump. It used to be driven from the tractor's gearbox. I wonder if I can try using something lot less powerful (an electric drill) to power it. I don't need 360 bar(5000 psi) of pressure at 20L/min it is used to making. I need a lot less. I'm not sure what pressure is suitable for those automotive oil filters and if they're even suitable.

I might have to think very carefully if I want to touch that sediment on the bottom of the oil sump. If I could just suck the oil out from above it, dispose off the sediment and add let's say 20l of new oil with minimum filtration that would be great, but I worry there will be tiny abrasive particles present that currently sit harmlessly near the bottom.

The most difficult is making the decision if and when to do it. That's why measuring acid number is such a good idea. The machine runs well, there are no particulates in the pressurised oil. So my main concern is not to mess it up.

Quote: Originally posted by Rainwater  

After the strainers come a 1 micron
Then its back into the shipping barrel. A sample is sent to the in house lab and with their blessing the oil is back in production. The strainers get changed every day or so, the filter, maybe once every other month.


That final step is going to be difficult for me. I think I'll have to use a cheap automotive paper oil filter as my last stage and hope that it works and the pump doesn't blow through it.

Rainwater - 18-5-2023 at 01:22

That mesh is awesome, it catches most of the particulates, but the key is its cleanable and saves the filters.

Cheaper filters are available.
McMaster is not cheap, but has everything. Shopping around will save a ton of money.
Quote:
. It seems having a pump push that oil is inevitable

If you can tack togethet some pipe and valves, you can fill it with oil, and apply compressed air. That will save on a pump and may already be available to you

unionised - 18-5-2023 at 03:23

Re. "But talking seriously. How to prevent oxidation while boiling?"
Not an issue in a vaccum.

bolek - 18-5-2023 at 23:01

I've ordered phenolphthalein to do the acid test. With a bit of luck it'll arrive today so I can do the test. I have pH strips, but I wanted to do the test following the proper method with phenolphthalein as marker in alcoholic media.

Quote: Originally posted by Rainwater  
That mesh is awesome, it catches most of the particulates, but the key is its cleanable and saves the filters.

Cheaper filters are available.
McMaster is not cheap, but has everything. Shopping around will save a ton of money.

Quote:

It seems having a pump push that oil is inevitable

If you can tack togethet some pipe and valves, you can fill it with oil, and apply compressed air. That will save on a pump and may already be available to you



Unfortunately we don't have McMastercarr here in Europe (technically they're present, but I never found a way to order from them as a person (not business). But there is a vendor that specialises in stainless meshes I ordered from before.

However, I'm having trouble converting 325x2300 mesh to metric(in microns) . Some conversion tables claim 325 mesh is 44 microns (they don't have 2300). If I just assume this is a number of openings per inch then I get 78 microns for 325 and 11 for 2300. So two sources differ. Which one is right?

This is a very good idea with compressed air, but it'll limit me to 6~7 bar (I have pressure vessels that can work with this pressure, it should be more than sufficient based on what you mentioned before)

Quote: Originally posted by unionised  
Re. "But talking seriously. How to prevent oxidation while boiling?"
Not an issue in a vaccum.


I expect the oil will outgas a ton of volatiles before any appreciable actual vacuum is reached (overwhelming my traps in the process). So wouldn't running a cheap inert gas through it be easier? (for example nitrogen I already have?) Probably not CO2. I'd much prefer it due to low cost (even lower than nitrogen), but I don't think it is inert enough.

Hopefully it'll not have to come to distilling it.

Edit: I checked steel mesh prices... Wow, the smallest one available here is 23microns (they call it 1000 mesh). The price is 80EUR per sq meter and the least one can buy is half a meter. The prices go down a lot as the openings get bigger. For example 50 micron mesh is 15eur per meter. But this is still pretty far from what I need.

Then 1 micron filters are very expensive...

I wonder if my 10 micron bag filter (supplied under pressure after filtering with 50 micron steel mesh) is the best I can do within my current budget. The question is if this will improve the situation or harm it?

It is very likely the pickup filter in the machine is probably filtering only stuff above 20 microns and there is a lot of smaller stuff on the bottom. Once it's disturbed and unfiltered it will gang around for a long time.

Probably unless I can find a way to filter it at 1 micron (or distill) it's better to leave it alone for a bit longer.

Also I was surprised to find out your typical automotive oil filter is only rated at 20~40 micron... I thought it would be much better, but it isn't.

Distillation starts to look more attractive to me now. I could do it outdoors, with just a copper tube as an air condenser. The main difficulty is in making it both fairly well sealed (to supply few l/min nitrogen) and make it openable to clean. Perhaps an old pressure cooker could be repurposed...

I might do a small scale experiment with some old oil I collected over the years first.

Edit2: I've done some calculations and it seems if the oil didn't outgass too much it should be possible to distill it at much lower temperature (let's say 150C) in moderate vacuum.

I think I'll need to do an experiment.



[Edited on 19-5-2023 by bolek]

Fulmen - 19-5-2023 at 00:20

Mesh is (supposed to be) 'wires per inch'. For fine mesh you will need to factor in the wire diameter which can be larger than the openings.

Rainwater - 19-5-2023 at 10:13

That mesh is spec'ed at 5 microns. $80 + s&h for a 12in×12in piece. I buy from there so I dont have to order the$30k 100ft roll.
A few years back we had a different setup the contained about 10 different strainers before the filters. It was a pain to service. Someone finally talked them into trying a super-sized fine mesh strainer and it made life much better. Many companies can filter that amount for a fee. If this is only for your home shop a more cost effective method would be building a polisher yourself.

These folks made a good one, you would need a better pump and filters
https://youtu.be/2lw6UmxJAB8

All this still doesn't address acidification

bolek - 20-5-2023 at 02:50

Quote: Originally posted by Fulmen  
Mesh is (supposed to be) 'wires per inch'. For fine mesh you will need to factor in the wire diameter which can be larger than the openings.


Thanks, that makes sense now.

Quote: Originally posted by Rainwater  
That mesh is spec'ed at 5 microns. $80 + s&h for a 12in×12in piece. I buy from there so I dont have to order the$30k 100ft roll.
A few years back we had a different setup the contained about 10 different strainers before the filters. It was a pain to service. Someone finally talked them into trying a super-sized fine mesh strainer and it made life much better. Many companies can filter that amount for a fee. If this is only for your home shop a more cost effective method would be building a polisher yourself.

These folks made a good one, you would need a better pump and filters
https://youtu.be/2lw6UmxJAB8

All this still doesn't address acidification


Polisher? Is this supposed to be a different word? I might be missing something, but it seems to me they only have an oil/water separator. They don't filter particulates from the oil.

Regarding a commercial company, this is the first thing I checked. Unfortunately around here the only companies that do that want to do your "oil asset management" and process it by cistern-load, not deal with a guy bringing in one barrell of oil to filter.

The oil in this machine should last 2500 hours. Withing a hobby shop that is many years. So hopefully I'll not have to deal with thus too often.

Regarding acidity, I'll know hopefully this weekend. However I already read a, scientific article that claimed the typical cutoff of 2.5mg KOH is very likely much too strict. They did corrosion tests with such "spent" oil and they discovered it doesn't promote corrosion within the duration of their tests so the recommendation was to up those limits.

Also, I've been collecting my vacuum pump oil for years (it's a, small pump, I have about 7L). I might use it for my distillation experiment.

Based on the stuff shared by people in this thread and what I found elsewhere any oil filtration setup to be complete needs a 1 micron (or close to it) filter. Such filters are a bit too expensive for me now.

So distillation under vacuum (to lower the boiling point to 120~150C) seems the best bet. Once tested with other oil I plan to get 5~10 L from the top of the sump. Distill this and see how long it takes (hopefully 10L can be done in a day with my small scale setup). Then reassess if this is feasible to continue.

Sulaiman - 20-5-2023 at 06:28

If you use vacuum distillation
(I have little experience)
it might be worth heating the oil to above 100C to drive off most of the water,
which would otherwise go to your trap(s) or pump,
then do vacuum distillation.

To process 10l a day, to me implies multiple smaller distillations per day, eg 3x 3.33l batches.
That would need a 5l flask...
Which I'd be worrying about while under vacuum.
An all metal still would probably be safer,
and I'd consider some sort of blast shield.

I'd probably heat a little oil in a spoon over a flame to get an idea of the oil's volatility and flammability etc.
Before starting any bulk or batch distillation.

[Edited on 20-5-2023 by Sulaiman]

bolek - 20-5-2023 at 08:31

Quote: Originally posted by Sulaiman  
If you use vacuum distillation
(I have little experience)
it might be worth heating the oil to above 100C to drive off most of the water,
which would otherwise go to your trap(s) or pump,
then do vacuum distillation.

To process 10l a day, to me implies multiple smaller distillations per day, eg 3x 3.33l batches.
That would need a 5l flask...
Which I'd be worrying about while under vacuum.
An all metal still would probably be safer,
and I'd consider some sort of blast shield.

I'd probably heat a little oil in a spoon over a flame to get an idea of the oil's volatility and flammability etc.
Before starting any bulk or batch distillation.

[Edited on 20-5-2023 by Sulaiman]


Thank you for your reply.

I have used 1L flasks under vacuum before. This time I plan to use 2L. This is a ground glass 3 neck borosilicate flask. I hope it'll be OK. I just have to ensure it has no air bubbles trapped in the glass (I had two such flasks, one has an air bubble, I have to ensure to use the other one). Using vessels I already have is of course preferred to having to fabricate from metal. I was going to fabricate, but mostly for high temperature resistance (I don't trust glass above 300C). Does anyone have any concern using a well made round borosilicate 2l flask under vacuum? I only used it for non vacuum distillation before and I always used smaller flasks (1L) under vacuum.

I've been looking for some materials about this being done by other people (redistilling used oil to base oil), but most I found is how to make diesel by pyrolysis from used motor oil.

I did however find this document: https://erdolrahmen.com/wp-content/uploads/2019/07/Used-oil-...

It contains various weird statements. For example this: "Process of used oil recycling begins with dehydration. From storage tank, used oil is transferred to reactor for dehydration where it is heated up to certain temperature to remove water & fuel (Which is also known as light ends). The process is done under 600-700 mmHg vacuum to decrease surface tension."
surface tension? seriously? Surely they mean lower the boiling point of the more volatile impurities.

Then they go on:
" After dehydration process, used oil is shifted to evaporator for distillation where it is heated to 320 °C - 350 °C under >759.5 mmHg vacuum"

This 320~350C sounds very close to atmospheric pressure boiling point. So why would they have to hear the oil that much under vacuum?

And finally:
"Base oil received after distillation process is known as ‘Raw Base Oil’ because it still contains impurities (mostly aromatics & ash contains), so it is taken further to bleaching process where it gets reacted with bleaching clay (clay-treatment process) or NMP solvent (solvent-extraction process). After bleaching process, we get final Recycled Base Oil."

It seems they overheat the oil during distillation which results in some decomposition products that than require this" bleaching". I hope I'll not have to go above 200C in temperature therefore my oil will be clean after distillation.

If anyone has done something like it, or you know of potentially helpful materials please let me know.

bolek - 21-5-2023 at 01:25

Well, distilling the used vacuum pump oil experiment was a complete failure unfortunately :-(

Being used to dealing with pretty small reagent amounts in my normal experiments I didn't want to push the safety side of it too far. So when the oil failed to boil at 170C under best vacuum my pump can pull (with all joints greased etc). I decided to stop. Perhaps going with a full liter wasn't the best idea for a first try.

This is the "Black gold" vacuum pump oil made in US BTW.

It is likely my hydraulic oil will boil much easier, but doing it in glass is probably not the best idea as mentioned by others.

bolek - 22-5-2023 at 07:26

I've been thinking about this, and before I give up and go buy that oil for it's very high price I might try making my own filter taking syringe filters and filter paper as inspiration.

I've built a pressurised sander many years ago. After cleaning I can use it with minimal mods to pressurise oil to ~5-6bar (80psi). I wonder if that's enough to send prefiltered oil through a super sized syringe-like filter I'm thinking of building.

I've plenty of 150mm filter papers including some with 3~5 micron pore size. Imagine a 160mm wide "supersized syringe filter". I wonder what the flow would be through such filter. If it could do let's say a liter per hour and lasted a couple of hours that would be feasible.

Rainwater - 22-5-2023 at 13:36

Dont underestimate sand filters. NaCl finely powdered is used for cooking oils

bolek - 22-5-2023 at 22:36

Quote: Originally posted by Rainwater  
Dont underestimate sand filters. NaCl finely powdered is used for cooking oils


This is interesting. I never heard that. I have to do some research into this topic (how to determine particle retention of such filters etc).