Large Volume vs. Small Volume CAT Processing

[kurtak]

Since I have been a member of this forum (now going on 4 years) every so often (a few times a year) someone will post the question of - "how do I process CATs on a large scale" - & leaching them is always the answer & then the next question is - "is it better to leach them as whole combs or crushed"

Leaching CATs on a large volume basis "is not" the way to process large volumes of CAT material - if it was that's how the big boys (like Johnson Matthey) would do it - but they don't - they smelt them & for very good reason - which I will try to explain in this thread

One of the reasons I was absent from the forum for a long time was that I was involved with a local scrap yard that is one of the bigger local CAT buyers - my involvement was due to my knowledge of recovering & refining PMs (which knowledge I owe thanks almost entirely to this great forum) the purpose of my involvement (as a joint venture) was to see if we could recover/refine the PGMs from CATs on a large scale & of course do so at a profit --- so in this venture I spent about 9
months focused on nothing but processing CATs with something like 1500 to 2000 hours put into R&D - & though I learned a lot I can't claim I know everything - so I don't know everything - which means I don't have all of the answers - which means this thread is about sharing what I did learn

The foremost thing I learned (though not the first) was that you can not leach CATs on a large volume scale - as a profit making venture by leaching them --- at least not if you are buying them at "whole CAT fair market value"

Now - that is not to say that you can't leach large volumes of CAT material & succeed in recovering PGMs to refine - in fact you can & will recover PGMs for refining - but that's not the real question - the real question is can you do it & make a profit & the answer to that is a big fat NO --- your only "hope" in doing it at a profit is in smelting

Its one thing to leach a few CATs on a small scale but its an entirely different thing to try to do it on a large scale !!!

For one thing - even on a small scale - you "will NOT" get all the PGMs by leaching & the greater the volume of material is that you try to leach the less effective leaching becomes - & this is especially true if you are trying to leach crushed material - In fact at this point let me say that you will have better success at leaching large volumes of whole CAT combs then trying to leach large volumes of crushed CAT material - also you can set up (the equipment needed) much cheaper to leach
large volumes of whole CAT combs then the equipment you will need to leach large volumes of crushed material

There are 2 main reasons you can not leach all the PGMs out of CATs - even on a small scale --- one is the solubility of the metals them selves (the Rh in particular & to some degree the Pt) & the other reason is in understanding the make up &/or construction of the CAT combs them selves

This is all the time I have today - It is going to take several postings for me to completely cover this & I will do my best to post something everyday - or at the very most every other day depending on what I have on my plate to make my world go round - so please bear with me before asking questions as I intend to post this as complete as possible & most questions should go answered by the time I am done

Kurt

 

[arsenic123]

A very very much needed thread. Thanks a lot for starting the thread and do take your time. We are all eager to learn but not in a hurry :lol:

 

[kurtak]

Yesterday I ended my post by saying

(There are 2 main reasons you can not leach all the PGMs out of CATs - even on a small scale --- one is the solubility of the metals them selves (the Rh in particular & to some degree the Pt) & the other reason is in understanding the make up &/or construction of the CAT combs them selves)

CATs (as we know) contain 3 of the 6 PGMs Pd, Pt & Rh - these are the metals we are after & as anyone that has read Hokes - watched lasersteves video &/or spent any time studying this forum knows the solubility/reactivity with acids of each of these metals is different - Pd is more reactive then Pt & Pt more reactive the Rh & this play's a part in how effective leaching each metal is - even though these metal are very finely divided in the CAT wash coat (we will talk more about this wash coat as it also plays a part in leaching)

The Pd being the most reactive is the first to dissolve - in fact - because it is so finely divided "some" of it will go into solution almost instantly with nothing but HCL (no nitric no CL) normally HCL will not dissolve Pd but in this case being so finally divided it will --- this makes leaching the Pd form CATs the easiest of the 3 PGMs (take a CAT comb that is high in Pd like a bread loaf or a Kia - put it in a bucket & pour HCL into the comb & you will get an instant reaction with an instant color change & a positive Pd stannous test)

The Pt being less reactive does not dissolve as readily & therefore requires an oxidizer (nitric or CL) along with heat & time to leach it from the CAT - the heat & time playing a part in the success of the leaching

The Rh being the least reactive is the tough one - as anyone that has read Hokes &/or watched lasersteves video knows - Rh is only slightly dissolved in AR &/or HCL/CL - even when finely divided - which is why you can effect a separation of Rh from Pd & Pt with AR in finely divided PGM black that contains all 3 metals - with only "some" Rh going into solution

Also for anyone that has watched lasersteves video you will notice that when he leaches the CAT beads in the coffee pot he points out the pinkish color of the beads after leaching with hot AR & he mentions that this is good indication that all of the Rh was not leached out - so it is "very" difficult to get the Rh out by leaching & that is part of the value

So - just in the solubility factor of each of these metals - is "in part" what makes leaching CATs on a large scale difficult to do & do so at a profit - when leaching you will get most (not all) of the Pd - much of the Pt ( but again not all) & only some of the Rh --- Because this (solubility) is not the only factor - the construction (how they are made) & there make up (what they are made of) also play's a factor in leaching the metals out of them --- this is what I will address in my next post

Kurt

 

[arsenic123]

Yesterday I ended my post by saying

The Pd being the most reactive is the first to dissolve - in fact - because it is so finely divided "some" of it will go into solution almost instantly with nothing but HCL (no nitric no CL) normally HCL will not dissolve Pd but in this case being so finally divided it will --- this makes leaching the Pd form CATs the easiest of the 3 PGMs (take a CAT comb that is high in Pd like a bread loaf or a Kia - put it in a bucket & pour HCL into the comb & you will get an instant reaction with an instant color change & a positive Pd stannous test)

Kurt

so once we add HCl in the comb or even crushed cats and leave it for say 1 day we can take out the liquid/filter/zinc addition and precipitate the PD? Also wouldn't heating the HCL will increase recovery and save time?

Keep them coming. Very informative. Thanks

 

[Slayer-PGM]

Thank you Sir Kurt for the topic. it was really much needed. I do have some questions as well. I will ask once you finish writing it.

 

[kurtak]

Construction & Make Up Of CATs

Understanding how CATs are made & what they are made of is important to understanding why leaching CATs on a large scale for profit venture does not work & is not the way to go --- again - this is not to say that you wont leach & recover PGMs from CATs by leaching them because you will - you just will not get it all & the larger the volume of material you are trying to leach the more your recovery is going to drop off (as I said earlier - leaching a few CATs is one thing but large volumes are a whole other thing)

The comb its self is made of a cordierite ceramic which is extruded to produce the comb with its many very small vanes so as to create a very large surface area for the exhaust gas to pass through & contact without taking up a lot of space

So if trying to leach whole CAT combs - it is these "very" small vanes that in part cause a problem with trying to leach out the PGMs (we will get to problem of this ceramic when doing crushed material later) but with whole combs the problem is in getting "fresh" acid to the surface area inside these vanes inside the comb - the very small nature of the vanes them selves hinder the ability of fresh acid getting to the surface area & if they are plugged with carbon, oil or melt down or even stop leak that was used on a cracked engine block it will be hindered even more

Another words when you first start the process the vanes fill up (if not blocked) with fresh acid which "starts" the leaching process - it dissolve "some" of the PGMs that are exposed at the very surface of the wash coat (we will get to the wash coat later - which also poses a problem) but the acid soon becomes spent - so you need to get more fresh acid into the vanes in order for the leaching to continue

On a small scale (5 gallon bucket) this is not a big problem - you can simply reach in with a good pair of chemical resistant gloves & pick the comb out - shake out the old acid & set the comb back down into the acid so it fills backup with fresh acid

But on a large scale - say a 35 gallon drum with combs stacked in it - that's not going to work --- you can set your drums up so that you can drain the leach 2 or 3 times a day into a holding tank - then ad some air pressure to help push the old acid out of the vanes & then recirculate the leach back into the drum --- however (even with air pressure to help push out the old acid) you will not get complete draining of all the vanes &/or nether will you get complete refilling of fresh acid into all of the vanes
& therefore you will not get complete leaching of the combs - which in turns means you are leaving values behind in the leaching process its self

In the same way that these vanes cause a problem in the leaching process - they also cause the same problem in the washing process when trying to wash the leach out of them --- so you need to set up a whole other operation for washing wherein you can wash each individual comb to get the leach out - & you are going to produce a HUGE amount of "very" dilute leach in this part of the process --- this means a huge increase in the volume of your solution &/or waste - which means you now have to
go to an evaporation process to reduce that volume for an effective zinc drop &/or reduction of waste

(you can do your zinc drop with out evaporating but the more dilute it is the slower it is & if very dilute it will produce a very fine precipitate that can take "days" to settle & tends to go right through filters)

Now lets talk about the wash coat --- this is where the real problem is in trying to leach CATs & the reason why you can not & will not leach all of the PGMs out of CATs

The wash coat is the coating that goes over the surface of the ceramic comb monolith (or substrate) & is where the PGMs are - however - the wash coat is not made of PGMs - rather it is another substrate in which the PGMs are distributed - & again this substrate is a ceramic in its nature & there are different wash coats depending on manufacture &/or country of origin (domestic or import)

The vast majority of the wash coat is the ceramic with a very small amount of PGMs distributed through out it (as well as some other metals) besides having the PGMs (& other metals) in it - it also serves the purpose of giving the comb monolith a rough surface there by giving the monolith an even greater surface area for the exhaust gas to pass over

Aluminum oxide (alumina) titanium dioxide, silicon dioxide or a mixture of silica & alumina are the ceramics used for making the wash coats & besides containing the PGMs they can/will also contain cerium, iron, manganese, nickel &/or copper (I have processed some "import" CATs where I recovered more copper then PGMs)

Its the ceramic foundation of this wash coat that prevents the complete leaching of the PGMs from CATs & that is due to the fact the PGMs are encapsulated in the ceramic wash coat there by preventing full contact of the acids with the metals --- That - combined with the solubility factor (Pt needing "hot" AR or HCL/CL to dissolve it & Rh being only slightly soluble) as well as the problem of getting fresh acid to the surface to work on the metals - makes it near impossible if not impossible to effect
complete leaching of the metals & the larger the volume of material you are trying to leach the more the problem is compounded (or the less effective the leaching)

So just in the leaching there are problems that hinder/prevent you from recovering all of the PGMs (or all of the value) & we still need to talk about washing/filtering where more values will be lost

Next I will cover working with crushed material - then washing/filtering

Kurt

 

[kurtak]

Thank you Sir Kurt for the topic. it was really much needed. I do have some questions as well. I will ask once you finish writing it.

Thank you for waiting till I am done writing to ask questions - this will help me stay focused on the work of writing this & I believe most questions will be answered as I write & then when I am done I will be more then glad to answer questions that are not answered in my original/completed posting

Kurt

Edit : to also say that waiting to ask questions will help keep this thread in an easy to follow format for readers in the future

 

[kurtak]

Sorry I will not have time to post on this today as I had several PMs to answer - if you can hold off on asking questions till I am done posting I believe most questions will be answered as I post & it will allow me to stay focused on posting

Thank you for understanding

Kurt

 

[Lou]

Not to torpedo the thread, but I think the best practice is not to leach automotive catalytic converters--there's a big graveyard filled with companies that have tried that and it's clear pyrometallurgy is the best practice. It generates substantial volumes of dilute PGM solution and can produce hazardous waste.
Really, the easiest thing to do is to sell them to an accumulator who will take them to the arc smelter.

Another factor in incomplete leaching that should be mentioned is the form in which the metals are present. Most engines are designed to run more towards rich rather than lean so it's unlikely to see much in the way of Rh/Pd oxides (which are considerably less acid soluble than the metals themselves). This rich operation means some carbonaceous material (coking) is likely to be present. Fine carbon like this holds values and diminishes recovery. Also working against the poor souls leaching it chemically is that the substrate (wash coat and the gamma-alumina monolith) are designed to have a high porosity and surface area, all of which leads to values being adsorbed onto the surface.

Thank you for providing valuable details on how converters are made, the location of values, and some discussion on the leaching.

 

[kjavanb123]

All,

I am sorry to put my nose here, but for pyrometallurgy method, there is a link in forum that talks about keryolite copper added to crushed cats melted in an induction furnace, I will find and post the link here.

Regards
Kevin

 

[kurtak]

Now lets talk about crushed CAT material - with crushed material you still have the problems of the solubility of the metals them selves (especially the Rh) & the fact that the PGMs are "encapsulated" in the wash coat - but you don't have the problem of the vanes preventing fresh acid getting to the surface area which hinders the leaching

However crushed material presents a new problem that cause's the same problem - The comb needs to be crushed so that "all" of the vanes are destroyed which leaves you with a very fine powder of ceramic - ceramic is heavy & therefore settles very fast & it packs tight in a matter of a few minutes - so what you end up with (very quickly) is a hard packed ceramic sediment with all of the fresh acid on top

Therefore you need to keep the material under continuous stirring - including with starting the stirring & then adding the material so that all the material stays in suspension in order for fresh acid to stay in contact with all of the material for the time needed for it to do its work

This in turn creates another problem - you are starting with a fine crushed material which settles & "packs tight" in the first place (which makes it hard material to filter &/or wash to begin with) & just like wet sanding a hard surface is more effective then dry sanding - so it is with this material - the continuous stirring cause's abrasion which brakes the material down even more (finer) & by the time you have run it long enough to complete the leaching process - you now have an "ultra fine" material which is even harder to filter/wash

Wick filtering has been suggested - but think of it in the terms of the ultra fine clay used for sealing the bottoms of ponds to keep the water form leaching out - so you would need a "very large" surface area filter pan to spread the material thin enough for even wicking to work on the original solution let alone the subsequent following washes

Washing & filtering is the other problem with leaching CATs - the fact here is there is absolutely "no way" you will filter & or wash all of the leach out Cat material - & this is true whether it is whole CAT combs or crushed material - the ceramic is porous so absorbs leach that wont wash out & you reach a point in the washing of diminishing returns where in all you are doing is diluting your leach without recovering enough value to justify the time & water used in washing

So ad the fact that you can't recover all of the leach in the filter/wash process to the fact that you won't get complete dissolving of the metals in the first place & the fact that these problems become compounded (greater) with the larger the volume of material you try to process - & the fact is you will be "lucky" to recover the value of the CATs if sold as whole CATs - let alone make a profit

The other factor that needs to be mentioned in values lost to oils, carbon & other organics - you can NOT eliminate this problem with whole CAT combs even in a high temp oven because you can't get the heat &/or air to the center of the combs (due to the vanes again) so oils &/or organics become carbon - which absorbs values that you can't recover - & with crushed material it needs to be spread thin enough for the oxygen to turn all organics &/or carbon to ash - which ether means incineration of "many small batches - or an incinerator large enough (& hot enough) to stack many trays in

This covers the bulk of the problems of leaching on a large for profit scale - so next I will move onto smelting

Kurt

 

[kurtak]

Lou

you are correct on all points & that of course is the short story

What I am trying to do with this thread is explain in detail why leaching does not work &/or answer the questions of - can't you do this &/or what if you do that

as well as creating a thread that future members can be pointed to - to answer their questions when they come up again


Kurt

 

[kurtak]

Sorry - I wont have time to post today

Kurt

 

[kurtak]

I realized after posting the other day that there are a few other things I should cover about "large volume" leaching of CATs before I move on to smelting

Reaction vessels - we are talking large volume leaching so we are also talking large size vessels & they need to be both resistant the strong oxidizing acid's & heat - large glass or titanium or glass lined stainless steal vessels are vary expensive so for average home refiner the only other choice is plastic (like 35 or 55 gallon drums with the snap ring lids) but they are limited in the number of times they can be used because though they are chemical resistant at ambient temperatures they become brittle
after a few runs (4 - 5 maybe 6) when heat is applied (which is needed to dissolve the Pt & Rh) so you ether need chemical resistant heat elements to go in the barrels or a larger tank with heat elements for a hot water bath to bring your leach up about 180F

Then you need fume control - the gasses (which are what dissolves the metals) produced by AR are bad enough & if you are using HCL/CL you are producing CL gas - VERY nasty & can kill you if not well controlled --- this means a fume hood/containment built (with scrubber) for the reaction vessels to be placed in (when working on loading & unloading) as well as a vacuum scrubber to deal with fumes during reaction

Chemicals - you are going to need a huge amount of HCL - a 35 gallon drum (which you can only get combs from about 15 CATs
in) is going to take about 23 - 25 gallons of HCL & you don't want to dilute it because you are going to be diluting it enough
as is when you add the washes to it

Buy the time you are done with the washing you are going to double (or more) that - which means an increase in waste to be treated
after the zinc drop &/or evaporation to reduce the amount of waste

Now there is a thing called counter current leach which I am not going to go into details on - but basically its where you use the leach from the first batch to start the second batch but then it needs to go to a second & even third leach - first leach being zinc dropped after second batch & the second leach from batch 2 used to start batch 3 & so on --- this helps to get more full use of the acid but only reduces the amount of waste by a small amount because what you save in the beginning you lose at the end by needing to run the last batch through a third leach to get complete leaching of the last batch

You are going to produce about 50 gallons (or more) of waste to process the combs from about 30 - 45 CATs which in turn amounts to a lot of lime or sodium hydroxide to adjust PH & treat the waste --- & you are going to use up "a lot" of zinc to recover your PGMs

This is all of what makes trying to leach CATs on a large volume with any hope of making a profit prohibitive & why smelting is the better choice - & even smelting has its own problems - but can at least be done with some hope of making a profit on a large scale - which what I will cover next

Kurt

 

[kurtak]

Smelting is the way to go if you are going to process CATs on a large volume with any hope of making a profit

However smelting also has its own set of problems - the biggest problem being short crucible life - the reason for this goes back to the ceramic the CATs are made of - you need to dissolve away the ceramic in order to smelt out & collect the PGMs


The only flux (at least that I am aware of) that will do this & do it effectively is sodium hydroxide - so the problem is that as much as the sodium hydroxide is effective in dissolving the ceramic (alumina for the most part) but it also attacks the silica in the crucible - it is also hard on the refractory in the furnace which leads to eventual relining of the furnace


They do make crucibles of inconel 601 for long life high temp application designed for use with strong alkali (like sodium hydroxide) I have only been able to find them in small lab size but if you could find them in large size (#40) in the long run they would be worth the investment as I am sure they would be quite spendy

The thing here is sodium hydroxide is dirt cheap when you buy it in bulk - & you need a lot of it - about 3 times the amount of CAT material you have - it only takes about 2 - 1 (flux - CAT material) to actually dissolve the ceramic but you need the additional flux so that it runs fluid enough in the crucible to get good circulation in the crucible for your collector metal (silver or copper) to be effective in collecting the PGM

Though copper can be used as your collector metal silver is a much better choice (& you need fair amount of collector metal) & this is due in part to the dynamics that take place in the crucible - as well as in the parting of the PGMs from the collector metal later

I will talk about the collector metal & the dynamics of what takes place in the crucible next

Kurt

 

[kurtak]

Ether copper or silver can be used as your collector metal & although copper is much cheaper then silver (there by making it a considered collector metal choice) silver is "by far" the better choice for your collector metal - this is due to what takes place in the crucible during smelting as well as the properties of the two metals them selves

First lets look at what we are trying to accomplish in the smelting - we are trying to collect a "VERY" small amount of PGMs from a "VERY" large amount of ceramic matrix in which the PGMs are literally locked up in - we are talking "less" then 10 grams of PGMS per kilogram of ceramic (so that is .00X kg PGMs per kg CAT material) so the first thing that needs to happen in the smelt is to dissolve away the ceramic to unlock the PGMs for collecting which of course is served by the NaOH (sodium hydroxide) as your flux (your not melting the ceramic - you are actually dissolving it in a reaction as the NaOH becomes molten)

It is important to understand that you want a finely granulated well mixed load (flux/CAT material/collector metal) going into your crucible to insure optimum collection of your PGMs - this includes having a fine granulated collector metal - the finer & more homogeneous the mix of the load going into the crucible the better the collecting of the PGMs is served & you want plenty of collector metal (the more metal in contact with the ceramic matrix the better the collecting)

The melting point of NaOH is very low (318 C) where as the melting point of your metals is very high which is why you want fine granulated collector metal well mixed in your load - NaOH is also "very" fluid when it becomes molten (although it becomes thicker as it dissolves the ceramic) so if you use large pieces of collector metal they would simply fall to the bottom of your crucible as the flux begins to melt without being molten to collect the PGMs

Its not that finer metal melts at a lower temp (though this is true in a "small" way) its that it takes less energy to melt a small piece of metal as opposed to melting a large piece of metal & your heat (energy) starts at the walls of the crucible & works its way into the center of the load - so you want your metal to start melting at the wall so it can preform the task of collecting as the melting moves towards the center of the load

There are a couple of reasons why silver is a far better collector metal then copper - first is that lower melt point metals act as a solvent to higher melt point metals there by lowering the melt point of the higher melt point metal - molten metals also act like a sponge in that they actually suck metal in (you can watch this happen if you have ever moved a large pool of molten metal around in a melting dish to pickup a non-molten bead in your dish)

Also lower melt point metals are "more fluid" then high melt point metals & the more fluid the better the sponge effect - another words the more fluid the better they are at collecting & this is what makes silver a better collector then copper

The melt point of copper is 1084 C & the melt point of silver is 961 C which is 123 C (or 221 F) lower then copper so it melts before copper to become the collector & it is more fluid - "much" more fluid which means a better sponge effect for collecting

Having good circulation in the crucible during the smelt is also important so I will cover that next

Kurt

 

[g_axelsson]

For deeper reading about reaction between molten lye and various metals, here is an interesting article.

View attachment The Reactions of Molten Sodium Hydroxide with Various Metals. - ja01601a004.pdf

Göran

 

[kurtak]

Good circulation in the crucible during the smelt is another important factor that needs to be considered - heat is energy - energy cause's excitement - excitement cause's movement & when fluid enough movement cause's circulation

Therefore - you need your flux (which is now becoming slag as it becomes molten & starts to dissolve the ceramic) needs to run fluid to insure complete collection of the PGMs

Molten NaOH its self is "very" fluid - however as it starts to dissolve the ceramic it becomes quite thick (turning to slag) therefore you need to use an excess of NaOH so that your slag runs fluid - if not your metals as they start to melt will melt into small balls that hang up in the slag with a barrier of slag around them preventing the sponge effect from doing it job of collecting (this is going to happen anyway - but you want to prevent the problem as much as possible)

Again this all starts at the walls of the crucible & works it way into the center of the load with things more fluid at the walls of the crucible less fluid at the edges of the load so your flux/slag needs to start running fluid enough so that as small balls of metal start to melt they can move (circulate) so they can bump into one another as well as circulate into the outer edge of the load so that larger balls are formed creating a larger surface area of molten metal to preform collection & become heavy enough to run down through the slag & pool at the bottom of the crucible

Now as you near the end of your smelt circulation becomes the most important - this is because it is near the end of the smelt that most of your flux/slag has become molten & most of your metals are pooled at the bottom of your crucible so you have a smaller amount of metal that needs to go molten yet in a greater amount of slag therefore forming small balls of metal distributed in a large amount of slag & therefore the more fluid the slag the better these small balls will circulate to the bottom of the crucible to contact the pool of metal at the bottom & be collected in the pool (there will always be some small balls that stay hung up in the slag - but the more fluid the slag - the better the circulation - the better the collecting) --- (the ball metal that hangs up in the slag is recovered later)

If copper is used as a collector metal this circulation is even more important which means a more fluid slag which in turn means the need for more flux - because copper is less fluid then silver when molten & thereby having less of a sponge effect it needs to have better circulation in order to make better contact to effect the collecting

There are just a few more things I need to cover before finished which I will cover in my next posting

Kurt

 

[Lou]

NO don't use NaOH for melting PGMs or doing a fusion on catalyst. More dangerous than aqua regia and less effective.

How this is done industrially is with cryolite as the flux and fine chopped copper wire inside of a furnace you could drive a small truck into.

 

[kjavanb123]

Kurt,

Thanks for your detailed post on CATs. As you mentioned also, I read on the forum that indeed cryolite is what they use for flux, and as for mixing the CATs powder and collector metal, I think using an induction furnace would be best option. I cant wait to try this smelting on my induction furnace when finally arrive.

Thanks
Kevin