Hygroscopic sweeps

[4metals]

A few weeks back an old client from Asia contacted me because he is having issues with getting a constant weight on a sweeps sample. This material, despite being dried, gains weight when exposed to the air. To the point where a sample weighed out on the balance is visibly gaining weight. (well you cannot actually see it, but you can watch the analytical balance increase in weight) This poses a problem for them because it represents a lot of material his clients ship him. Apparently the material is refining residues mixed with waste hydroxides which has been incinerated, crushed and sifted before he gets it. The typical solution is to dry it to a constant weight in an oven in the lab and cool it to room temperature in a dessicator. Sometimes this works long enough to weigh out a sample and flux it for assay. But he said within seconds of his 5 gram sample being placed on the balance the weight increased.

I had heard of this issue many years ago in the '80's and a Romanian assayer I worked with in NYC told me a trick he had used for similar problems when he worked in Romania. The fix was to blend the entire lot with two ingredients which stabilized the weight. Actually it stabilized it to the point where later lab heating and desiccation did not change the weight.

The magic fix was adding DOT 5 silicone brake fluid and stearic acid to the sweeps and blending it thoroughly. DOT 5 silicone (as it is called here in the USA) is a silicone based fluid that is hydrophobic (which means it repels water). Stearic acid is a saturated carbon long chain fatty acid which is insoluble in water. Together these two ingredients worked to do the trick for my Asian friend as well as he indicated on a
Christmas card I just received from him.

The quantities to start with are 3 fluid oz of the DOT 5 and 3 ounces of stearic acid for every 1000 oz of sweeps. (68.5 pounds). This ratio always worked for me in my jewelry sweeps days but if you have a severe case of hygroscopic sweeps, I'm sure an increased quantity of both chemicals could solve it.

This may be a problem for most of us that doesn't need fixing so many may ignore this post. But maybe someone out there can find this useful and that is why I posted it.

 

[Pastorman75]

This is a great insight trick @4metals !!
Thx for this precious knowledge u shared here!
Often the hygroscopic phenomena is due two causes:
A) chemical adsorption of water from air
B) microscopical shape of particles that encourage condensation of water in air (radius is relayed to minimal "surface tension")
I had these behaviour in ashes come out from incinerator in a big watch manufacturer in Switzerland! It happened into ash mixer before sampling. In that application we just seal better the closing lip of machine and change the working procedure because they charge ashes still warm.

Many thanks again for you experience!! Bye

 

[goldshark]

Could another cause be from the hydroxides absorbing CO2 from the atmosphere?

 

[Yggdrasil]

Could another cause be from the hydroxides absorbing CO2 from the atmosphere?

Many Hydroxides is hygroscopic.
And will soak up water from the air.

Edit to correct text

 

[nickvc]

4metals couldn’t your client or others simply weigh a sample out and then simply dissolve the values out and then simply cement them out with zinc dust? My thinking been that you would have metals to melt and then simply assay the button or if they have access to an AA machine get a reading from the solution.

 

[Martijn]

A few weeks back an old client from Asia contacted me because he is having issues with getting a constant weight on a sweeps sample. This material, despite being dried, gains weight when exposed to the air. To the point where a sample weighed out on the balance is visibly gaining weight. (well you cannot actually see it, but you can watch the analytical balance increase in weight) This poses a problem for them because it represents a lot of material his clients ship him. Apparently the material is refining residues mixed with waste hydroxides which has been incinerated, crushed and sifted before he gets it. The typical solution is to dry it to a constant weight in an oven in the lab and cool it to room temperature in a dessicator. Sometimes this works long enough to weigh out a sample and flux it for assay. But he said within seconds of his 5 gram sample being placed on the balance the weight increased.

I had heard of this issue many years ago in the '80's and a Romanian assayer I worked with in NYC told me a trick he had used for similar problems when he worked in Romania. The fix was to blend the entire lot with two ingredients which stabilized the weight. Actually it stabilized it to the point where later lab heating and desiccation did not change the weight.

The magic fix was adding DOT 5 silicone brake fluid and stearic acid to the sweeps and blending it thoroughly. DOT 5 silicone (as it is called here in the USA) is a silicone based fluid that is hydrophobic (which means it repels water). Stearic acid is a saturated carbon long chain fatty acid which is insoluble in water. Together these two ingredients worked to do the trick for my Asian friend as well as he indicated on a
Christmas card I just received from him.

The quantities to start with are 3 fluid oz of the DOT 5 and 3 ounces of stearic acid for every 1000 oz of sweeps. (68.5 pounds). This ratio always worked for me in my jewelry sweeps days but if you have a severe case of hygroscopic sweeps, I'm sure an increased quantity of both chemicals could solve it.

This may be a problem for most of us that doesn't need fixing so many may ignore this post. But maybe someone out there can find this useful and that is why I posted it.

My first thought was to dissolve the hydroxides with HCl, rinse and dry the residue? I am probably oversimplifying the case. Not all of these particular hydroxides may dissolve in HCl.
Thinking about it, this would also leach some metallic base metals and your starting weight will be smaller in stead of rising, but constant.
Raising the pH of the HCl wash would result in hydroxides forming again> can be measured to some extent in a controlled environment?
If not, hat does make a good assay result nearly impossible. No way to tell what was the 100% you started with.

 

[4metals]

All of this type of low grade residue is smelted with copper. Since it comes into the shop in multi one ton gaylords, it is impractical to chemically leach it prior to smelting. (Their shop is primarily set up to smelt and refine copper) Their goal, is to be able to accurately fire assay the material and, by treating the incoming lot, determine a stable settlement weight. And I’m sure they add a pretreatment charge to cover the cost of chemicals and labor.

Since these are low grade sweeps running as high as 10 oz per ton, one might say a few pounds of water won’t matter much, which I originally said, but when I used this technique on jewelers polishing sweeps that were hygroscopic the sweeps ran as high as one ounce per pound, so settlement weight mattered.

 

[Lou]

We have seen this phenomenon on activated carbon containing solvents that were hygroscopic (think long chain polyols). Their vapor pressure was much lower than water so they would not dry at 130 C to a constant mass. They were also inconsistently present in the catalyst bid. Our solution was to just rinse 30K lbs of really expensive catalyst with water and dry it.

 

[4metals]

Our solution was to just rinse 30K lbs of really expensive catalyst with water and dry it.

Ugggg! Then there was the fun part of dealing with the rinse water.

 

[orvi]

We have seen this phenomenon on activated carbon containing solvents that were hygroscopic (think long chain polyols). Their vapor pressure was much lower than water so they would not dry at 130 C to a constant mass. They were also inconsistently present in the catalyst bid. Our solution was to just rinse 30K lbs of really expensive catalyst with water and dry it.

Adsorption of various organic substances to large surface areas of various carrier materials is very painful. I encounter this phenomena on a daily basis, since I perform heterogenous reactions very frequently. Zeolites (molecular sieves) and activaated carbon with whatever catalyst on the surface... Just as examples.

We had one story in our lab which happened many years ago including molecular sieves. As we needed very dry solvents to perform specific reactions, we needed to optimize the process of drying (work time vs. cost). Distillation of tetrahydrofuran or methanol/ethanol from sodium was effective, but tedious, wasteful and hazardous. On the other hand, molecular sieves worked like a charm, no distillation needed, just buy, pour in the solvent and let it sit for few days. Only disadvantage was that good sieves were pricey...

There was a "genius" idea - why won´t we recycle the mol sieves ? Just put them in the drying oven, crank it to the activation temperature (for these around 400 °C), solvent will evaporate and it will be done.
So they did exactly this. And set off two firealarms in matter of 20 minutes
Solvents bind to the surface and pores of these zeolites very very strongly, and it is practically impossible to evaporate all of the solvents out of them. And as temperature rise, solvent start to decompose and smoke... Rendering this operation absolutely unusable

 

[Lou]

Adsorption of various organic substances to large surface areas of various carrier materials is very painful. I encounter this phenomena on a daily basis, since I perform heterogenous reactions very frequently. Zeolites (molecular sieves) and activaated carbon with whatever catalyst on the surface... Just as examples.

We had one story in our lab which happened many years ago including molecular sieves. As we needed very dry solvents to perform specific reactions, we needed to optimize the process of drying (work time vs. cost). Distillation of tetrahydrofuran or methanol/ethanol from sodium was effective, but tedious, wasteful and hazardous. On the other hand, molecular sieves worked like a charm, no distillation needed, just buy, pour in the solvent and let it sit for few days. Only disadvantage was that good sieves were pricey...

There was a "genius" idea - why won´t we recycle the mol sieves ? Just put them in the drying oven, crank it to the activation temperature (for these around 400 °C), solvent will evaporate and it will be done.
So they did exactly this. And set off two firealarms in matter of 20 minutes
Solvents bind to the surface and pores of these zeolites very very strongly, and it is practically impossible to evaporate all of the solvents out of them. And as temperature rise, solvent start to decompose and smoke... Rendering this operation absolutely unusable

You should have tried with steam. Steam has been used to regenerate sieves and activated carbon to great success.

 

[orvi]

You should have tried with steam. Steam has been used to regenerate sieves and activated carbon to great success.

Yeah, I know now. I "researched" this toppic among organic chemists on various other forums, and steam treatment, or repeated water washing is apparently common and effective practice. However, this one "smoky" experience was enough to persuade the persons in charge that no other "trials" will be tolerated. I somehow understand them, they are responsible for safety measures, and do not want to take any chances...

However, this "wonderful" idea wasn´t mine. I pointed my doubts about it before some bunch of other guys did what they did. I left the school and I am now working for private company, where this is not an issue anymore. Now, I am dealing with other challenges.

I included the story just to spread awareness