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.
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.