I am trying to prepare a homogenized salt mixture from 2 salts of certain proportion. Lets say that the melting point of the salts are about 300 °C & 400 °C. It is a bit difficult for me to heat above 400 °C. Hence for preparing a homogenized salt mixture, is it mandatory to heat the mixture well above the melting point of both the salts? Before heating the mixture, if we do proper mixing by ball milling, is it useful to reduce the temperature to which it should be heated?
It is not mandatory . If you mixed both salts well and keep in the temperature above liquidus line. However, to get homogenous liquid ,a prolonging heating is expected.
I really do not see why solid salts require heating for good homogeneous mixing. I learnt ,from industrial chemistry, that mixing solid fertilizers (such as an ammonium phosphate + ammonium nitrate + potassium chloride) is done by combining them in proper amounts inside an electrically-driven special mixer. The resulting mixture is subjected to a test on small sample and then the mixed fertilzer is bagged.
Yes; it is mandatory. It is only at their respective melting point s that the crystal structures of the two salts will collapse COMPLETELY ( not before ) and they become liquids .Now It is as good a state as if we have two liquids,i.e. the two solids will be completely miscible when they become liquids. Their mixing will be completely homogeneous to result in ONE PHASE of two solids; which ,in fact, is required during the conduct of the experiment.
It is not mandatory . If you mixed both salts well and keep in the temperature above liquidus line. However, to get homogenous liquid ,a prolonging heating is expected.
Just curious, why are you having trouble getting over 400 C? What is your heating medium? You could try using molten tin as a heating medium, and high-temperature electrical heating tape could be used as a heat source. Just be sure to use a lot of insulation to prolong the life of the tape.
After this, look at a phase diagram for your salts, if one is published. In some cases the melting point of the mixture is between the melting points of the salt, and in some cases the melting point might be above BOTH salts, depending on the thermodynamics of the solution (note Kurcharski's comment about the liquidus line).
Adding on to what Kucharski Marian and Michael Nolan said, in case you are not familiar with phase diagrams, this is really where the answer lies and is a materials science and engineering topic worth reviewing. There are databases of phase diagrams that may cost a lot, but the content is valuable. See, for example, http://www.nist.gov/srd/nist31.cfm If you have access to a good library you might find the information there, or perhaps you can find an old paper copy of this classic reference for less money.
Depending on the chemical nature of the components, you can have a mixture that melts at a temperature below that of either component, as well as above or in-between. In my previous work with small quantities of salts whose mixture had a lower melting point than either of the components, I did not have to use ball-milling. Simple mixing of the grains was enough, and the melting temperature of either pure salt did not need to be reached as melting occurred at a lower temperature even though there was certainly granularity and the particles were not extremely fine.
The nature of the solids and the force in the ball mill, may locally achieve the concentrations given by the liquidus line, at a lower Bulk temperature. Processing at a lower temperature often results in a longer time to reach a homogeneous bulk mixture.
If your salts do not have nearly identical crystal structures, their phase diagram will probably contain a eutectical point, which is at a temperature below both melting temperatures. Please follow Tanya's advice and look up the phase diagram.
But what do you do with the molten mixture? If you let it cool down, and if you are not exactly at the eutectical composition, the resulting solid will have an inhomogeneous composition.
So the proposal of using a ball mill or electrical blender (at temperatures below the melting and eutectical points!) is worth considering.
- Badges
- Science topic