Salt Roast - A summary
10th August 2019
1 September 2019
I’m not sure how familiar everyone is with the Salt Roast Process, why we use it and what it does or indeed how it is affected by different inputs so I’ve put together a brief description for everyone. [Interested readers may subsequently wish to read Alfacomp's article on the Vametco plant visit from earlier this year.]
Basically, as we get it out of the ground our Vanadium is in the rocks, but not evenly spread through the rocks, it concentrates in a crystalline matrix along with Iron and Titanium in a mineral formation that we call titaniferous-magnetite or just magnetite. When we get reports of the Magnetite content of our assets it is usually quoted as V2O5 in magnetite, so some may be wondering if it is already V2O5 why do we go to all the hassle of the salt roast process to make V2O5, isn’t there an easy way to just separate it out?
Image by Bushveld Minerals
The answer sadly is no there isn’t. The Vanadium is held in a complex crystalline matrix in which Iron, Titanium, Vanadium and substantially Oxygen atoms are rigidly packed together. Importantly there are a few others in there too notably Aluminium, Chromium, Calcium and Manganese in much lower abundance but the presence of these “impurities” is quite important later.
Way back in the stone-age, cavemen probably first encountered “metal” as a strange shiny, malleable “rock” that they found at the bottom of their Fires. They had accidentally “smelted” metal from metal ores by having used the metal ore rocks in and around their fires as containment or for cooking (hot rocks packed with bound food and buried is an ancient cooking method). It was from these observations that the first metal production almost certainly emerged. So why can’t we just “Smelt” out our Vanadium. Well as it happens – we can and this is actually part of the process by which most Vanadium worldwide is produced, as a secondary product in the Iron and Steel Industry.
The typical titanomagnetite smelting operation entails (very simplistically) feeding the titanomagnetite with coal into a pre-reduction kiln. The prereduced material is then conveyed to a furnace where it is smelted to produce a vanadium-bearing pig iron metal and a by-product titania (TiO2)-rich slag. The vanadium-bearing pig iron is tapped into a shaking ladle where it undergoes oxidisation of vanadium into the slag phase. The iron metal from the ladle is tapped off for the production of steel.
The remaining vanadium slag is then tipped out, cooled then crushed and the iron entrainment is removed by means of an overhead magnet leaving Vanadium (not very pure) behind. More typically that Vanadium Slag (at anywhere between 20%- 33% Vanadium) is put through a salt roast process as we are about to discuss instead. It’s worth pointing out that Vanadium made through this classic “secondary” production process has a recovery of about 55-60%, so very wasteful compared with our Primary production method.
Right, onto Salt Roasting……. Those rocks containing in part the magnetite matrix are crushed up and then passed over magnets to pull off the magnetite part leaving behind the other rocks. This concentrate is then ground down to a consistent particle size typically somewhere between 30 and 200 microns (um) the actual size is something which is established for each salt roaster independently and is a function (in part) of the relative contents of each element.
Image by Bushveld Minerals
Most chemistry is surface area related and so the smaller the grind, the bigger the relative surface area, now go too small and the effort you put into making it small also generates energy which in turn causes unwanted oxidation of the surface which impedes the chemistry you want to happen later. Too big and the surface area to volume is too big to get all the chemistry you want going and you get lower yields.
Anyway, once ground up, the Magnetite concentrate is mixed with Sodium salts – why Sodium Salts ? Because we are endeavouring to make something that will be soluble in water so we can dissolve it away from all the things we don’t want and separate it away. Now our target is to create SodiumVanadate which is a molecule containing Sodium, Vanadium and Oxygen. Not that it matters but the formula is 2NaVO3 and the reaction we use to create it is shown below: (remember the Vanadium is in the form of V2O5 locked into the Magnetite Matrix).
Na2CO3 + V2O5 - - > 2NaVO3 + CO2
There are other chemicals formed along the way of course with the Iron and Titanium in the Magnetite and also with the other impurities we mentioned. Some of these are themselves “Vanadate” salts i.e. Vanadium and Oxygen containing salts of the other elements. These however are insoluble in water but the fact that they are formed at all is a problem because obviously that wastes Vanadium that we can’t get out later. As it happens one such Impurity is Calcium Vanadate which by a happy coincidence undergoes a secondary reaction in the presence of Sodium salts to form yet more Sodium Vanadate (I won’t bore you with the reaction).
Image by Alfacomp of The Bushveld Perspective.
Importantly now, these other reactions creating other (unwanted) chemicals can to some extent at least be controlled by something as simple as the temperature of the Kiln. All chemical reactions require some sort of energy to kick them off and by holding the temperature of the Kiln in a place that is optimal for our preferred reaction between Vanadium and the Sodium Salts we can optimise the process. I’ll come back to this later.
OK so that is the “Roasting” part of the process, next come the “Leaching”. Basically, the Sodium Vanadate (2NaVO3) is dissolved in water. Think of this like separating sugar from sand. If you had a mix of sand and sugar, you could add water, dissolve the sugar leaving behind the sand. Filter off the Sand and then evaporate the water away to get your sugar out of the sugary water solution. Ta-Da separated. Well this is exactly what we are doing.
The insoluble salts are left behind and our Vanadium containing Sodium Vanadate is dissolved in the water. The water is then evaporated down to form something we call the pregnant solution, this just means a concentrated solution. Unlike my sugar example, we don’t just evaporate off the water, it is a more efficient extraction to now throw in some Ammonia salts to change our soluble Sodium Vanadate into insoluble Ammonium Vanadate which just falls out of the solution as a precipitate. The most common and abundant Ammonium Vanadate salt formed is Ammonia MetaVanadate which you will have read about referred to as AMV.
Ammonium Metavadate produced at Vametco. Image by Alfacomp of The Bushveld Perspective.
The AMV is a saleable product and there is a market for it, but we typically go through a couple more basic chemistry steps to create out end products either the V2O5, V2O3 (or at Vametco a mixture of the two we call Modified Vanadium Oxides – MVO).
Modified Vanadium Oxides – MVO. Image by Alfacomp of The Bushveld Perspective.
Now all that stuff before was to give a background to something I have been looking into which I think is quite important for us at BMN. You will recall that I said that the optimisation of the process to get out the Vanadium was dependent on a number of things not least of which was the impurity profile and make-up of the Concentrate going into the process.
Oh by the way remember I said that the secondary process was quite wasteful and could only typically achieve a recovery of 55-60%. Well our primary production is capable of achieving over 80% recovery of the Vanadium at optimised settings. Anyway you will I hope be able to see that if we have significantly different inputs from our in ground assets i.e. Vametco/Brits & Mokopane then we would need to run different set ups reducing our flexibility.
Bushveld Projects - VTM Impurity Breakdown
What this means for us is that with some sensible Run of Mine Management our processes at both Vametco and at Vanchem can run essentially to the same set-ups and that where ore comes from is then far less important. Also this gives us incredible flexibility and opportunity. Vanchem opens the doors to a new and exciting world of diversified Vanadium products as well as secured manufacture across multiple sites and the chance to schedule maintenance and output to an optimal business plan. I can’t wait.
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