How to Improve Phosphoric Acid 85% leaching of Natural Titanium Rutile without increasing impurity?

Question

Using magnetically separated natural titanium rutile from a mine in South Africa to leach titanium for oxide production using H3PO4 85%. (The reason for using H3PO4, is that it separates out any Fe from the leachate making the Fe residue soluble and allowing it to be washed out with water after gravity separation.) After leaching the rutile sand under reflux at 270 C° twice, the rest of the natural rutile sand left over does not produce leachate any more with 100% H3PO4(85%).

This is a very high grade natural rutile with 93% Titanium. The Natural Rutile on a white plate.

After some research and experimentation, I found out that the Natural Rutile that wont leach has very strong bonds that the strength of the H3PO4 simply can't break and oxidise. It was suggested that I try adding 5 too 10% of Nitric Acid to the H3PO4 which would help break/crack the rutile and allow leaching to commence. The problem with this was that the Nitric Acid lowered the boiling point of the H3PO4, and also increased the fume/evaporation rate of the acid sol. I don't have a pressurised reaction vessel or chamber for the process and under simple reflux this now became a problem.

It was suggested to use 5% Nitric Acid(65%) and 10% Sulfuric Acid(98%) with the Phosphoric Acid(85%), the Sulfuric Acid brought the boiling point up nicely and almost stopped the Nitric Acid from fuming out (Only slight fuming that did not escape the condenser easily), this worked very nicely, but the new problem is that prior to adding the Sulfuric Acid, all my amorphous leachate after washing was a brilliant white. After adding the Sulfuric Acid the washed leachate is a white with a very slight yellow tinge to it, which I can't seem to remove and get it back to the brilliant white that is required for the Ti oxide.

H3PO4 only leachate after separation, which is a brilliant bright white after washing and drying.

The leachate color after adding sulfuric acid

Can any of you suggest an effective way forward that does not require specialised acid or chlorine reaction chambers? Or a simple chemical route to clean the yellowish leachate and make it a brilliant white? I am open to any suggestions.

Thank you for your time and consideration.

Answer 1

Here is something that I read in a hydrometallury text (see Hydrometallurgy in Extraction Processes, Vol I, by C. K. Gupta and T. K. Mukherjee, Page 15) as an aid in dissolving ores based on the science surrounding so-called activity coefficients.

It centers on simply increasing the effectiveness of an acid dissolution by actually using the same (or more dilute) parent acid combined with a related salt at high concentration. So, for example, in place of strong $\ce{HCl}$ employ strong or weaker $\ce{HCl}$ with say $\ce{FeCl3}$ (where the higher valence chloride ion is apparently more effective than say $\ce{FeCl2)}$.

In the current context, try working with $\ce{H3PO4}$ at varying concentrations and a select phosphate salts in high concentrations.

As such, you may be able, as per your question, actually "Improve Phosphoric Acid 85% leaching of Natural Titanium Rutile without increasing impurity".