The following sources (Source 1, Source 2) both cover the extraction of Vanadium (specifically Vanadium Pentoxide, $\ce{V2O5}$) from petroleum residue (petcoke and flyash respecitvely).

However when looking closer at the processes describer there seems to be some differences which I cannot fully explain. In the process described by Lucinda C. Paris-Marcano in the patent linked above, the petroleum coke is mixed with a sodium hypochlorite solution and concentrated nitric acid solution, which after filtration is referred to as Solution 1. It goes on as follows:

Solution 1 having a pH of about 3.0 is neutralized and alkalinized with a 10% $\ce{NaOH}$ solution to obtain a mixed solid precipitate containing essentially all the $\ce{Ni, Co}$ and $\ce{Fe}$ extracted from the flexi-coke. This precipitate is filtered, washed and preserved for further $\ce{Ni}$ or $\ce{Co}$ recovery.

The second filtrate, Solution 2, contains essentially all the vanadium extracted from the flexi-coke, in the form of sodium vanadate.

Solution 2 is heated to boiling and then acidified by adding carefully nitric acid up to pH 1-2. Red vanadium pentoxide ($\ce{V2O5}$) precipitates.

In the second source, Dr. Mohammed Al-Zuhairi lays out a similar process, where a Vanadium laden solution is obtained by treating the flyash with sodium hydroxide ($\ce{NaOH}$). This solution is then treated as follows (no attempts made to clean up the original english):

50 ml was taken from Vanadium-laden solution and poured into conical flux to equivalent acidity as the vanadium-laden solution was of (pH 12) when drops of Nitric acid of (0.5 M) concentration were added while continuing stirring process by using magnetic stirrer with (20 r.p.m) sinning to make solution acidity at (pH 2) by adding (22 ml) of Nitric acid for each (50 ml) of the V-laden solution. Then, precipitation process was done by adding ammonium hydroxide of (4 M) concentration while continuing stirrer to obtain ($\ce{NH4VO3}$, ammonium metavanadate) was kept in ceramic container and then it was put in the oven for calcination process under (690 °C) for 30 minutes [Daniel Kittelty, 2001]. The calcination resulted in (V2O5).

My question:

As far as I understand, both processes start off with a solution of sodium metavanadate ($\ce{NaVO3}$). Why then does $\ce{V2O5}$ precipitate when $\ce{HNO3}$ is added to a boiling $\ce{NaVO3}$ solution in the first example, while the other process requires the addition of ammonium hydroxide and a rather energy intensive calcination?

As a curious amateur I'm sure I'm missing something here, but an explanation as to what's going on and why the methods differ as they do would be highly appreciated.


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