Assuming that you are discussing a typical thiobacillus ferrooxidan you should be controlling the pH between 2.0 and 2.5 with the temperature about 28-35C.

However, when iron arsenic sulphide is present you will need a second treatment tank and a neutralization process at higher pH levels.

The addition of limestone occurs the As precipitation (during biox step) which doesn't permit to us a suitable measure of As in solution. Is there any other reagent capable to replace the limestone?

I dont know if this will help, but when I was processing flotation concentrate by BOIX, we used a weak calcrete slurry instead of limestone. This is not as aggressive a pH modifier, and reduces the risk of localised areas of high pH in the slurry that might result in arsenic precipitation.

This is the option I was looking for. Please tell me if you use it in a industrial or pilot scale ?

I have used it in both. I tried lime in a BIOX pilot plant, bur found that the lime immediately caused a precipitate on the surface of the lime particles due to slow mixing. We used caustic but it was too expensive. With the operating plant, we used in a calcrete grinding mill and ground calcrete to a slurry -100um (nominally) and diluted and added on a timer. and this worked well.

Agree, calcrete slurry gives the best ability to control pH within tight limits, As is mostly fixed as insoluble As at pH 2.0. If NaOH is used to neutralize BIOX residue before cyanidation, As can become As soluble at high pH (not good for tailings), and NaOH also retard Au adsorption to activated carbon.

I once worked at the same type of plant where we made a change to reduce the % sulfide target in flotation by a couple of percent. There was enough acid consumption in the carryover gangue minerals so we didn't have to add calcrete at all. This was beneficial as occasionally the operators would overdose calcrete with the manual dosing system. I don't know if that strategy would be applicable to your flowsheet.