How to acid-wash sand?
I have just started working with sand columns and was also surprised that I could not find any valid and trustworthy guidelines or process breakdown for acid-washing sand.
However, I found this paper, which may answer some of your questions. In short, acid washing (or leaching) is supposed to remove impurities (mostly iron impurities) from the sand. 

From the introduction of the paper: 


  Iron is one of the most troublesome impurities in silica
  sand because it impairs the transparency of high-quality
  glass, e.g. for tableware, and optical glass; it impairs
  transmission in optical fibers; it adversely affects the production
  of pure silicon products, e.g. silicon metal for
  photovoltaic application, silicon carbide. One of the main tasks in the purification of silica sand
  is to further reduce iron compounds to a required level
  by a highly efficient method.


In table IV in the results section you can see a comparison between several acids with regard to their efficiency in removing iron and aluminum impurities. According to the data, sulfuric and phosphoric acids should be preferred. From what I understand, however, diluted HCl solution may be preferred by some, probably due to better price and availability, but that's just an assumption.

As for the procedure itself, I heard about adding a solution of 1% HCl to the sand, stirring and letting it stand for a few hours, and then washing with distilled water and decanting ~10 times, and then drying in an oven overnight.  I don't have any valid reference for it, though.