Assuming the waste was from a chloride solution.
Several salts can be white, some of the more common ones you may be dealing with like lead, silver, and copper.
Very hot water will help to put lead chloride into the wash solution, so you can wash lead out.
leaving copper salts and silver salts, elemental gold, or any other valuable metal.
Copper I chloride (a fairly insoluble white powder), will form soluble Copper II chloride with fresh HCl and air (coloring the solution blue, green, or almost brownish-black depending on dilution and acidity, as well other factors as to what valence or concentration have you.
Silver will not be more soluble in hot water, and very little (almost non will go into solution with the HCl acid and air, although it is fluffy and needs time to settle (while you are washing the lead out with hot water).
Lead salt tends to form needle-like crystals and can look more clear (than white), whereas silver chloride (unexposed or reduced by light to dark crystals)) is more white and light and fluffy than the lead chloride salts.
Once washed of free acid the white silver chloride can turn a violet-black from exposure to the sun.
To save on generating excessive toxic waste, you can reuse the hot water used to dissolve more lead salt, removing the lead chloride by letting it cool precipitating and decanting the solution from the lead crystals, and reheating it for reuse again, using two different portions of water will speed the process as when one is hot the other is cooling...
copper metal will only go into solution as ions if it is oxidized or made to lose electrons where something else in a solution that wants those electrons to be reduced itself, the acid will oxidize or take electrons from the copper is oxidized giving up electrons to the acid, the copper colors the solution with ions, the acid is reduced the chloric acid gains electrons it shares with copper to form a salt of chloride ions in solution.
Copper I chloride can be put back into solution (oxidized) with electrons from HCl changing the valence of the copper ions or salts, here again, the acid is reduced in the chemical reaction.
At this point forget about pH, acids being consumed will affect the pH, as the acid is reduced to salts along with your copper, once the copper consumes all the free acids it will give up electrons to the values in solution (if any like gold) and some copper will be consumed or be converted to ions as the gold ions get an electron (from the copper) the gold is reduced to elemental gold and will form a black impure gold powder once enough gold atoms cluster together to overcome the gravity of the solution and precipitate, some of the black gold powder may stick to your undissolved copper and need to be brushed free from the remaining copper buss bar.
Read the book again to see if say's C.M. Hoke.
Or are you learning from Pole.s or Poke.s book?
