First off, I'm not sure how you're planning to get a phosphate salt from mixing things with hydrochloric acid (if you are thinking of phosphoric acid, there are various OTC sources, but I've already written this whole post on HCl). I am too lazy to research this myself, but if >>13085
is right and the bones/antlers are ammonium carbonate, this should give you ammonium chloride and carbon dioxide. I could be missing something, that anon could be wrong, or you could be leaving out part of your procedure, but I'd double check if I were you. The same concept also applies if what you're using is calcium carbonate (think seashells, and I'm pretty sure human bones are calcium carbonate too).
Also, hydrochloric acid is a stronger acid than acetic acid, so mixing NaCl with acetic acid will give you nothing (this is why sulfuric is used - it's stronger than HCl, so you get sodium sulphate plus hydrochloric acid - the stronger acid always displaces the weaker one and forms a salt, leaving the weaker one as the free acid). Moving on...
>I've been trying to think of a method to produce food-grade HCl at home
Don't make it. You can buy HCl at very high concentrations. HCl, at room temperature, is a gas, so you will purchase it as a solution in water. The maximum theoretical concentration is somewhere in the 30% range, and it turns out that "muriatic acid" at your local hardware store (used for cleaning concrete and shit) is just hydrochloric acid solution in water at a concentration of high 20's or low 30's of percents, obviously with specifics depending on brand and batch (safety note: due to the high concentration, the bottle will release HCl fumes every time you open it, so keep it away from your fucking eyes; also, don't store it near anything metal, because the fumes leak out slowly over time and cause corrosion).
>but anon, concrete cleaner isn't food grade
You're right, it's not. It will, at the very least, contain iron chloride as an impurity, hence the yellowish-greenish color that you'll notice. But here's the key: iron chloride, and any other impurities, are not gaseous. Dessicate the solution, liberating HCl gas, then bubble the gas through some distilled water. If you aren't very precise, it won't end up highly concentrated and if you aren't good at this, you'll probably lose some, but it's a great way of purifying it
Dessicating the solution is easy. Basically, you're just going to dump some muriatic acid onto something that absorbs water. You want to do this carefully and slowly and preferably in a covered container, or you will get hurt. I would expect that dehydrated epsom salts work, as they are an effective dessicant for other purposes, but dry CaCl2 is more commonly used. You can also use concentrade H2SO4 (sulfuric/sulphuric acid). Unlike HCl, sulphuric acid is a liquid at RT, and really likes to absorb water. So if you just slowly drip aqueous HCl into relatively dry H2SO4, it'll suck all the water out and release gaseous HCl. You are, of course doing this in a covered container with a tube leading out the top, so the gas passes harmlessly into the tube, then down into a container of distilled water, dissolving in the water and giving you what you desire.
>the only readily available source of sulfuric acid I can think of in the home is from a car battery
First off, you can buy battery acid at the hardware store, separate from a battery. To concentrate this, boil til the fumes turn white. The internet can give you info on concentrating from there.
But, there is, in many areas, a better source.
Have you ever been to the hardware store and seen the drain cleaner (not toilet bowl cleaner, drain cleaner) that is a bottle inside a plastic bag with shitloads of warnings all over it? That's VERY highly concentrated H2SO4 (>90% - it will cause severe chemical burns, as well as severe heat bruns from rapidly dessicating your skin, so be fucking careful). It's often reasonably pure, but for use as a simple dessicant, the purity is irrelevant - it never touches your final product.