Saltpeter

Saltpeter is a mixture of alkali nitrates, including potassium nitrate, sodium nitrate and calcium nitrate. It is extremely valuable in early chemistry as the most common source of the nitrate (NO₃ anion) until the haber/bosch/ostwald chain becomes available.

Uses

• Essential for efficient production of sulfuric acid via the chamber process
• Feedstock for nitric acid
• Essential for the production of blackpowder

Natural occurrence

Saltpeter occurrs naturally in two common forms:

• As the a mineral (chilean saltpeter, niter) in high deserts and dry evaporated lakes
• As a constituent in compost or manure

Hazards

• Nitrates taken in excess can lower blood pressure
• Nitrates are oxidizers

Production

Extraction

Starting with nitrate rich soil (such as compost) the soil is leached, the nitrates are isolated, then converted to saltpeter, and the saltpeter recovered.

Leaching

Extract soluble compounds from the substrate. The substrate can be reused afterward.

1. Gather one unit by volume of dried substrate in a clean container with a filtering drain in the bottom
2. Gather 2 units by volume of water
3. Repeat twice
   1. Plug the bung
   2. Pour 1 unit of water into the substrate
   3. If necessary, stir
   4. Wait for one hour the first time, overnight the second time
   5. Unplug the bung and drain
   6. Filter the solution
   7. Retain the filtrate. It contains saltpeter
   8. Return the residue to the substrate
4. Recycle the residue

Purification

Recrystallization

Recrystallization is effective for saltpeter

Simple

1. Add calcium oxide in excess to the dilute saltpeter solution
2. Boil half of the solution off
3. Filter
4. Discard the residue. It is calcium hydroxide, calcium carbonate, denatured organics, and poorly soluble salts
5. Evaporate the solution
6. The residue is crude saltpeter.

Isolation

The solution contains a lot of different soluble materials at this point, both organic and inorganic. The isolation step attempts to remove the bulk of compounds that do not fit the pattern

[Ca,K,Na][NO₃]

This is not a complete isolation since chlorides and nitrites also pass through. It does remove the majority of common adulterants. This process could be repeated as the volume of the solution decreases.

Step	Action	Optional?	Followed by	Things removed	Things added
1.	Boil the solution to ½ its original volume	no	let stand, cool, skim & filter*	Easily denatured organics, poorly soluble salts	none
2.	Add epsomite in large excess (up to 10% by mass)	yes	filter	precipitates calcium, lead, barium and strontium	magnesium and sulfates
3.	Add fresh calcium oxide in large excess (up to 25% by mass) and stir vigorously	no	let stand, cool, skim & filter	Precipitates carbonates and sulfates, as well as magnesium	small amounts of calcium hydroxide
4.	Aerate vigorously	no	filter	precipitates any remaining lime	none
5.	Add ground charcoal and stir vigorously	yes	filter*	adsorbs many organics, including those attacked by the calcium hydroxide which remained soluble, as well as metallic salts (tin, etc)	none
	*The residues from these filtering steps can be returned with the substrate to the composting process

See Also

• LeConte Process
• LeConte, Joseph (1862) "Instructions on the Manufacture of Saltpeter" (local copy)
  Charles P. Pelham, State Printer for South Carolina
  link courtesy University of North Carolina, last accessed 20-Jul-2014.
• Rains, Geo. W. (1861) "Notes on Making Saltpetre from the Earth of the Caves" (local copy)
  Daily Delta Job Office, New Orleans, Louisiana
  link courtesy Google, last accessed 20-Jul-2014.