Potash

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Potash is an ambiguous term, like "lye", that has different meanings dependent on context. In this project it simply refers to the soluble fraction of wood ash. It is a light-colored powdery solid composed of a mixture of various materials, primarily potassium carbonate with substantial amounts of sodium carbonate.

Character

The general components of soluble ash include a variety of salts composed of

  • Sodium, Potassium, Calcium, Magnesium, Iron
  • Carbonate, Sulfate, Chloride

With potassium carbonate and sodium carbonate being the vast majority. In potash from most woods the K:Na ratio is at least 10:1. If halophile grasses (such as barilla) are used, the percentage of sodium rises.

Economics

Real World Prices: (as of 12/27/12)

    $0.40-0.45/kg, raw (mined potash). At 3% yield, that's about $1 per 100kg of ash processed.
    $1.00-1.50/kg, crude (90-95%). At 3% yield, that's about $1 per 40kg ash processed.

Uses

Primary

Hazards

  • Some amount of hydroxides of sodium and potassium will also be present during production.

Production

Extraction

  1. Gather 1 unit by volume of loose, fine wood ash
  2. Gather 2 units by volume water
  3. Mix the ash and 1 unit of water in the ash container
  4. Wait until the material settles
  5. Check
    1. There should be three layers: (charcoal floating on the top, grey/brown tinted water in the middle, and ash at the bottom)
    2. The water should feel slightly soapy/slick
  6. Skim and discard the charcoal from the top
  7. Decant clear liquid (potash solution)
  8. Add another unit of water and mix again
  9. Filter
  10. Discard the residue of insoluble ash
  11. Evaporate the filtrate leaving potash
    Check: Taste a small amount potash has a distinctive flavor.
  12. The insoluble ash can be added to soil to aerate it, but is alkaline, so too much could be a problem

Purification

whitening

It is common for potash made directly from wood ash to be brown rather than white. This is because of impurities in the ash. The most common method to remove this is simply to wash the potash with small amounts of distilled water. Typically the red/brown impurities have much higher solubility than potash, so rinsing a little water through the powdered potash will dissolve the adulterants, leaving behind white powder. This can be done relatively effectively by placing an absorbent layer (cotton, paper) at the bottom of the potash container and adding a small amount of water or steam to the top. The water trickles down, carrying the impurities with it. Replacing the absorbent layer and adding more water will repeat the improvement.

bicarbonation

Dissolved sulfates or chlorides will have constant solubilities, but carbonates can be converted to bicarbonates, which have lower solubilities.

  1. Heat the potash above 200°C for an hour
  2. Repeat
    1. Prepare a room-temperature saturated solution of the potash
    2. Bubble air or carbon dioxide through the solution for several hours or overnight.
    3. Filter
    4. Return the filtrate to the solution
    5. Retain the residue. It is primarily potassium bicarbonate (KHCO3)
  3. Until no more precipitate forms from bubbling
  4. Heat the residue (the bicarbonate) above 200°C for an hour
  5. The heated material will be purified potash

simple fractional crystallization

Assuming there are no more than 10% impurities with solubilities higher or lower than the potash, use fractional crystallization to separate them.

  1. In boiling water, prepare a saturated solution of potash
  2. Boil until 10% of the original weight of materials has precipitated
  3. Filter
  4. Discard the residue
  5. Boil until 80% of the original weight of the materials has precipitated
  6. Filter
  7. Retain the residue. It is purified potash
  8. Discard the filtrate.

Testing

cf. solash process.

By solubility

Principle

Testing the solubility of the material at room temperature is a decent way to estimate the amount of sodium vs potassium in the material.

  • Pearl ash will dissolve slightly more than its own weight in water. (1100g in 1L of water) at 20°C
  • Soda Ash will dissolve sligntly more than 20% of its weight in water (210g in 1L of water) at 20°C

NB: This curve is not linear at 20°C, but it is close to linear at 100°C. If the testing below can be conducted using water near its boiling point (and the temperature can be maintained through the mixing) then the number of iterations will more closely mirror a specific fraction of pearl ash to soda ash.

Process

  1. Divide the potash sample into six equal units (by mass)
  2. Place 6 units of distilled water in a container
  3. Mix one unit of potash into the container
    Check: At this point, it should all dissolve. If it does not, either the water or the sample contains substantial contaminants
  4. Repeat the following
    1. Count repetitions (R) starting at 1
    2. Mix one unit of potash thoroughly into the water
  5. Until not all the potash dissolves, or you are out of potash
  6. The value of R will lie between 1 and 5

Interpretation

By specific gravity

Principle

Specific Gravity (g/cc)
°C K2CO3:Na2CO3 kg/L
28 100 0 1.7
28 0 100 1.4

The density of a saturated solution of potassium carbonate is higher than that of a saturated solution of sodium carbonate.

Process

  1. Prepare a saturated solution of the potash to be measured
  2. Using a hydrometer measure its density
  3. Using a thermometer measure its temperature
  4. If you have prior records for the measured density and temperature temperature
    1. Consult prior records to find out what the K:Na ratio is.
  5. Else
    1. Prepare a saturated solution of sodium carbonate
    2. Prepare a saturated solution of potassium carbonate
    3. Bring the temperature of all three saturated solutions to the same temperature
      NB One way to do this is to immerse their containers in a third container of some liquid
    4. Using a hydrometer measure the density of the reference solutions
    5. Interpolate a value
    6. Record all three densities and the temperature
  6. EndIf

Storage

Disposal

  • Potash can be disposed of into the soil. Do not dispose into waterways.

See Also

References