Chemical Precipitation
An insoluble compound formed by the reaction of soluble fatty acid soaps with divalent calcium ions promotes the attachment of ink particles to air bubbles. Use of a calcium soap collector in paper recycling mill flotation cells ensures that hydrophobic ink specks are bound to froth rather than remaining in the pulp slurry. This chemical reaction requires a controlled presence of hardness ions to ensure that the sodium soap transitions into the inactive, insoluble collector form.
Froth Flotation
Separation of ink from paper fibres occurs when air is injected into the pulp slurry, creating bubbles that rise to the surface. A calcium soap collector modifies the surface chemistry of the released ink particles, increasing their hydrophobicity and ensuring they cling to the rising bubbles. The resulting ink-rich foam is skimmed from the top of the flotation cell, while the hydrophilic wood fibres remain in the aqueous phase.
Without this mechanism, fine ink particles tend to stay dispersed in the water, which reduces the brightness of the recycled paper sheet. The size of the generated bubbles and the speed of their ascent must be coordinated with the precipitation rate of the chemical collector to prevent ink redeposition on the paper fibres.
Calcium Requirement
Process water must contain a sufficient concentration of calcium ions to complete the chemical transformation of the added soluble soap. If the water is too soft, the collectors do not precipitate fully, leaving unreacted soap that can cause excessive foam and poor ink separation. Mill operators generally maintain water hardness between one hundred and two hundred parts per million of calcium carbonate.
This range provides a reliable chemical balance that secures optimal flotation performance without causing scale accumulation on deinking equipment.
