1. Working Principle

The whiteness of kaolin is mainly reduced by ferric iron (Fe³⁺) present in hematite and limonite, as well as trace organic matter and carbonaceous substances, which cause the ore to turn yellow and grey.

Thiourea Dioxide gradually releases strong reducing hypo sulfurous acid (H₂SO₂) under weakly alkaline conditions:

- It converts insoluble Fe³⁺ into soluble Fe²⁺, which is then removed via washing and filtration to realize iron removal and whitening.

- It also reduces and decomposes colored organic matter and carbonaceous impurities for further whiteness improvement.

The reaction proceeds mildly without damaging the crystal structure of kaolin, featuring stable whitening effect and low reversion yellowing tendency.

2. Core Advantages versus Sodium Dithionite（Sodium Hydrosulfite)

ž   Higher whiteness: Under the same dosage, the whiteness is 2% ~ 9% higher.

ž   Lower consumption: 1% Thiourea Dioxide is equivalent to 2% ~ 2.5% sodium dithionite, effectively reducing chemical cost.

ž   Excellent stability: Solid Thiourea Dioxide is stable at room temperature, non-flammable and odorless; its aqueous solution decomposes slowly for safe storage and transportation.

ž   Environmental friendliness: No release of hydrogen sulfide or sulfur dioxide. The wastewater contains qualified sulfide content and low AOX.

ž   Low corrosion: Far less corrosive to carbon steel and 304 stainless steel compared with sodium dithionite（Sodium Hydrosulfite).