Chemicals for pulp and paper

A kraft mill cooks wood chips in white liquor, a solution of sodium hydroxide and sodium sulfide that dissolves lignin and frees the cellulose fibers. Sodium hydroxide 50% makes up caustic losses in the liquor cycle and supplies alkali to extraction and bleaching stages. Most modern bleach plants run elemental chlorine free (ECF) sequences built on chlorine dioxide, with hydrogen peroxide reinforcing the alkaline extraction stages, while totally chlorine free (TCF) sequences rely on oxygen, ozone and peroxide. A chelant such as EDTA ties up metals ahead of a peroxide stage. After bleaching, sodium bisulfite removes residual oxidant, and sulfuric acid trims pH and splits tall oil soap into crude tall oil.

On the paper machine, papermaker's alum sets rosin size and helps retention, and coagulants such as ferric chloride clean mill water and effluent. Chemical choice affects brightness and fiber strength, and impurities that reach the recovery cycle cost evaporator and boiler uptime.

In a pulp mill, grade questions are mostly about the impurities a chemical brings into a closed loop. Membrane-cell sodium hydroxide carries far less sodium chloride than diaphragm-cell caustic. Chloride accumulates in the recovery cycle, where it promotes stress corrosion cracking of stainless steel and sticky deposits in the recovery boiler. In the bleach plant, the main threat to hydrogen peroxide is the iron, manganese and copper already in the pulp, which is why mills chelate before the peroxide stage. Technical-grade sodium bisulfite is the usual antichlor. Mills making food-contact board need every additive cleared for that use, so some buy food-grade sodium metabisulfite for those lines. Using an uncleared additive where the end customer requires a cleared one can get a production run rejected.

The EPA Cluster Rule set combined water and air standards for pulp and paper mills (40 CFR Part 430 effluent guidelines and 40 CFR Part 63, Subpart S). It moved bleached kraft mills to elemental chlorine free bleaching and set limits on dioxin and adsorbable organic halides (AOX) in effluent. Paper and paperboard that touches food falls under FDA 21 CFR 176.170 for aqueous and fatty foods and 176.180 for dry foods, which list the substances allowed in the sheet. Chemicals in US commerce must be on the TSCA inventory or covered by an exemption., and OSHA exposure limits and the Hazard Communication Standard cover workers handling caustic, acids and oxidizers. Check your discharge permit and customer specifications with your environmental team.

• Buying caustic on strength alone. "50%" says nothing about chloride. Ask whether it is membrane or diaphragm caustic, and check chloride on the COA before it goes into the liquor cycle.
• Running peroxide without chelation. Without EDTA or another chelant, metals decompose the peroxide before it brightens, and the mill raises the charge to hold brightness.
• Skipping the impurity check on food-contact lines. An antichlor bought without checking heavy metals on the COA can put board out of specification for food-contact testing.
• Overstocking hypochlorite or freezing caustic. Hypochlorite loses strength in storage, and 50% caustic starts to crystallize at about 54 °F in unheated tanks.