Carbon dioxide in pulp/paper mill stocks: fix or fizz?

The use of carbon dioxide for neutralising pulp mill black liquor residuals and controlling paper machine stock pH has increased dramatically over the past decade. It is replacing, or supplementing, alum and sulfuric acid as the chemical of choice because it does not add any metal or sulfate ions to the water system; an important consideration with the trend to closed loop water systems in paper mills. However, dissolved and entrained gases can create production problems and paper quality faults at the wet end of the paper machine. Pin holes (where there’s a bubble of gas, there no fibre!), drainage loss and poor formation are some of the more noticeable impacts on paper quality, whilst sheet breaks and pump cavitation can create runability problems. Entrained gases in stock systems have historically been due to mechanical inclusion of air in falling stock, leaky pump glands and poor piping design. Air has a limited solubility in water (~ 120 mg/L), whilst carbon dioxide has a solubility of 1450 mg/L in water at 25C and 1 atm. pressure. Thus carbon dioxide can create up to 12 times more dissolved gas than air, which will erupt out of solution when the stock pressure is reduced to one atmosphere as occurs at the flowbox slice. So while entrained air was a minor annoyance, entrained carbon dioxide can be a major problem. Up until now, the method for measuring entrained gas yielded a total entrained gas result and did not distinguish between air and carbon dioxide. Recent developments have resulted in a new test procedure which allows the measurement of both total gases and CO2. The method is simple, fast and reproducible as long as the carbon dioxide pressure-solubility ratio is borne in mind – i.e. correct sampling is a critical step in obtaining accurate results. This paper discusses the development of the test method and the background chemistry of carbon dioxide addition to paper machine stock systems. INTRODUCTION The rise of carbon dioxide as a key chemical in the pulp and paper industry has been one of the most important steps in allowing mills to close-up their water systems. The acidic properties of carbon dioxide are being used to replace alum and sulfuric acid for neutralisation of pulp mill stocks and for adjustment of paper machine pH. The beauty of carbon dioxide in this role is that it leaves no residual sulfate or aluminium ions in the system, thus allowing the water to be used many more times before treatment is required. The other potential source of carbon dioxide in paper machine waters is the gas liberated when calcium carbonate filled papers are recycled under acidic conditions. Carbon Dioxide Chemistry The presence of CO2 gas (i.e. bubbles) is governed by its equilibrium with dissolved CO2 (carbonated water) as shown below.