Foam is used in a diverse industry fields due to its useful rheological and mechanical properties. In addition to viscoelastic behaviour of liquid foams they are also able to flow and deform like a liquid. Air bubbles are utilised in wastewater treatment and min-eral flotation processes. For example, paper industry uses flotation processes for ink removal from the newspaper. Foam is also a good carrier material for different agents.
Textile industry exploits foams in a fabric finishing processes where dyes and chemicals are mixed with foam and applied to a textile. Dye colours migrate smoothly with foams and drying is faster and less energy consuming than with a liquid based carriers. Other applications can be found in cosmetics, oil recovery, multilayer laminates and compo-sites.7
Paper industry uses wet web forming in a paper making process. Fibres and other com-ponents of paper are mixed with a large amount of water and transferred from the head box to the web of the paper machine forming section. In the water forming method, a dilute suspension of water and fibres is essential since fibres tend to pile up, tangle, curl and flocculate which leads to non-uniformity of the paper sheet and loss of quality and mechanical properties. However, the water forming method is very usable when short and stiff cellulosic fibres are used even though the water consumption is high and a lot of energy is needed for the drying section.7
The technique, where foam acts as a carrier for fibres was developed and used first time in 1970’s. The technique was named the Radfoam process after its inventor Ben Rad-van. The method is based on an idea that the fibres readily bind to foam bubbles. 0.75 % - 1 % of fibres (w/w) are suspended in a foam containing 60 % - 70 % of air and applied on the wire of the paper machine through the head box. Foam generation is done with a surfactant which is added to the water-fibre suspension and air is injected into the solu-tion until the desired volume is achieved. The bubbles forming the foam suspension are usually 20µm – 100µm in diameter and foam is pseudoplastic. Foams high viscosity at low shear forces and low viscosity at high shear forces guarantee that the fibres are at-tached to the bubbles and do not move across each other until the suction boxes remove the water and the foam collapses, and fine fibre dispersion is achieved.138
Replacing water with foam could decrease the water and energy consumption, thus making the process more cost-effective. The more diverse range of raw materials could be used, including long fibres, and there would be no need for retention chemicals.
Foam forming can produce papers sheets of a higher uniformity and bulk comparing to wet web forming, yet the strength properties are better with water-laid sheets. Again it is possible to regain the strength loss with wet pressing. Another way to improve strength properties is to add micro-fibrillated cellulose (MFC) to the foam-fiber suspen-sion.138,139
4.3.1 Surfactants in foam forming
In spite of successful research work in the 1970’s the foam forming method did not pique any interest until several decades later. The need for more cost-effective and di-verse techniques for paper and board making has awaken the foam research. Develop-ment of new paper products made of new raw materials, including nanoparticles, nano-cellulose and long fibres, need a sophisticated manufacture technique and foam method could provide that. Yet, many technical and production challenges need to be settled before the technique can be fully implemented.138,139
Surfactants are one keen interest of the foam research. The surfactant used in the first experiments done in the 1970’s was not revealed in the articles. Research now focuses on the influence of surfactants in foaming properties of fibre suspensions and chemical interactions between surface active agents, fibres and other papermaking materials.138,139 According to the latest research published in 2014 [Mira, Lappalainen]
the foaming properties of a mixture of commonly used CTMP pulp and a surfactant was dependent on the sur-factant concentration. One of the sursur-factants investigated was SDS (sodium dodecyl sulfate) and the experiments showed that both the foamability and the liquid drainage rate altered according to the SDS concentration. Critical micelle formation of SDS was not affected by the fibres or other filler chemicals, but the foam generation and the liq-uid drainage rate was slower and lesser than in pure water-SDS solutions. Also, the foam generation of SDS-CTMP suspension stops when the concentration reaches 0.5 x
CMC. One possible explanation for these findings is that fibres and additives in the SDS-pulp suspension are physically interfering the adsorption of SDS molecules on the surfaces of gas bubbles and slowing down the liquid drainage rate.138
Also other surfactants, such as MixSAES (a mixture of alkyl and ethoxylated alkyl sul-fates) and C8/C10Gluc (a mixture of short chain alkyl glucosides), were tested in the experiments. Results showed that the foamability cannot be entirely explained by sur-factants properties like ionic character, critical micelle concentration or structure of the hydrophilic head. The foamability is more likely an effort of rightly proportioned mix-ture of surface active agents with the suitable molecular strucmix-tures. In spite of these conclusions, anionic surfactants seemed to be most effective agents in foam genera-tion.138
The effect of surfactants on the mechanical properties and quality of the paper was also investigated. It was observed that paper samples made of foam-pulp were bulkier and had a better formation than normally formed paper made of water-pulp. Different foam-ing agents had differfoam-ing effects on the bulk, and the formation was particularly affected by ionic surfactants. Tensile strength (the in-plane mechanical properties) was almost the same between foam and water formed samples, but out-of-plane properties (Scott bond delamination energy) were considerably lower. Delamination properties are im-portant factors for the grade and functionality of the board. Dryness achieved by wet pressing was better with foam formed samples than water formed samples and influence of different surfactants was notable. Retention of filler chemicals was better with non-ionic surfactants than annon-ionic surfactants.139
5 FOAMING PROBLEMS AND ELIMINATION OF FOAM
This chapter covers the foaming problems of a modern pulp and paper industry process waters and effluent waters of wastewater treatment plants (WWTPs). Also, the foam elimination methods, including defoamers and different physical techniques, are cov-ered briefly.