Development of filter media treatments for liquid filtration

Woven monofilament, multifilament, and spun yarn filter media have long been the standard media in liquid filtration equipment. While the energy for a solid-liquid separation process is determined by the engineering work, it is the interface between the slurry and the equipment - the filter media - that greatly affects the performance characteristics of the unit operation. Those skilled in the art are well aware that a poorly designed filter medium may endanger the whole operation, whereas well-performing filter media can make the operation smooth and economical. As the mineral and pulp producers seek to produce ever finer and more refined fractions of their products, it is becoming increasingly important to be able to dewater slurries with average particle sizes around 1 ¿m using conventional, high-capacity filtration equipment. Furthermore, the surface properties of the media must not allow sticky and adhesive particles to adhere to the media. The aim of this thesis was to test how the dirt-repellency, electrical resistance and highpressure filtration performance of selected woven filter media can be improved by modifying the fabric or yarn with coating, chemical treatment and calendering. The results achieved by chemical surface treatments clearly show that the woven media surface properties can be modified to achieve lower electrical resistance and improved dirt-repellency. The main challenge with the chemical treatments is the abrasion resistance and, while the experimental results indicate that the treatment is sufficiently permanent to resist standard weathering conditions, they may still prove to be inadequately strong in terms of actual use.From the pressure filtration studies in this work, it seems obvious that the conventional woven multifilament fabrics still perform surprisingly well against the coated media in terms of filtrate clarity and cake build-up. Especially in cases where the feed slurry concentration was low and the pressures moderate, the conventional media seemed to outperform the coated media. In the cases where thefeed slurry concentration was high, the tightly woven media performed well against the monofilament reference fabrics, but seemed to do worse than some of the coated media. This result is somewhat surprising in that the high initial specific resistance of the coated media would suggest that the media will blind more easily than the plain woven media. The results indicate, however, that it is actually the woven media that gradually clogs during the coarse of filtration. In conclusion, it seems obvious that there is a pressure limit above which the woven media looses its capacity to keep the solid particles from penetrating the structure. This finding suggests that for extreme pressures the only foreseeable solution is the coated fabrics supported by a strong enough woven fabric to hold thestructure together. Having said that, the high pressure filtration process seems to follow somewhat different laws than the more conventional processes. Based on the results, it may well be that the role of the cloth is most of all to support the cake, and the main performance-determining factor is a long life time. Measuring the pore size distribution with a commercially available porometer gives a fairly accurate picture of the pore size distribution of a fabric, but failsto give insight into which of the pore sizes is the most important in determining the flow through the fabric. Historically air, and sometimes water, permeability measures have been the standard in evaluating media filtration performance including particle retention. Permeability, however, is a function of a multitudeof variables and does not directly allow the estimation of the effective pore size. In this study a new method for estimating the effective pore size and open pore area in a densely woven multifilament fabric was developed. The method combines a simplified equation of the electrical resistance of fabric with the Hagen-Poiseuille flow equation to estimate the effective pore size of a fabric and the total open area of pores. The results are validated by comparison to the measured values of the largest pore size (Bubble point) and the average pore size. The results show good correlation with measured values. However, the measured and estimated values tend to diverge in high weft density fabrics. This phenomenon is thought to be a result of a more tortuous flow path of denser fabrics, and could most probably be cured by using another value for the tortuosity factor.

Siistausmassan saostuksessa kiekkosuodattimella käytettävän suodatinväliaineen testaus ja kehittäminen

Työn aiheena on siistausmassan saostuksessa käytettävä kiekkosuodatinväliaine ja tarkoituksena selvittää lankamateriaalien ja pinnoitusaineiden vaikutusta väliaineen likaantumisherkkyyteen ja suodatusteknisiin ominaisuuksiin. Siistauslaitosten ongelmana on suodatinväliaineen likaantuminen massassa olevien tahmo- ja painoväripartikkelien johdosta. Tällä hetkellä siistauslaitosten kiekkosuodatinpussien valmistuksessa käytetään pääasiassa polypropeeni- ja polyvinylideenifluoridimonofilamenttilankoja, joiden lianhylkivyys on osoittautunut huonoksi. Uusiksi lankamateriaaleiksi tutkimuksiin valittiin Easy Kleen I, Easy Kleen II, Hyflon, Halar ja Polyesteri. Tulosten perusteella Easy Kleen I ja II monofilamenttilangoista kudotun suodatinkankaan lianhylkivyys on polypropeeni- ja polyvinylideenifluoridimonofilamenttilangoista kudottua kangasta parempi. Suodatusteknisissä ominaisuuksissa ei materiaalien välillä havaittu merkittävää eroa. Easy Kleen monofilamenttilangat näyttävät soveltuvan myös keittokutistumiltaan ja mekaanisilta ominaisuuksiltaan kiekkosuodatinpussien materiaaleiksi. Polypropeeni- ja polyesterimonofilamenettilangoista kudottujen suodatinkankaiden pinnoittamisessa käytettiin hydrofiilisiä ja hydrofobisia pinnoitusaineita. Tulosten perusteella on mahdollista parantaa suodatinkankaan lianhylkivyyttä pinnoittamalla. Pinnoitettu kangas on myös suodatusteknisiltä ominaisuuksiltaan toimiva. Ongelmaksi suodatinkankaiden pinnoittamisessa saattaa kuitenkin muodostua pinnoituslämpötila, jossa kangas kutistuu asennuskelvottomaksi.

Surface characterization of chemically modified fiber, wood and paper

Inorganic-organic sol-gel hybrid coatings can be used for improving and modifying properties of wood-based materials. By selecting a proper precursor, wood can be made water repellent, decay-, moisture- or UV-resistant. However, to control the barrier properties of sol-gel coatings on wood substrates against moisture uptake and weathering, an understanding of the surface morphology and chemistry of the deposited sol-gel coatings on wood substrates is needed. Mechanical pulp is used in production of wood-containing printing papers. The physical and chemical fiber surface characteristics, as created in the chosen mechanical pulp manufacturing process, play a key role in controlling the properties of the end-use product. A detailed understanding of how process parameters influence fiber surfaces can help improving cost-effectiveness of pulp and paper production. The current work focuses on physico-chemical characterization of modified wood-based materials with surface sensitive analytical tools. The overall objectives were, through advanced microscopy and chemical analysis techniques, (i) to collect versatile information about the surface structures of Norway spruce thermomechanical pulp fiber walls and understand how they are influenced by the selected chemical treatments, and (ii) to clarify the effect of various sol-gel coatings on surface structural and chemical properties of wood-based substrates. A special emphasis was on understanding the effect of sol-gel coatings on the water repellency of modified wood and paper surfaces. In the first part of the work, effects of chemical treatment on micro- and nano-scale surface structure of 1st stage TMP latewood fibers from Norway spruce were investigated. The chemicals applied were buffered sodium oxalate and hydrochloric acid. The outer and the inner fiber wall layers of the untreated and chemically treated fibers were separately analyzed by light microscopy, atomic force microscopy and field-emission scanning electron microscopy. The selected characterization methods enabled the demonstration of the effect of different treatments on the fiber surface structure, both visually and quantitatively. The outer fiber wall areas appeared as intact bands surrounding the fiber and they were clearly rougher than areas of exposed inner fiber wall. The roughness of the outer fiber wall areas increased most in the sodium oxalate treatment. The results indicated formation of more surface pores on the exposed inner fiber wall areas than on the corresponding outer fiber wall areas as a result of the chemical treatments. The hydrochloric acid treatment seemed to increase the surface porosity of the inner wall areas. In the second part of the work, three silane-based sol-gel hybrid coatings were selected in order to improve moisture resistance of wood and paper substrates. The coatings differed from each other in terms of having different alkyl (CH3–, CH3-(CH2)7–) and fluorocarbon (CF3–) chains attached to the trialkoxysilane sol-gel precursor. The sol-gel coatings were deposited by a wet coating method, i.e. spraying or spreading by brush. The effect of solgel coatings on surface structural and chemical properties of wood-based substrates was studied by using advanced surface analyzing tools: atomic force microscopy, X-ray photoelectron spectroscopy and time-of-flight secondary ion spectroscopy. The results show that the applied sol-gel coatings, deposited as thin films or particulate coatings, have different effects on surface characteristics of wood and wood-based materials. The coating which has a long hydrocarbon chain (CH3-(CH2)7–) attached to the silane backbone (octyltriethoxysilane) produced the highest hydrophobicity for wood and wood-based materials.