Todd Kulis stands with his poster on Use of Paper Mill Waste in Ceramics: Comparing Paper Mill Ash with Traditional Fluxes presented at May 2003 CEER Annual Meeting

Materials Research

Utilization of Paper Mill Waste in Ceramic Products

Investigators: Dr. David A. Earl and Dr. Christopher W. Sinton

Industrial Collaborators: International Paper and Dal Tile

Each year 112 kraft-process paper mills across the US produce approximately 1.5 million tons of waste material from their energy and chemical recovery processes. The waste stream can cost US companies millions of dollars for disposal and pose an environmental burden for landfills. The goal of this project is to utilize some or all of this waste stream as a raw material for manufacturing ceramic products, thereby creating an industrial practice for environmental sustainability. Tile manufacture has the most potential of the ceramic products because: 1) the ash is most readily compatible with the starting materials; 2) production volumes are high; 3) there are multiple manufacturing locations across the US; and 4) the raw material specifications are relatively broad.

The work will initially focus on incorporating boiler fly and bottom ash in ceramic tile products. The use of dregs and grit from the recausticization process will also be investigated. The work will consist of technical, economic, and environmental studies of substituting these wastes for traditional minerals used in tile (e.g. wollastonite, whiting, talc, and feldspar). Preliminary work at Alfred University produced high quality ceramic tile samples with boiler bottom ash substituted for 25% of the traditional raw materials. The bottom ash and the other three products will be substituted for raw materials in tile body formulations and composition-processing-properties relationships will be quantified with multiple regression models. The models will allow each ceramic manufacturer to optimize the quantity of waste from a specific source that can be incorporated in their formula, while maintaining or exceeding industrial quality standards. The data and models developed during the study will also establish the process by which other ceramic manufacturers can utilize the waste as a raw material.

The economic and logistical feasibility studies will involve collecting data on the sources, amounts, characteristics, and consistency of wastes from at least four mills. The price range of the waste materials will be estimated and will be compared to existing industrial formulations to estimate the ceramic market potential and savings/income for paper mills. An environmental assessment will consist of a simple life cycle assessment. The factors that will be taken into account in the assessment include the energy and environmental costs of mining, transportation, and processing of virgin minerals relative to the mill wastes.

Final Report: Utilization of Paper Mill Waste in Ceramic Products

EPA Grant Number:  R83042001-1
Center: CEER at Alfred University
Investigators: Earl, David A. and Sinton, Chris
Institution: Alfred University
Project Period: September 1, 2003-August 31, 2005
Research Category: Congressionally Mandated Center

Description and Objective of Research: The main objectives were to: 1) characterize paper waste material from at least three sources, 2) study the performance of ceramic tile body formulations containing variable amounts of waste materials, and 3) estimate the environmental and economic impacts of using waste materials.  Ceramic tile products, with formulations incorporating boiler fly and bottom ash, were studied to evaluate and quantify composition-processing-properties relationships.  Tile manufacture is used in this investigation because: 1) the ash is most readily compatible with the starting materials, 2) production volumes are high, 3) there are multiple manufacturing locations across the U. S., and 4) the raw materials specifications are relatively broad.

Background: The kraft process for manufacturing paper is one of the most widely used processes in the world.  In this process, wood chips are chemically digested to release the cellulose fibers, which are held together by lignin.  The resulting liquid from this process (‘black liquor’) is concentrated and then burned in a recovery boiler, where organic solids are burned for energy (steam and electric power), and the process chemicals are removed and recovered.  There are four streams of waste from the recovery process:  boiler bottom ash and fly ash, and recaust grit and dregs.  Bottom ash and fly ash are produced from the combustion of the black liquor in the boiler.

There are 112 kraft-process mills in the United States and 19 in Quebec/Ontario. Mills can produce 10,000 to 20,000 tons/year of ash and recaust waste, which is placed in landfills.  There is potential to use some of this paper mill waste as a raw material for the ceramic industry.  A study in Thailand has shown the use of paper mill waste to manufacture bricks.  Other studies have shown the use of coal fly ash in the production of glass-ceramics.  Paper mill ash is more desirable for ceramics, because of much lower iron and heavy metal concentrations and higher CaO content compared to coal ash.

Preliminary work at Alfred University had identified similarities between paper mill waste and tile raw materials, and some tile prototypes have been produced using bottom ash waste.  The composition and phases of a bottom ash sample indicate potential for partially replacing common whiteware raw materials, including wollastonite, whiting, quartz, clay, and talc.  Replacing wollastonite and whiting would be of particular interest to tile manufacturers as wollastonite is one of the most expensive raw materials used (over $200/ton), while whiting tends to cause glaze bubble defects due to its large loss-on-ignition (LOI~ 50%) above the glaze melting temperature.  The US tile market used about 5.3 million tons of body material last year.  If paper mill waste replaces 25 weight per cent of the raw materials, yearly consumption would be 1.3 million tons for US tile companies. 

Methods: Bottom ash and fly ash samples from International Paper Co. in Ticonderoga, NY, were characterized.  The ash composition was found to have significant variations in SiO2, Al2O3, and CaO, important oxides with respect to ceramic (porcelain) compositions, in samples received over a four-month period.  Although the composition within single samples was uniform, variations over time (and among sites) could lead to complications in optimizing ceramic body formulations on an industrial scale, unless the ash is blended to a constant composition before shipment or the ceramic formulation is modified each time the ash composition changes.  

Particle size distribution analysis was used to determine the amount of milling required to successfully substitute ash for traditional raw materials.

All of the ash samples were examined using qualitative X-ray diffraction (XRD) to identify crystalline phases.  The major phases were quartz (SiO2) and calcite (CaCO3), which are commonly present in raw materials for ceramic products.

The sintering/melting behavior of the ash materials was characterized using a hot-stage microscope (HSM) and the results were compared to typical porcelain batch materials.
Experimental compositions incorporating paper mill ash were calculated and batched for an initial target composition based on industrial porcelain.  An “ideal” batch containing bottom ash was calculated, as well as two other formulations, with higher and lower bottom ash content. Compositions for the experimental formulas are shown below in Table I.  The compositions were prepared into a slurry, filter pressed and extruded into rods and bars to be fired and tested.

Table I.  Calculated compositions for experimental formulas.

 Results:
Particle Size Distribution:  As-received ash samples were tested for particle size distribution (PSD), then milled to achieve the size range of traditional raw materials.  The PSD of the milled bottom ash samples compares well with traditional ceramic raw materials (D50 = 10-15 µm).  The Androscoggin ash has a smaller mean size due to a smaller and more uniform initial size distribution as received.  PSD results are shown below in figure 1.

Figure 1. Particle size distributions (PSD) of the milled bottom ash samples and G-200 feldspar.

           
XRD:  X-ray diffraction analysis of bottom ash samples revealed the presence of quartz (SiO2), calcite (CaCO3), corundum (Al2O3), a calcium aluminosilicate, and a sodium aluminosilicate.  Fly ash samples were found to contain quartz, calcite, Monticellite (a calcium-magnesium silicate), Shortite (a sodium-calcium carbonate), and a sodium aluminosilicate.  Diffraction patterns are shown in figures 2(a), (b).
XRD of fired samples showed that the ash bodies form most of the same crystalline phases as a typical porcelain.  A significant amount of cristoballite, which does not appear in typical porcelain bodies, forms in the ash bodies as well.  Compared to porcelain compositions, less glassy phase forms in the ash bodies, thereby inhibiting sintering (filling of pores).

Figure 2(a).  X-ray diffraction pattern: bottom ash sample.

Figure 2(b).  X-ray diffraction pattern: fly ash sample.

Hot-stage Microscope:  Comparison of sintering behavior of the ash materials with typical porcelain batch materials using HSM showed lower (by approx. 100oC) sintering temperatures for bottom ash than for feldspars and nepheline syenite (traditional fluxes).  Sintering progress as observed by hot-stage microscope is shown below in figure 3.

Figure 3.  Hot-stage micrographs of traditional materials, ‘ideal’ ash compositions, and ash waste samples.

Chemical analysis:  Results of chemical analysis, shown below in Table II, reveal that the paper mill ash has higher amounts of “fluxing” oxides and carbon than traditional raw materials as well as lower SiO2 and Al2O3 levels.  Incorporating ash into traditional ceramic formulations therefore will require systematic adjustments of the other batch materials to achieve the target composition.

Table II.  Analyzed compositions for ash and traditional raw materials.

Porosity:  Samples were fired at a typical porcelain firing schedule of 2.5o C/min.
with a 2.5 hr. soak at maximum temperature.  Fired samples were measured for apparent porosity, water absorption, and bulk density using the Archimedes method.  Measurement data are shown below in Table III.  The ash bodies showed much higher porosity and lower bulk density than the control composition (a typical porcelain).

           
Table III.          Samples fired at 1290oC                 Samples fired at 1350oC

Laboratory and Pilot Trials (July 2003-December 2004):  Body trials were focused on porcelain because the ceramic tile industry has moved towards porcelain bodies over
the past few years, and other whiteware compositions (dinnerware, high-voltage porcelain, sanitaryware) are porcelain as well.  Initial experiments using an RO + R2O balancing technique in porcelain bodies show that the ash bodies produce higher porosity (lower glassy phase) than the traditional compositions.  Samples were fired using a typical porcelain firing schedule.  The ash bodies were found to have higher porosity and lower bulk density than the traditional composition.  A significant amount of cristoballite, which does not form in typical porcelain bodies, was found the ash bodies as well.  SEM micrographs of a traditional composition (‘control body’), bottom ash composition, and fly ash composition fired to 1290oC are shown below in figures 4(a-c).

       
Figure 4(a). Control body fired at 1290oC (1000X).

       
Figure 4(b). Bottom ash body fired at 1290oC (1000X).

          
Figure 4(c). Fly ash body fired at 1290oC (1000X).

Experimental Tile Bodies with Bottom Ash Paper Waste: Ash body formulas were designed to increase the amount of glassy phase present.  Bottom ash waste was ground to pass through a 65-mesh screen and ball-milled in dry state.  Four batches with different formulas were prepared.  Batch compositions are shown below in Table IV.

Table IV. Composition of the batches in grams

For each formula 30 samples (buttons, 1”dia. x 0.3”h., 8 g wt.) were pressed (at 6370 psi).  The samples of formulas #1 and #2 were heat treated at 1000°C for 4 hrs then cooled slowly (in the furnace) to 50°C. The color of samples for both formulas is light gray, with a smooth surface texture.  The samples for formulas #3 and #4 were heat treated at 1200°C for 8 hrs, then cooled slowly to 50ºC. The surface texture of formula #4 samples is smooth, but for #3 is very porous. The color of samples is gray with dark brown particles (most likely associated with flint pebbles).
For measurement of water absorption, samples with the formulas 1, 2 and 4 were dried and weighed, boiled in water for 5 hrs., then weighed again.
           
Table V.  Percent water absorption.

Experimental glazing of green body samples: 
First glazing: Dried green samples of formulas 1and 2 were glazed with 0.6g of a leaded glaze then heat treated at 1100ºC for 8 hrs. and cooled slowly to room temperature. Surface texture of the glazed samples was smooth with no stress apparent in the glaze layer.  The percent water absorption of the glazed samples was 11% for formula #1 and 12% for formula #2.

Second glazing: Dried green samples of formulas 1, 2, 3, and 4 were glazed and heat treated at 1100ºC for 8 hrs.  Samples from formulas 2 and 4 showed smooth surface texture, from formula 1, a small amount of surface porosity, and from formula 3, a very porous surface texture.  The percent water absorption for formula #4 was 8%.  Shrinkage during heat treatment at 1100oC was 1% for formulas #1 and #2, and 5% for formula #4.

Conclusions: 
Substituting paper mill ash waste for traditional raw materials in porcelain tile bodies was not proved successful in this study, primarily due to compositional makeup and sintering behavior. 

The composition of the ash waste varies significantly over time at the same source.  Because of this variation raw materials must be continuously tested and blended, based on chemical analysis data, to achieve consistent batching. 

The test compositions developed in this study exhibited poor sintering behavior with insufficient glassy phase development and the formation of a cristoballite phase which resulted in a decrease in thermal shock resistance.  The fired samples also showed high porosity and low density when compared with traditional porcelain compositions, and did not perform well in glazing trials.

References:
“Effect of substitution of fly ash for quartz in triaxial kaolin-quartz-feldspar system”, K. Dana, S. Das, W. Kumar Das, Journal of the European Ceramic Society (in press).

 “Physico-chemical characterization of slag waste coming from GICC thermal power plant”, A. Acosta, M. Aineto, I. Iglesias, M. Romero, J.Ma. Rincon, Materials Letters 50 (2001) 246-250.

“Use of coal fly ash for ceramics: a case study for a large Spanish power station”, I. Queralt, X. Querol, A. Lopez-Soler, F. Plana, Fuel Vol. 76, No. 8, pp. 787-791, 1997.

 “Use of soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix”, A. Tucci, L. Esposito, E. Rastelli, C. Palmonari, E. Rambaldi, Journal of the European Ceramic Society 24 (2004) 83-92.

Publications / Presentations:  Poster presentation, Alfred University, May 2004.

Key Words:  characterization, fly ash, bottom ash, ash utilization, porcelain, waste materials, tiles, paper mill waste, ceramic products

CEER is funded in large part by the United States Environmental Protection Agency.

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