Waste-containing clinkers: Valorization of alternative mineral sources from pulp and paper mills

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Abstract

Waste streams generated from pulp and paper mills show high potential to be used as alternatives raw materials for production of clinker. In this context, this work aims to evaluate the use of such materials to obtain hydraulic binders, which are referred to as Ecoclinkers and Ecocements. Lime mud, wastewater treatment process sludge and biomass ashes were characterized, and five formulations (F1–F5) were developed by applying the Bogue equations. These compositions were processed at three different clinkering temperatures (1350, 1440 and 1455 °C). The formulated Ecoclinkers and a reference hydraulic binder were characterized by X-ray diffractometry to identify the crystalline phases, which were subsequently quantified by the Rietveld method. The results showed that crystalline phase characteristics of clinkers and cements, and compression strengths of ∼20 MPa after 28 days curing time were obtained, confirming the potential use of the investigated waste as alternative materials for production of clinker.

Introduction

The production of pulp and paper is growing every year. In 2013, paper production was 403 Mt, while cellulose production was 179 Mt (SFIF, 2014). USA, China, Canada, Brazil, Switzerland, Finland, Japan, Russia, Indonesia and Chile are the world’s largest pulp producers (Bracelpa, 2014).

Increased production of paper and pulp generates correspondingly increased amounts of waste, and the study of environmentally friendly alternatives for waste valorization is extremely important to the sustainable development of this industry.

The most common paper production process is the Kraft process, which uses alkalis (NaOH and Na2S) for digestion and separation of cellulose and lignin (Castro et al., 2009). The process is divided into six main stages: pulping, washing, evaporation, combustion, clarification and causticizing. Dregs are generated in clarification, and lime mud and grits are generated in causticizing (Pöykiö et al., 2006). Within the Kraft process, the wastewater treatment process (WWTP) generates sludge and power generation units generate biomass ashes from burning biomass and organic-rich waste (Modolo et al., 2011a).

In general, such wastes are considered non-hazardous (García et al., 2008, Ribeiro, 2010, Siqueira and Holanda, 2013) and show high alkalinity, typically with pH >10 (He et al., 2009, Mäkitalo, 2012).

When improperly disposed of, those waste can show a significant impact on the environment, damaging the water, soil and air. Thus, the waste must be intensively researched to mitigate their negative impacts and, much more than this, can allow the search for their valorization.

Many reports have been published showing the use of these wastes as potential industrial sources of the minerals SiO2, Al2O3 and CaO, and their further use in obtaining cement-based materials through clinkerization (Buruberri et al., 2015, Castro et al., 2009). Waste from other pulp and paper mill processes has also been investigated (Raupp-Pereira et al., 2008).

Lime mud is a calcium carbonate-rich waste (Jia et al., 2013, Qin et al., 2015, Sun et al., 2013, Zhang et al., 2013), and the similarity between this waste and commercial CaCO3 encouraged several studies in building materials. Buruberri et al. (2015) replaced CaCO3 by lime mud in the manufacturing of clinker, while Modolo et al. (2011b) evaluated the properties (fresh and hardened) of lime-containing mortars in Portugal.

Biomass ashes are rich in silica (Berra et al., 2015, Esteves et al., 2012, Ramos et al., 2013, Siddique, 2012) and can be considered raw material for the obtainment of cement-based materials (Berra et al., 2015, Esteves et al., 2012, Rajamma et al., 2009). Many works have analyzed the durability, mechanical strength and other properties of ash-based building materials (Garcia and Sousa-Coutinho, 2013, Ravina, 1998). Ramos et al. (2013) presented results of the recycling of this waste in the production of mortars and concrete.

WWTP sludge is a waste rich in organic matter (Gallardo et al., 2010, Gea et al., 2005), which has shown potential use in power generation (Chen et al., 2014, Kaluža et al., 2014, Nurmesniemi et al., 2007). Moreover, such wastes can be used for producing cementitious-based materials (Modolo et al., 2011a, Modolo et al., 2011b, Siqueira and Holanda, 2013, Zhang et al., 2013). Although some environmental impacts associated with incineration have been reported (Halonen et al., 1993, Korhonen, 2001, Oral et al., 2005) some authors advocate its use as a suitable solution for destination instead of sending to a landfill.

The heat treatment of waste from the pulp and paper industry in clinker kilns can be considered a co-processing treatment, when the industrial kiln operating parameters are taken into account. According to the Brazilian Solid Waste Policy (Law n. 12305/2010) (BRAZIL, 2010), some of these parameters, such as high temperature and holding time are higher than those used in traditional incineration, which ensures the environmentally safe disposal of hazardous waste (ABCP, 2016).

The present work aims to study new alternative materials for clinquerization. Thus, lime mud, biomass ashes and WWTP sludge were used as mineral source for obtaining clinker and cement, i.e. compositions were formulated only using waste. Taking into account all those aspects, even the edge of the industrial productive reality, such clinkers and cements can be designated as Ecoclinker and Ecocement, respectively

Section snippets

Experimental

This work was divided into two main steps: obtaining the Ecoclinker from the waste and functionalization of the Ecocement in mortars.

Waste characterization

WWTP sludge shows a high water content (∼86 wt%), higher than that recommended for cements (<40 wt%) (Buruberri, 2014, Mehta and Monteiro, 2005). However, such high water content does not affect the preparation of the proposed formulations, because lime mud and biomass ashes show very low water content (∼26 and ∼23 wt%, respectively).

Particle size distributions of the waste (Fig. 2) show d50 of 14.6, 20.7 and 80.0 μm for lime mud, WWTP sludge and biomass ashes, respectively, which can be considered

Conclusions

Lime mud, biomass ashes and wastewater treatment process sludge were studied for the manufacture of clinkers, named Ecoclinkers, because they contain the main components necessary for clinkerization (Al2O3, SiO2 and CaO). XRD patterns of the Ecoclinkers showed the main crystalline phases commonly found in commercial cements and the reference material. Formulation F4 fired at 1455 °C was chosen to produce the Ecocement (95 wt% of Ecoclinker and 5 wt% of gypsum), which was used to manufacture

Acknowledgements

The authors are very grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Brazil) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil) for funding this work.

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