Non-process Elements Research Articles

Total and extractable non-process elements in green liquor dregs from the chemical recovery circuit of a semi-chemical pulp mill

The total heavy metal concentrations in green liquor dregs investigated in this study were clearly lower than the current Finnish statutory limit values for fertilizer used in forestry. This is an advantage if applying for an environmental permit to utilize the green liquor dregs. They contained only carbonate minerals, were strongly alkaline (pH 11.7), and had a neutralizing value of 34.2% (Ca equivalents; d.w.), according to which 1.1 tonnes of green liquor dregs would be required to replace 1 tonne of a commercial ground limestone product produced by SMA Mineral Ltd. These properties support the utilization and re-use of this residue, for instance as a soil conditioning agent. Before such use, it would be necessary to assess the mobility of non-process elements in the dregs. Three-stage sequential extraction procedure development by the European Community Bureau of Reference (BCR) was therefore carried out, in which elements in the residue were fractionated between acid soluble (CH3COOH; BCR 1), reducible (NH2OH–HCl; BCR 2) and oxidizable (H2O2+CH3COONH4; BCR 3) fractions. Except for Co and Ni, the highest extractable concentrations of non-process elements (Al, As, Ba, Be, Cd, Cr, Cu, Fe, Mn, Mo, Pb, Sb, Se, V and Zn) were in the oxidizable fraction, although certain non-process elements were also extractable and quantitatively detectable in fractions BCR 1 and BRC 2. The results are discussed from various perspectives and in relation to observations in the literature concerning the release of heavy metals and sulphur from the sample matrix under different extraction conditions.

Nonprocess-element control in the liquor cycle using an ash leaching system

To lengthen run time between washes and reduce thermal cycling of the recovery boiler, the Evadale, Texas, mill has been attempting to reduce the levels of nonprocess elements (NPEs) in the liquor cycle. The mill has periodically purged electrostatic precipitator ash in an effort to reduce and control potassium (K) and chloride (Cl) concentrations. Additional efforts to lower chloride concentrations involved changing the makeup chemical from diaphragm- to membrane-grade caustic and improving the operation of the chlorine dioxide generator spent acid salt filter. The change in makeup chemical NPE content successfully reduced the amount of boiler plugging due to sticky ash formation, but did not provide the desired amount of time between boiler washes. In an effort to further reduce the amounts of potassium and chloride in the liquor cycle, an ash leaching system was installed. The leaching system partially dissolves precipitator ash in hot water. The potassium and chloride are selectively leached into the liquid phase and remain in solution. Sodium sulfate crystals are separated from the leaching slurry using a decanting centrifuge and returned to the liquor cycle via a mix tank. The potassium- and chloride-rich liquid is sewered. Since the implementation of these nonprocess-element reduction methods, the chloride content of the incoming electrostatic precipitator ash has been reduced by roughly 55% and the potassium content by approximately 50%. Approximately 80% of the soda entering the ash leaching process is recovered into the liquor cycle.

Effect of pH and washing on calcium and magnesium distribution between pulp and filtrate

AbstractThe impact of pH and washing procedure on calcium and magnesium distribution between pulp and filtrate in pulp processing was investigated. Depending on media pH, the elements are present in the form of ions (Ca2+, Mg2+) and hydrated oxides (Ca(OH)2, Mg(OH)2). Distribution was monitored using binary systems of CaCl2 and MgCl2 dissolved in deionized water or Ca2+ and Mg2+ ions present in filtrates from an industrial pulp mill. Complying with the relevant standards in force, Ca and Mg contents are expressed as CaO and MgO. The study was aimed at obtaining mathematic relations between pH and calcium and magnesium, as two important non-process elements, sorption on pulp. Distribution of the elements between pulp and liquid (filtrate or water) was determined with analytical procedures and simulation of pulp washing. It was found that both pH and filtrate composition influenced sorption/desorption of the elements on/from pulp. Filtrate from a pulp mill, mainly content of organic substances in liquid phase, affects their sorption on pulp and desorption to liquid. The higher the pH value the more efficient the sorption of the elements, magnesium being sorbed on pulp in higher amounts than calcium. Prevalence of the sorbed magnesium is preserved even at lower concentrations of both elements. Distribution of the elements between pulp and solution is expressed through mathematic relations which, in turn, can be employed to purposefully modify or optimize the distribution between the solid (pulp) and liquid (filtrate) media. Moreover, rationalized exploitation of mathematical relations enables balancing the elements within a cellulose manufacture, regulating media recycling, and predicting an eventual impact on technological processes.

Aspects on metal ion distribution models in pulp washing simulation

The closure of the liquid system in a pulp mill has increased the concentration of metal ions in the process as compared to traditional plants. These ions commonly referred to as non-process elements, NPE, enter the pulp mill with the wood chips, the water supply and the pulping chemicals. The presence of these ions may lead to various problems, e.g. incrust formation, filtration failures and reduced bleaching efficiency and the objective of the pulp washing operation is to remove a major part of these metal ions from the pulp. The objective of this work was to study the influence of sorption of metal ions on the simulation of pulp washing. A process simulator, ASPEN Plus, has been used to describe the different steps in a displacement drum washer. In order to take account of the sorption of metal ions on the pulp fibres blocks with FORTRAN subroutines have been developed and included. The simulations studies show that the washing results are quite similar for all models at high concentration levels. Hence, the adsorption does not contribute significantly except for low concentration cases. The influence of the sorption model and thus also the choice of model is important only in washing stages where the concentration of ions is very low. Comparison with data for an industrial displacement drum washer indicated the usefulness of the simulation model in evaluation of the operation. It was found that the displacement-washing step was far from ideal probably due to uneven liquid distribution and formation problems. For washing of solids other than paper pulp, e.g. lime mud, the effect of sorption metal ions on the washing result is less pronounced and a simpler model can be used with good results.

Evaluation of white water reuse in the bleaching process for reducing fresh water consumption

The main objective of this work is to study the technical viability of using the effluent generated in paper machines (white water) in the wash presses of the bleaching stage, reducing fresh water consumption. As a case study, the industrial process of Ripasa S.A. Celulose e Papel was evaluated. White water rate is about 700 m3/h and it is not possible to reuse all this volume in the bleaching stage without causing operational problems (fouling in tubes and clogging in the screens). A mass balance of the bleaching unit was developed in an electronic spreadsheet in order to evaluate the possibility of reducing fresh water consumption, using only a fraction of the available white water in the wash presses. To achieve this objective some physical-chemistry properties of the white water stream and of other streams of the process were determined. The maximum concentration of some non-process elements (Si, Ca, Mn and Fe), which could accumulate in the process, were determined in order to establish some parameters to allow process integration of the streams involved, considering operational constraints. The results obtained have shown that it is possible to reduce approximately by 13% the consumption of fresh water and this methodology has been satisfactory.

An integrated approach to the optimisation of water usage and discharge in pulp and paper plants

This paper is aimed at developing a systematic methodology for the cost-effective reduction of water usage and discharge in pulp and paper plants. As the industry moves towards increased system closure, the build-up of Non-Process Elements (NPEs) leads to serious consequences on the process equipment. In response, this paper achieves the following: a mathematical model to track water and primary NPEs throughout the pulping process; rigorous targeting for water usage and discharge; a systematic framework for water reduction using mass integration strategies including no/low cost techniques as well as capital-based techniques; an optimisation model for the optimisation of allocation, recycle and separation of aqueous streams. A case study for water management in a kraft pulping process has been solved to highlight the usefulness of the devised approach.

Sorption of kraft lignin from spent liquors on pulp fibres

In this investigation, the sorption of kraft lignin (KL) from spent liquors, membrane filtrated and model KL liquors on fully bleached softwood kraft pulp fibres, was studied. The time, temperature and alkalinity during the sorption experiments were chosen to reflect the conditions of black liquor impregnation of chips before the cooking operation. The amount of sorbed KL was estimated by brightness and kappa number measurements. A linear correlation between the k value and the kappa number was found. Among the parameters investigated, the concentration of metal non-process elements was found the most crucial for the sorption of KL on pulp fibres under black liquor impregnation. The influences of KL concentration, metal ion profile and KL molecular weight distribution on the KL sorption behaviour under black liquor impregnation are discussed.

Systematic reallocation of aqueous resources using mass integration in a typical pulp mill

This work deals with the application of mass integration to the Kraft process in an effort to minimize the fresh water usage. Due to the recycle and reuse of various aqueous streams present in the given process, there is a build-up of non-process elements (or NPEs) which has to be addressed. The approach consists of gaining process insights from the flow sheet via a graphical solution strategy followed by mathematical optimization. A targeting approach is described. The source sink diagram and path diagram are included in the analysis. A rule based on the lever arm and material balance is used to determine the sequence of steps at fresh water minimization. The graphical approach is easy to apply and allows the user to determine initial solutions to the problem. This is followed by a detailed mathematical optimization formulation for the problem and the solution strategy to be used to converge on the final solution. A case study dealing with the Kraft process, and chloride being the targeted NPE, is considered to demonstrate the potential benefits of the approach. Significant reductions in fresh water demands and wastewater discharges are observed for the Kraft process along with the allocation of NPEs. Any build-up of NPEs due to the recycle and reuse of various aqueous effluent streams to replace fresh water is accounted for via the inclusion of path diagram equations in the mathematical optimization. The principles developed are generic in nature and can easily be extended to other pulp processes as well.

Intrinsic Metal Binding Capacity of Kraft Lignins

The metal removal properties of kraft lignins were examined for a series of SW kraft pulps. The addition of purified kraft lignins to pulp fibers was shown to result in the removal of 20 to 40% of nonprocess elements, such as Mn, Mg, and Ca, in pulp. The effects of concentration and type and amount of kraft lignin applied were explored. These studies illustrate that structural differences in the kraft lignin affect the metal removal from pulp. Factors such as extent of delignification and cooking technology influence metal removal properties. In addition, we have demonstrated that a correlation exists between metal removal and the content of carboxylic acids and condensed phenolics in the kraft lignin applied.

Effluent reduction and control of non-process elements towards a cleaner Kraft pulp mill

This work deals with the reduction of aqueous effluent and fresh water demand in a typical Kraft process to make the process cleaner and environment-friendly. The recycle and reuse of various aqueous streams present in the given process leads to a buildup of non-process elements, or NPEs. The challenges posed by this problem are solved using the holistic framework of mass integration. The approach consists of gaining process insights from the flow sheet followed by a rigorous mathematical optimization. A case study, dealing with the Kraft process, is looked at to demonstrate the potential benefits of the approach. Different NPE separation techniques, specific to the pulp and paper industry, are considered as part of the analysis. Various solution strategies are generated to achieve significant reductions in fresh water demands and waste water discharges for the Kraft process. The major changes in the process involve only piping changes. The principles developed are generic in nature and can be extended to other pulp processes as well.

Electrodialysis for bleach effluent recycling in kraft pulp production: Simultaneous control of chloride and other non-process elements

In the United States, more than 50 million tons of kraft pulp are produced per year. A strong effort by kraft pulp producers has been under way for several years to reduce water use. The in-process recycling of the acidic bleach effluent carrying metals, transition metals, calcium, and often large amounts of chloride is still a significant challenge. The simultaneous selective removal of chloride, and detrimental cations from the acidic bleach effluent of bleached kraft pulp mills by electrodialysis was investigated. The laboratory results with actual effluents show that very good removal levels can be achieved in an economical process. The chloride removal from acidic bleach effluent will aid significantly in keeping chloride levels acceptable in the recovery cycle. Aux Etats-Unis, plus de 50 millions de tonnes de pâte kraft sont produites chaque annee. Les producteurs de pâte font d'enormes efforts depuis plusieurs annees pour reduire l'utilisation d'eau. Le recyclage dans le procede de l'effluent de blanchiment acidique qui contient des metaux, des metaux de transition, du calcium et souvent de grandes quantites dechlorure, reste un defi important. On a etudie le retrait selectif simultane par electrodyalise du chlorure et des cations nocifs de l'effluent de blanchiment acidique d'usines de pâte kraft blanchie. Le retrait de chlorure de l'effluent de blanchiment acidique aidera considerablement a garder des niveaux de chlorure acceptables dans le cycles de recuperation.

Biosludge Incineration in a Recovery Boiler

An activated sludge process is an effective tool against effluent emissions in a pulp mill. It has only a few features which can be regarded deficiences. One of them is that effluent treatment of a modern pulp mill creates some 10-20 tonnes dry solids of biosludge per day. This sludge is difficult to burn due to its high moisture content. The most common way is to mix biosludge with primary sludge, to dewater the mixture in presses and finally to burn it in a solid fuel boiler. This type of sludge treatment incurs rather high costs and does not produce any net energy. Also combustion emissions vary depending on the boiler type. The Metsä-Botnia Kemi Pulp Mill was the first mill in the world to burn biosludge in a recovery boiler. The system start-up was in 1993 and it has been in operation ever since. Mechanically dewatered biosludge is mixed with weak black liquor and concentrated in a conventional evaporation plant equipped with a pressurized superconcentrator unit. In a modern recovery boiler, firing conditions are well controlled and monitored. Better emission control than in most bark fired boilers is achieved. Accumulation of nonprocess elements, corrosion, plugging, scaling and some other operational problems were expected. A lot of experience has been gathered during the years of operation and reviewed in this presentation. The achieved benefits of the system are discussed. Disposal of biosludge in a recovery boiler offers an economically and environmentally attractive alternative. Probably the best evidence from this is the fact that Metsä-Botnia has applied the same process solution in the recent reconstruction of the recovery departments at the Jouteno Mill.

Separation methods for closed-loop manufacture of bleached kraft pulp — overview of an EU project

A summary of an EU project concentrating on separation processes for the closed-loop manufacture of bleached kraft pulp is presented. The project includes four separate tasks in which the need and function of specific “kidneys” are studied. In the first task, the reuse of condensates from black liquor evaporation in pulp bleaching has been studied, including a detailed characterization of the condensates. The discharge of accumulating non-process elements (NPEs) from acidic or neutral bleach filtrates was the focus of the second task of the project. Precipitation of NPEs occurs during a separate pre-evaporation of the filtrates to about 20% DS content. Further precipitation may be obtained by pH adjustment to a level where the precipitation and separation properties can be optimized to favour the discharge of some specific NPEs (e.g., silicon). The third task concentrates on the discharge of accumulating compounds from alkaline bleach filtrates by the use of ceramic nanofiltration membranes developed within the project. In the fourth task an overall system analysis of the closed cycle pulp mill is made.

Elementos minerais em madeiras de eucaliptos e acácia negra e sua influência na indústria de celulose kraft branqueada.

No ramo de papel e celulose os efluentes são um dos principais problemas e o consumo de água é crítico. Os circuitos internos estão sendo fechados e as fontes de contaminação precisam ser conhecidas. Uma das fontes de contaminação destes efluentes são os minerais da madeira, matéria-prima neste tipo de indústria. Neste trabalho foi avaliada a quantidade de alumínio, cálcio, cobre, ferro, potássio, magnésio, manganês, sódio, níquel e silício em madeiras de cinco espécies florestais, plantadas na região de Guaíba, RS, compreendendo Acacia mearnsii, Eucalyptus dunnii, Eucalyptus globulus, Eucalyptus grandis, Eucalyptus saligna. As amostras de madeira foram tratadas com digestão ácida e com carbonização em mufla, solubilizando em ácido para serem analisadas por ICP (Inductively Coupled Plasm). Foi observado que a quantidade de elementos é considerável e os principais elementos encontrados foram K, Ca, Na, Al, Mn, e Si. As espécies Eucalyptus grandis e Eucalyptus saligna mostraram melhor crescimento. Por outro lado, Eucalyptus globulus e Eucalyptus dunnii mostraram menor crescimento e maior teor de elementos. Pelo fato dessas espécies introduzirem maiores quantidades de elementos no processo de fabricação, devem conduzir a maiores problemas industriais. A Acacia mearnsii mostrou o menor nível para ferro, manganês e níquel, sendo uma boa opção para uso em branqueamento com oxigênio, ozônio e peróxido. O silício observado estava em baixa quantidade, o que leva a afirmar que há, na prática industrial, muita contaminação da madeira explorada com solo já que se encontram altos valores de sílica nos circuitos de licores. A entrada dos elementos minerais estudados em uma fábrica de celulose via madeira varia de 4 a 6 kg por tonelada de polpa não-branqueada produzida. Cuidados especiais devem ser tomados com as espécies com madeiras mais ricas em cinzas e elementos minerais e mais baixas eficiências nutricionais, como foi o caso de Eucalyptus dunnii e Eucalyptus globulus.

Biosludge incineration in a recovery boiler

An activated sludge process is an effective tool against effluent emissions in a pulp mill. It has only a few features which can be regarded deficiences. One of them is that effluent treatment of a modern pulp mill creates some 10-20 tonnes dry solids of biosludge per day. This sludge is difficult to burn due to its high moisture content. The most common way is to mix biosludge with primary sludge, to dewater the mixture in presses and finally to burn it in a solid fuel boiler. This type of sludge treatment incurs rather high costs and does not produce any net energy. Also combustion emissions vary depending on the boiler type. The Metsa-Botnia Kemi Pulp Mill was the first mill in the world to burn biosludge in a recovery boiler. The system start-up was in 1993 and it has been in operation ever since. Mechanically dewatered biosludge is mixed with weak black liquor and concentrated in a conventional evaporation plant equipped with a pressurized superconcentrator unit. In a modern recovery boiler, firing conditions are well controlled and monitored. Better emission control than in most bark fired boilers is achieved. Accumulation of nonprocess elements, corrosion, plugging, scaling and some other operational problems were expected. A lot of experience has been gathered during the years of operation and reviewed in this presentation. The achieved benefits of the system are discussed. Disposal of biosludge in a recovery boiler offers an economically and environmentally attractive alternative. Probably the best evidence from this is the fact that Metsa-Botnia has applied the same process solution in the recent reconstruction of the recovery departments at the Jouteno Mill.

Sodium aluminosilicate crystal formation in alkaline solutions relevant to closed cycle kraft pulp mills

AbstractMetal ions, such as those of Al, Si, and Ca known as non‐process elements (NPEs), are a major concern in the development of technology for achieving closed cycle operation in Kraft pulp mills. For example, the build‐up of NPEs such as Al and Si ions causes sodium aluminosilicate complex formation in the recovery cycle of Kraft pulp mills. These complexes precipitate on process equipment and are very hard to remove. The purpose of this study is to supply basic data on the precipitation conditions for such complexes. These data can be used to develop process models for design and optimization of closed mills. The precipitation conditions of sodium aluminosilicate complexes in synthetic green and white liquors were determined. In the experiments, the effects of varying the A1/Si ratio and concentrations of OH‐, Na2S, CO32‐, and SO42‐ were studied. The structure of the precipitates was identified by X‐ray diffraction, thermo‐gravimetry and chemical analysis. The precipitates were found to have the structure of hydroxysodalite and/or sodalite.

Low Effluent Processing in the Pulp and Paper Industry: Electrodialysis for Continuous Selective Chloride Removal

Pollution prevention is currently a major focus of the United States pulp and paper industry. Significant process changes are inevitable to implement low effluent processing. The kraft pulping process is prevalent for the production of wood pulp. About 50 million tons of wood pulp are produced annually in the United States alone using the kraft process. Water consumption is currently roughly between 30 and 200 m3 of water per ton of air dry bleached kraft pulp. In-process recycling of water is now being implemented by many mills to reduce the use of increasingly scarce water resources and to reduce the need for waste-water treatment. Mass balance considerations and industrial experience show that nonprocess elements, which are detrimental to the kraft process, such as chloride and potassium, will quickly build up once water use is significantly reduced. High concentrations of chloride and potassium can cause corrosion and lead to more frequent mill shutdowns due to fouling of heat exchanger surfaces in the kraft recovery furnace. Electrodialysis with monovalent selective anion and cation exchange membranes was explored here to selectively remove chlorine as sodium and potassium chloride from a feed stream with very high ionic strength. Experiments with model solutions and extended tests with the actual pulp mill materials were performed. Very good selectivities and current efficiencies were observed for chloride over sulfate. The outstanding performance of the process with actual mill materials containing organic and inorganic contamination shows great promise for rapid transfer to the pilot scale. This work is an example of the usefulness of membrane separations as a kidney in low effluent industrial processing.

Non-process elements in a bleached kraft pulp mill with a high degree of system closure - state of the art

Non-process elements in a bleached kraft pulp mill with a high degree of system closure - state of the art