Showing papers by "Carlos Negro published in 2006"

Methodology for flocculant selection in fibre–cement manufacture

Abstract: In the Hatschek process used to produce fibre–cement products, it is necessary to use a suitable flocculant when asbestos is substituted by pulp fibres. The right selection of flocculant is crucial in the industrial process due to its effects on mineral fines retention, dewatering and formation and, as a consequence, on the overall efficiency of the machine. This paper presents a two-step methodology for flocculant selection in the fibre–cement manufacture. The first step is based on the study of the flocculation processes and the flock properties, using a focused beam reflectance measurement (FBRM). This technique allows the study of flock size, flock stability and flock resistance to shear forces, reflocculation tendency and reversibility of the flocks, as well as the optimal flocculant dosage for each particular fibre–cement suspension. The second step uses a drainage vacuum tester to study retention and dewatering. The two techniques give important and complementary informations that allow a proper selection of the best flocculant for the fibre–cement manufacture.

Monitoring of Dissolved Air Flotation by Focused Beam Reflectance Measurement

Abstract: Aggregation and removal of detrimental substances from peroxide-bleached, fiber-free thermomechanical pulp water by dissolved air flotation (DAF) were studied for different chemicals. A focused beam reflectance measurement (FBRM) instrument was used to continuously assess the aggregation and removal of dissolved and colloidal substances (DCS) during batchwise DAF of process water. FBRM is particularly suitable for assessing the aggregation of DCS during the coagulation process prior to flotation. Differences between flocculants, as well as between different flocculant doses, can be observed. The FBRM results were in good agreement with turbidity measurements and with determination of wood resin and pectic acids by gas chromatography. General trends in the flotation process were also observed. However, monitoring the aggregates during flotation is more complicated due to the interference of air bubbles. To determine the overall flotation efficiency, a two-probe system with measurement points before and aft...

Fluoride speciation in stainless steel pickling liquor

Abstract: Mixtures of hydrofluoric and nitric acids are used in the pickling of stainless steels, in order to remove oxide scale, generating pickling waste liquors with high HF concentration. Species appearing in this mixed acid solution are fluoride-metal complexes with high stabilization constants. This work discusses about the modelling of equilibrium reactions that could take place, like complexation of iron and chromium. As observed in the literature, 31 species could be present in pickling liquor with 25 equilibrium reactions. Activity coefficients are calculated using Bromley's methodology that depends on cation–anion interaction parameter B. New parameters at 25°C have been calculated, using Nelder and Mead Simplex Algorithm. The new model shows that FeF3, CrF2+ and Ni2+ are the main metallic forms in the solution and undissociated hydrofluoric acid has a high relative concentration, while free fluoride practically does not exist in solution. In neutralization processes, free fluoride is released from undisocciated hydrofluoric acid. Therefore, high fluoride complexes are formed, which could precipitate instead of metal hydroxides.

Purification of peroxide-bleached TMP water by dissolved air flotation

Abstract: Papermakers are continuously searching for better ways to overcome the problems related to accumulation of pitch and polygalacturonic acids (pectins) in process waters. Dissolved air flotation is a technique that could be used as an internal cleaning stage to remove these substances. This study investigated removal of pitch and galacturonic acids from peroxide-bleached, fiber-free thermomechanical pulp (TMP) water by dissolved air flotation (DAF). Both inorganic coagulants and organic flocculants were applied to aggregate the detrimental substances before flotation. The smallest chemical doses that caused aggregation were determined before DAF experiments by turbidity measurements. We also used focused beam reflectance measurement (FBRM) to assess aggregation. Up to 90% of pitch could be removed by DAF at pH 5, determined by gas chromatography and turbidity assessment. At pH 8, the anionic charge of detrimental material increased, accompanied by increased consumption of cationic chemicals. In addition, resin acids dissolve at pH 8, impairing their removal. Aggregation and flotation should therefore be carried out at acidic conditions to increase the aggregation of resin acids and to avoid excessive chemical consumption. Pectins were selectively aggregated. The results indicate that understanding the chemistry of aggregation and flotation is necessary for selection of flocculation chemicals, adjusting process conditions, and for obtaining an acceptable cleaning efficiency. Application: This study provides valuable information for papermakers who might install a DAF cleaning stage in aTMP process and for optimizing the DAF functions.

Optimization of the Fiber Cement Composite Process

Abstract: This study reflects the success of combining focused beam reflectance measurement (FBRM) techniques and artificial neural networks (ANN) to make predictions of fiber cement properties, to optimize the industrial process. Three neural networks have been developed. The inputs of these networks are the FBRM sensor measurements and the densities taken from formed sheets. The outputs are final product properties, related to product resistance. With this work, a good prediction of final properties has been achieved. The conclusions reached with the analysis of the results of neural networks can be used in establishing optimal process conditions. The obtained results demonstrate that the FBRM probe can be considered to be a good soft sensor for predicting the on-line fiber cement resistance.

Evaluation of an Alternative Flocculation System for Manufacture of Fiber−Cement Composites

Abstract: Flocculation has been a key issue for fiber cement manufacturing since cellulose or poly(vinyl alcohol) (PVA) fibers are used to replace asbestos. Due to the complexity of flocculation, many fiber cement companies face difficulties in optimizing it, which leads to unpredicted production problems and lower process efficiency. This paper studies the behavior of a dual system (PFR/PEO) and compares it with two anionic polyacrylamides commonly used in this industry. The work was carried out using a focused beam reflectance measurement as a sensor to monitor the flocculation process in real time. Results show that the proposed alternative dual system induces the formation of larger but less stable flocs than the ones obtained with the anionic polyacrylamides. A flocculation mechanism was proposed for the dual system consisting of the initial adsorption of the phenol−formaldehyde resin (PFR) onto the particles, which provides junction points for bridge formation by poly(ethylene oxide) (PEO). The flocculation k...

Flocculation of pulp fractions induced by fluorescently- labelled pdadmac

Abstract: Successful modelling of the papermaking process permits to build up models on the behaviour of paper chemicals, based on the knowledge of the interaction between the different pulp fractions and the chemicals. The paper presents the results obtained with a fluorescence-labelled polydiallyldimethylammonium chloride (PDADMAC) using a focused beam reflectance measurement (FBRM) to monitor the interaction between the different pulp fractions. Results show that, in terms of mass balance for polymer adsorption, the fibers are capable of retaining more polymer than needed for pulp flocculation. In the presence of fibers, fillers particles would adsorb on them yet, in time, fillers would be detached while the polymer would remain on the fibers.

Modelling and simulation in pulp and paper industry: Current state and future perspectives

Abstract: Paper industry is facing today the challenges of being simultaneously flexible towards customers, efficient in the use of capital intensive assets, and ecologically sustainable both when using resources and utilities and with respect to waste handling. However, the production lines are not flexible enough to meet today's requirements because of some complex dynamics and rigid operational practices. The benefit of applying modelling and simulation techniques in pulp and papermaking manufacturing processes will favour a better understanding of the mechanism involved in the processes and their control loops. This will allow papermakers to find solutions for currently pending problems in the paper industry, such as fast grade changes, improvement of paper quality, optimising the wet end chemistry, enhancing runnability and reducing emissions by improving process design, process monitoring and decision support during operation. Due to the importance of the topic at European level, a COST Action has been initiated for promoting the development and application of modelling and simulation techniques in pulp and paper manufacturing processes. In this paper, a review of the state-of-the-art on modelling and simulation in the pulp and paper industry is presented, alongwith further research needs.

Use of modelling and simulation in the pulp and paper industry

Abstract: In pulp and paper the modelling and simulation of the pulp production processes was the first major application. Since then several other applications have been realised though papermaking has never been on the lead concerning the use of computer-based modelling and simulation. The complex nature of the materials is one of the most demanding challenges and the biggest hurdle for any electronic description of the papermaking process or paper itself. So other industries took the lead and it is now for the paper industry to learn from these industries what can be done with the help of computers to control or optimise processes or to design new grades. The application of results gained with the use of modelling and simulation techniques in pulp and paper manufacturing processes has helped the industry to, for example, reduce emissions and increase the productivity and cost-efficiency of the processes. Still there are many important tasks open. A better understanding of the mechanisms of the processes and thei...