FAIR-CT96-1360 Separation Methods for Closed-Loop Technology in Bleached Kraft Pulp Manufacture

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FAIR Area 1.3 - Forestry-Wood Chain : Paper/Pulp : Process Engineering : Pulping : Separation/Fractionation

	Contract No:	FAIR-CT96-1360
	Date Prepared:	November 2000
	Source:	Final Report Abstract

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Final Report Abstract

Introduction

The pulp and paper industry has a great potential to develop its operations into a truly eco-cyclic production system. However, a high degree of system closure in the process leads to problems due to the accumulation of different substances. It is therefore necessary to find ways to handle the accumulation of the most troublesome substances.

The overall objective in this project was by separation methods find specific solutions to some of the earlier identified "bottlenecks", related to closed-loop kraft pulp production. One way to handle accumulation of troublesome substances is to introduce internal separation techniques, which clean the process streams in the pulp mill. Another issue of great importance for closed-loop operation is to solve the water balance in a reasonable way, which means, for example re-uses of evaporator condensates.

Activities

Reuse of condensates fro black liquor evaporation Analytical methods were successfully developed for quantitative determination of the known main organic impurities in black liquor evaporation condensates. In addition, detailed gas chromatography - mass spectrometry investigations resulted in identification or partial characterisation of several poorly known or totally new condensate impurities. Based on their chemical structures, it can be expected that some of them (particularly thiols) may contribute to odour of the condensates. On the other hand, it is known that the thiols are easily oxidised by typical bleaching chemicals, such as ozone and hydrogen peroxide. Thus it is reasonable to assume that their typically low concentrations disappear during the bleaching reactions.

Industrial condensates from black liquor evaporation were used as wash water during bleaching of kraft pulp in laboratory trials. The effects on several pulp properties were monitored. Most of the studies were carried out with typical secondary condensates from a traditional technology mill. In addition, the effects of more contaminated condensates were searched for by carrying out selected treatments with the mixtures of secondary + foul condensates, and secondary condensate + black liquor. The latter system was used to simulate malfunction of the evaporator.

The principal result from all the experiments simulating single bleaching stages was a relatively unexpected finding according to which the use of the condensates as wash water during bleaching does not increase off-odour levels of the pulps (according to the analytical methods used in this project). This is apparently due to the capability of the applied bleaching chemicals to oxidise the main odour-causing compounds to less harmful species (although some losses due to evaporation, especially during pulp drying, are also possible). It was similarly found that the use of secondary condensates does not have detrimental effects on viscosity, kappa number, and brightness of the pulps. However, more contaminated condensates naturally had several negative effects. Separate experiments indicated that the clean condensate fractions have only little, if any, effect on the consumption of the bleaching chemicals.

The principal results from all the experiments simulating entire bleaching sequences were equally promising. In these studies, the condensates were applied as wash water at the second last stage - the promising results here naturally indicated that the condensate use at any earlier stages should be possible.

Handling of acidic and neutral bleach plant filtrates Bleach plant filtrates from neutral (Q) and acidic (DO) bleaching stages were treated for the removal of non-process elements (NPEs). Removal of silicon was of special interest as its controlled removal is difficult in today's pulp mills. The treatment included evaporation at 55 ·C, precipitation and separation. The idea was to adjust the pH in the filtrates to about 10-11.5 and form a precipitation of the troublesome elements, followed by a separation of the precipitate. This could be done either on an unconcentrated or concentrated (evaporated) filtrate.

Evaporation was carried out at two different pHs, at pH 6 and at pH 10.5, during which filtrate properties were detected at several DS contents. Large proportion of NPEs (especially silicon) could be precipitated in both cases. For Q filtrate NPE removal at pH 6 may be sufficient enough, but if necessary a further improvement can be achieved by pH adjustment to about 10. For D_o filtrate the higher pH was clearly better, but similar results were achieved by raising the pH to about 10 after evaporation at pH 6 and after evaporation at pH 10.5. No scaling problems appeared, but some problems in the circulation of the high amount of solid material were observed. Organic material co-precipitated with the NPES, but did not appear to cause any difficulties in the precipitation process.

The CST values, indicating the filtration resistance of the precipitates formed in the filtrates, were high, comparable or even higher than the values obtained with green liquor dregs. Slightly lower CST values could be obtained by the addition of lime mud to the filtrates.

The effect of storage (solid residence time) of the filtrates/precipitates on their filtration properties was studied by ageing them for 5 and 24 h in the same temperature in which the concentration by evaporation was carried out. Lower filtration resistance, indicated by lower CST values could be obtained after ageing of Q filtrate, regardless of the pH during evaporation. In the case of Do filtrate the effect of ageing depended on the pH during evaporation. Evaporation at pH 6 and ageing resulted in better separation properties than achieved directly after evaporation. The result was the opposite if the evaporation was carried out at pH 10.5. Lime mud was not added to the filtrates in these experiments.

Continuous crystallisation of Q filtrate was simulated by repeated batch experiments in the laboratory after mill scale evaporation, mainly to find out the effect of increased magma density on the solid-liquid separation properties. The experiments were carried out at pH 8-12. Fairly good settling properties of the precipitate were achieved, especially at pH 10. It was possible to obtain lower CST values after continuous crystallisation than after batch-wise precipitation. NPE removal efficiency was similar in both cases.

Handling of alkaline bleach plant filtrates Tight mineral membranes were developed for operation in bleaching stage filtrates containing hydrogen peroxide. Mineral membranes with cut-off around 3000 daltons were used in successful pilot trials for separation of organic substances from a bleach plant filtrate (an EOP stage) in an ECF sequence. At the end of the project also a PO stage filtrate from a TCF sequence was treated in a pilot trial. The membranes used had in this case a cut-off around 5000 daltons. The COD reduction was about 30% and the retention of hydrogen peroxide was max. 5-10%, which means that the bleaching agent can be recovered and improve the economy of an installation. In general the results, for handling accumulating troublesome organic substances, was promising regarding a high flux and long operating time between washing of the membrane surface.

Bleaching results, from a laboratory study, show that addition of concentrated EOP filtrate into a first D stage had negative effect on the brightness and reagents consumption. However, the effect was small due to the rather low level of carry over which can be expected in a modem mill with effective washers. On the other hand, pulp brightness was improved and reagent consumption decreased when, by membrane filtration, cleaned EOP filtrate was added. These effluents were also able to replace caustic soda in order to obtain the optimal pH at the end of D stage. Another interesting finding was that recycling of D filtrate to the D stage resulted in increased chemical consumption in order to obtain brightness.

From the long time pilot trials it has been calculated that it is possible to decrease the water consumption for the ECF bleaching. In a specific case, with high efficiency membrane filtration membranes, the fresh water saving could be about 4500 M³ /day. The thermal balance showed then an excess of steam production of 40 t/day in relation to a conventional kraft pulp mill. The increase of black liquor sent to evaporation needed to increase the evaporating capacity and the burning capacity of the mill about 10%. Mainly magnesium and calcium were removed by the membrane filtration at the alkaline conditions studied. The other NPEs will accumulate in the circuits.

Results have proven that both the coagulation/flocculation and the ultra/nanofiltration technologies can be considered as an available kidney to remove organic materials from the alkaline bleach plant effluents. They have shown the possibilities to recycle alkaline effluents partially cleaned without any negative influence on chemical consumption and pulp brightness.

Treatment of alkaline bleaching effluent in a membrane bioreactor was also studied and resulted in more than 50% reduction efficiency. It also allowed a COD load that corresponds to an effluent residence time four times lower than operation of activated sludge. The COD reduction efficiency was 10% higher, during all the trials for membrane bioreactor treatment, compared to activated sludge treatment.

Process integration of separation methods A system analysis was carried out including results from all main activities within the project, Closed cycle operation of the kraft pulp mill without the use of any additional separation methods will result in process disturbances and imbalances regarding the intake and out-take of sodium and sulphur according the simulations carried out.

Calcium silicate will precipitate in the lime mud due to mill closure. Our simulations showed that the silicon content could be drastically decreased in the NPE-kidney in closed-loop operation, but not to an extent where calcium silicate precipitation in the lime mud could be completely avoided. However, with only small liquor losses in the brown fibre or recovery area, the risk for calcium silicate precipitation would disappear. In the closed-loop ECF case, the content of chloride in the white liquor was high. A separate kidney to remove chloride would be required, since large quantities of ESP dust would have to be purged otherwise. Also, to control the sulphur balance a large purge of ESP dust will be demanded in the closed-loop cases. A more sophisticated technique to control the sulphur balance would therefore be necessary both in the ECF and TCF cases. -

In closed-loop bleach plant operation, the build-up of COD in the bleaching stages leads to an increased chemical consumption. By using membrane filtration, this negative effect could be strongly reduced in an ECF sequence where EOP stage filtrate was recycled to the post oxygen washing system. The risk for scaling of calcium carbonate (EO and EOP stages) and calcium oxalate (Q_l or D_o stages) would increase by increased closure. Addition of magnesium would be recommended in order to avoid the formation of calcium oxalate.

The integration of the proposed separation methods will have a relatively small effect on the overall energy situation in the mill. The heat surplus (steam) will decrease with 0.9 GJ/t₉₀ from about 6-7 GJ/t₉₀ due to the pre-evaporation of the bleach plant filtrates. The power consumption of the membrane filtration unit was estimated to 20 kWh/t₉₀. This should be compared to the total electrical power surplus of 600-700 kWh/t₉₀ that can be achieved in a modem mill.