SE517587C2 - Procedure for leaching electric filter ash from a soda boiler - Google Patents

Abstract

The invention relates to a process for purifying electrical filter ash from chlorides, in particular, by means of leaching and subsequently separating off crystallized substance, which crystallized substance is returned to the process for forming black liquor. A good separation of solid phase in the form of crystalline Na2S04 is obtained by means of a repeated gentle agitation of slurried electrical filter ash in a leaching tank with a total dwell time of between 1 and 5 hours, directly followed by centrifugation of the leached electrical filter ash. The solid phase is supplied to the black liquor prior to final evaporation and/or recovery in the recovery boiler. More than 70 % of the chloride content in the electrical filter ash and 70 % of the potassium content are efficiently leached out, and an improved utilization of process chemicals is obtained.

Description

ansa: rush 20 25 30 517 ä87 oo ou in its entirety or subsets thereof. There are also other processes for the recovery of Na2SO4 where the electricity ash is first completely dissolved and then crystallized by evaporation in a crystallizer with subsequent separation of solid phase and liquid.

Filtration technique SE, C, 504374 shows a procedure in which the elter ash is treated in a pressurized elter, where both filtration and washing of the elm cake and any leaching take place in one and true vessel. This process gives a good result as long as the filter can be kept free from clogging. Methods for cleaning the filter are required, where the cleaning takes place either during operation or in case of temporary interruption of the filtration process.

U.S. Pat. No. 3,833,462 discloses a process in which leaching takes place at pH 3-6 and 40-80 ° C, followed by filtration in a filter.

In Tappi Proceedings, Chemical Recovery Conference 1992 in Seattle, pages 329-350, "Recycle of Bleach Plant Extraction Stage Ef fl uent to the Kraft Liqour Cycle presents B.Blackwell and A.Hitzroth experiences from applied leaching processes. In a Harmac system a leaching of The electricity ash in water at a dry matter concentration of around 15% by weight and at pH 5, which low pH was pointed out, constituted a significant corrosion problem.

The leached liquid was altered in an alter (belt alter) and this alter showed significant clogging problems due to small particles of organic material clogging the alter cloth. A proposed measure was to use a larger filter, which in itself should extend the operating time, but sooner or later the filter must be cleaned. Just over 89% of the sodium chloride included in the electricity ash was stated to be leachable from the electricity ash.

The cooling technique SE, A, 95 04281 shows a method which involves cooling. Here, the leaching should take place at a pH below 10 and a temperature above 20 ° C, typically at 30 ° C, so that the total amount of slurry material undergoes a cooling to a temperature below 20 ° C. This process involves a cooling requirement with concomitant energy consumption. SE; A, 9603972 also shows another method for leaching the electric ash where the electric ash first undergoes a leaching at at least 32 ° C, preferably in the vicinity of the boiling point of the aqueous solution, and where a first crystallized substance after separation is returned to black liquor. lakningjl fi lteraskaldoc: won »win 10 20 25 30 5173587 The leachate obtained from the process then also undergoes cooling to about 10-15 OC, after which a second crystallized substance is also returned to the black liquor. This system provides good leaching but requires a large energy requirement for the cooling, even if the cooling is only required for the smaller amounts that the leachate constitutes.

The problem with the known solutions is that they have one or two of the following disadvantages; -energy-intensive, resulting in high operating costs; -requires expensive and complicated devices / systems; - contains filters that easily clog and that make continuous operation difficult; does not give a sufficiently high degree of bleeding of chlorides and potassium on recycled process liquid, or does not give an equal degree of recovery of Na2SO4 from the electric filter ash.

BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to obtain an improved recovery process for the useful chemicals of the electricity, mainly Na2SO4, which does not have the disadvantages (s) of the prior art.

The improvement mainly refers to a lower energy consumption but also an improved degree of recycling of these useful chemicals, without a high proportion of mainly chlorides but also potassium being included.

Another purpose is to obtain a continuous process which does not include alterations, which alteration processes cause clogging problems and the need for regeneration of the alterium diet. An optimized process which ensures a favorable particle formation should not require alteration devices.

This object is achieved with a method according to the core drawing part of claim 1.

Additional features and aspects and advantages of the invention will be apparent from the following claims and from the following detailed description of some embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 which shows an apparatus set with which the inventive method can be applied in the leaching of electric ash and subsequent separation of solid phase. lakningjl fi ltemskaldoc other true: 20 30 517 4587 o c n v ao DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Figure 1 shows an apparatus set which can be used according to the inventive procedure.

In a first leaching step 1, electric ash and liquid for leaching and recycled liquid phase are added from subsequent separation steps and possibly chemicals so that a suitable mixture is obtained.

The liquid for leaching can suitably be process water from a bleaching plant, dryer, evaporating condensate or scrubber condensate. Water can also be used.

In cases where the electricity ash contains high levels of Na2CO3, H2SO4 can be added. Typically, such an investment is at a level of around 0.1 kg per 1.0 kg of ash, ie in proportions of about 5-15% by weight.

The electricity ash can typically have a composition corresponding to; Component Weight% § w% 1 Na2sO4 see Na2CO3 5 NaCl 2 KCl 7 The ash that is added to the leachate contains chloride and potassium ions. The chloride content in the slurry is 2.0-7.0% by weight.

For optimal leaching process, a pH value in the range pH 8-13, preferably 1 1-12, is established in the leaching tank, thus a strong alkaline environment.

The mixture in the leaching tank must not exceed 1600 kg / m3, which thus sets the upper limit for the leaching process. If the consistency becomes too high, which is already beginning to be seen at 1570 kg / m3, the mixture becomes increasingly difficult to pump and agitate with the stirrers.

In particular, the initial mixing with the electrolyte with the initial stirrer 11 is made more difficult at these concentrations. A suitable lower level of the concentration corresponds to a mixture at 14 DEG-155 DEG kg / m

The temperature should be kept in the range 65 -1 05 ° C, where a higher temperature in the range 85-105 <> C is most favorable for the leaching process. An optimal temperature both from the process and the device point of view is at about 9500 leaching jl lterascaldoc: nare »urine 10 15 20 25 30 517 §87 If leaching takes place under mild agitation at these process conditions (pH, concentration and temperature) for a long time, in the range 1- 5 hours, preferably 2 hours, favorable process conditions are formed for crystal growth of mainly solid phase of Na 2 SO 4.

It has been found that the formed crystals are normally distributed around a particle size of around 200 .mu.m, which is a surprisingly good result to be compared with what can be obtained with the energy-intensive cooling technique where under optimal conditions crystal formation can be achieved with particles distributed around 300 microns.

The leaching tank is designed so that repeated mild agitation is obtained over the entire volume.

An initial agitation preferably takes place with an initial stirrer 11 in connection with the supply of the electric ash. Thereafter, the mixture passes at least one additional agitation zone and preferably two agitation zones.

This can be obtained by inserting at least one double bottom 13 in the middle of the leaching tank. In an outlet from this double bottom 13 an intermediate stage stirrer 12a is arranged and in connection with the outlet of the leaching tank an output stage stirrer 12b. The residence time in the leaching tank between agitations when the mixture is not subjected to direct agitation should amount to at least 30% of the total residence time in the leaching tank.

Intermediate stage stirrers thus act on the slurry electric ash when it has had a residence time in the range of 30-70% of the total residence time in the leaching tank, and the slurry electric ash is exposed to a final stage stirrer in a lower part of the leaching tank adjacent to the leaching tank. The agitation should be gentle so that already crystallized particles do not break, and for intermediate stage and final stage stirrers a propeller-type mechanical agitator can be suitably used in the respective position, both agitators driven by the same shaft, which is driven at a moderate speed in the range 50- 200 rpm, preferably about 80 rpm.

The sluice thus obtained is further pumped via the pump 4 to a centrifugation step, here a decanter centrifuge 2, where particles formed, solid phase with Na 2 SO 4 are separated out as a dry fraction. By using a decanter centrifuge, a continuous process is obtained which does not need to be interrupted for regeneration of the equipment (due to clogging, etc.).

The decanter centrifuge comprises in a known manner a rotating screw body 20 on which a screw thread 21 is formed. Around the writing body 20 a housing 22 is arranged which also rotates.

The screw body is given a rotation R1 and the housing a co-rotating rotation R2, where typical speeds are R1 = 3400 rpm, and R2 = 3350 rpm, thus the screw is given a slightly higher leaching scale x - of 10 20 25 30 517 587 6 - '. . . Rotational speed which contributes to the screw thread 21 of the rotating body slowly feeding the crystallized particles towards the outlet 24.

The slurry is fed to the decanter centrifuge via the inlet 23, and due to the rotation of the housing 22 the leached liquid is thrown towards the inside of the housing, where the heavier solid particle fraction forms the outer layer 28, due to the centrifugal action, and the liquid fraction forms the inner layer 29. The liquid fraction leaves the decanter centrifuge via the overflow drain 25, since the screw discharges the fixed fraction towards the outlet 24. Because the liquid fraction's overflow drain 25 is at a shorter distance from the axis of rotation than the outlet 24, the fixed fraction will mainly be transported by the screw thread 21 towards the outlet 24.

The solid fraction thus obtained, which is predominantly Na 2 SO 4, with the decanter centrifuge obtains a dry content in the dry matter fraction in the range 58-97%, and is then fed to a slurry vessel 3 where mixing takes place with, for example, black liquor / BL for further recovery, either via the evaporation (not shown) or directly to the recovery boiler 18. In the solid phase, most of the heavy metals present in the electricity ash, such as Ba, Pb, Cd, Co and Mn, are also recovered. The heavy metals are returned to the lye cycle of the pulp process and separated in the process section that is most favorable.

The liquid fraction, with its content of chloride and potassium ions, can partly be returned to the leaching tank 1 or partly 26 to the drain for external purification or further work-up.

In order to further leach out the solid fraction from chlorides, a washing zone can be arranged in the decanter centrifuge. In the figure this is shown by a distribution ring 27 for washing liquid, which opens up radially inwards towards the passing layer 28 of the solid fraction. The washing liquid, preferably pressurized, may suitably be the condensate (cond.) Which is otherwise added at the top of the leaching tank in connection with the initial slurry of the electric ash.

The washing liquid addition is advantageously done at a position in the decanter centrifuge where a separation effect on liquid is formed even after the position of the washing liquid addition.

Through the process according to the invention, it has been possible to separate chloride and potassium from the electricity ash, and return the useful process chemical Na2SO4 to the process. In an application where one starts from an elter ash with the following composition; lakningjl fi lteraskaldoc 517 587 7 Component weight% Cl 8.1 K 4.0 Na 30.4 5 S04 57.0 C03 0.5 we have succeeded in recovering up to 80% of the electricity of the electricity and 85% of the content of the electricity of S04.

The chlorides are soaked via the liquid fraction from the decanter centrifuge, where 0.77 tonnes of liquid fraction per tonne of ash is sent to sewage or further reprocessing.

The liquid fraction separated from the decanter centrifuge can typically have a content of 7% by weight of chlorides, as well as 3.5% by weight of potassium, thus avoiding that these levels build up in a closed process. 70% of the electricity and potassium content of the electricity can be separated from the electricity as a result of the process. »Plus> |||; la1ming_el fi1 teraska2.doc

Claims (1)

1. anta:: spwn 10 15 20 25 30 517 587 8 o o ø o Q o. ,. I .- PATENT CLAIMS. Process for purifying electric filter ash from recovery boilers from mainly chlorides and potassium by leaching and subsequent separation of solid phase, which solid phase is returned to the pulping process, the method comprising a first leaching step where elteraskani first slurries a leach tank at a temperature in the range 65-105 ° C, preferably about 95 ° C and at a pH in the range 8-13, preferably pH 11-12, and where the pH can be adjusted by adding H 2 SO 4 and suitable process liquid, whereby a slurry or ash with a dry matter level in the range 15-40% by weight is obtained. , characterized in that the process comprises and that the slurry or agitation passes and is treated in a leaching tank in at least two agitation zones, preferably at least three agitation zones, in which agitation zones the slurryed orlash undergoes repeated mild agitation in the leaching tank between a first leaching tank and a total leaching tank. interval 1-5 hours, preferably about 2 hours, and that the slurry or ash between agitations is not subjected to direct agitation for a period of at least 30% of the total residence time in the leaching tank, after which the leached ash is fed to a separation step in which the leached ash undergoes a separation or crystallization. substance by a centrifugal separation, from which centrifugal separation a fraction of dry substance and a fraction of liquid are obtained, the dry matter obtained being reintroduced to the leaching cycle or to further leaching steps with subsequent separation of solid phase from the liquid phase, and where the liquid fraction is returned to the process. . Process according to Claim 1, characterized in that the centrifugal separation takes place in a decanter centrifuge with the capacity to obtain a dry content in the dry matter fraction which amounts to a level in the range 5 to 97%. . A method according to claim 1, characterized in that the repeated mild agitation is obtained by passing the slurried electric ashtray a first initial stirrer arranged in the upper part of the leaching tank which mixes the electric ashtray with supplied liquid, an intermediate stirrer in a middle portion of the leaching tank. had a residence time in the range of 30-70% of the total residence time in the leaching tank 517 587 9 leaching tank, and that the slurried electric ash was exposed to a final stage stirrer in a lower part of the leaching tank adjacent to the leaching tank outlet and sliding outlet outlet. propeller type agitator which is operated at a moderate speed in the range 100-200 rpm, preferably about 80 rpm. A method according to claim 2, characterized in that the decanter centrifuge comprises a washing zone acting on a separated dry substrate fraction, where washing liquid is supplied to the formed layer of solid fraction and leaches additional chlorides and potassium before discharging the solid fraction from the decanter centrifuge. Method according to claim 2 or 3, characterized in that the leaching tank is located in a closed circuit in which is included in the series the leaching tank which receives electric ash, pump for feeding to separation phase, decanter centrifuge for solid phase separation, mixing vessel for mixing solid phase and black liquor, recovery boiler for combustion of black liquor with solid phase content, or for separation of electric ash from the flue gases of the recovery boiler, and separation of the electric filter ash from the electricity for recycling to the leaching tank, and where this closed circuit is fed condensate and H2SO4 to the leaching waste water with potassium and chloride-rich liquid was bled out of the circuit. arna; : ann lakning_el fi ltemsku2.doc