Background of the Invention

The invention relates to an improved method for operating a pressurized disc filter for the filtering of causticization liquors containing lime mud particles, and relates also to the pressure disc filter as such.

Prior Art

In older filtering processes for the causticization process developed during the 60-ies and up to 80-ies predominantly gentle filtering techniques were used with atmospheric drum filters or large settling tanks. Filtering out the small lime mud particles from the alkaline slurries obtained from the causticization vessels, or from lime mud wash thereafter, was considered very tricky as these lime mud particles were so small and residual lime mud particles in the white liquor produced must be kept at a minimum. Therefore, most often atmospheric drum filters were operated with a rather low differential pressure applied over the lime mud cake captured in the filter surface.

With increasing demand on productivity and reduction in investment costs in the causticization field, as pulp mills became bigger, grooving from a normal pulp production of 1500-2000 adt/day to 4-fold that capacity, these atmospheric drum filters became colossal in size, and hence the investment. Disc filters were then considered as an alternative as they provided more effective filtering area per foot print area of the filter, but still these disc filters were atmospheric filters with associated lower filtering capacity due to the problems with small lime mud particles. Generally, disc filters have been used in a multitude of application areas, ranging from fruit juice filtering to cellulose pulp washing. Atmospheric disc filters using drop legs for application of a small differential pressure over the filter surface have been used, and are disclosed in, for example, US 6,258,282 and US 4,676,901, where a liquid level in the vat is above the rotating filter shaft. Kvaemer, now Valmet, developed pressurized disc filters for the causticization area in the 90- ies (and these were patented in, e.g., US 4,929,355), which provided a major production increase in effective filtering area per foot print area of the filter. In these pressurized disc filters a precoat-filtering technique was applied, where a precoat of lime mud was built up on the filter media, typically a filter cloth with a dense web. During the buildup of the precoat no clear white liquor could be obtained, but after formation of the full precoat thickness a clear white liquor can be obtained. in conventional pressurized disc filters in the causticization process, developed in the 90-ies, the sluny to be filtered has always been established below the rotating filter shaft, with more than 50 % of the total filter media located in the pressurized gas phase. The objective is to assure high degree of displacement of liquor content through the precoat by the passing gas phase. This results in high dryness of the lime cake scraped off and catching of most of the alkaline liquors in the filtrate flow. Examples on pressurized causticization disc filters with liquid level below a rotating filter shaft can be found in, e.g., US 7,005,067 (GL&V, 2006), which comprises lime mud chute designs; US 8,518,273 (Kvaemer 2013, now Vaimet), which uses white liquor for Squirt™ operations; and US 2007/0221345 (Kvaemer 2013, now Vaimet), which discloses the addition of oxygen to a filter compressor. But today, when pulp production levels may regularly exceed 8000 adt/day, also the pressurized disc filters become colossal in size and the operating costs for a pressurized disc filter reaches considerable part of operating costs. Also, it becomes problematic to find a single compressor with a capacity that can pressurize these large capacity disc filters. One such operating cost of major concern to mill owners is costs for electricity, and the costs associated with pressurizing the disc filter using a compressor. Consequently, there is a need for an improved method for operating a pressurized disc filter for causticization liquors containing lime mud particles as well as a need for an improved pressurized disc filter for the filtering of causticization liquors containing lime mud particles.

Summary of the Invention

The above-mentioned objects are achieved with a method for operating a pressurized disc filter for causticization liquors containing lime mud particles, and with a pressurized disc filter for causticization liquors containing lime mud particles according to the independent claims. Preferred embodiments are set forth in the dependent claims.

Herein, the term "causticization liquors" is used, and this term include all kinds of alkaline liquors handled in the recovery area of a pulp mill, said causticization liquors being alkaline in varying degree from weak alkaline liquors to strong alkaline liquors, all having a content of varying degree of lime mud particles. A causticization liquor can thus be: a) a green liquor that is slightly alkaline and obtained from dissolving tanks where melt from the recovery boiler is mixed with weak liquor and/or alkaline process water; b) a causticized green liquor from the causticizing train, from which liquor strong white liquor is filtered out, and c) a lime mud washing after white liquor extraction, where said lime mud is reslurried in weak liquors and/or in alkaline process water. Further, by alkaline process water is meant any process filtrate obtained in a pulp mill, and can include wash filtrate from alkaline wash stages after cooking or alkaline bleaching stages. In all these filtering positions lime mud particles are present, either as a filter media for green liquor filtering or as a particle content in the slurry to be filtered.

In all of these filtering positions using pressurized disc filters a precoating technique is necessary in order to obtain the intended clear alkaline filtrate, using a precoat of lime mud particles to catch the lime mud particles that are present in the alkaline slurry to be filtered.

The invention relates to a method for operating a pressurized disc filter for causticization liquors containing lime mud particles and comprises the following steps:

feeding causticization liquor containing lime mud particles to a pressure vessel, a lower portion of which comprises a vat;

rotating a hollow shaft with filter discs arranged on said hollow shaft having a filter media on said filter discs and a hollow interior of said discs in communication with the rotating hollow shaft via filter disc outlets;

receiving alkaline filtrate in a storage tank from to the interior of the hollow shaft;

pressurizing the interior of the pressure vessel using a compressor,

collecting lime mud being scraped off by doctor blades from said filter media into lime mud chutes; and

establishing a precoat of lime mud on said filter media by arranging a distance between the edge of the doctor blade and the surface of the filter media, and is characterized in that a level of causticization liquor containing lime mud particles is established in the vat of the pressure vessel and said level located above the upper surface of the rotating hollow shaft and below the doctor blades, and in that the level of causticization liquor is established such that less than 40 %, but more than 10 %, of the total surface of the filter media is exposed to the pressurized gas phase in the pressure vessel. By these method steps a smaller compressor can be used, as the pressure losses through the exposed precoat surface are reduced in proportion to reduction of precoat surface exposed to the pressurized gas phase. In the inventive method is preferably also the compressor pressurizing the interior of the pressure vessel withdrawing a gas phase from the storage tank and feeding pressurized gas phase to the gas phase established above the level of causticization liquor. The gas phase thus needs less venting to gas destruction systems, as the gas phase may contain malodorous gases. According to a preferred embodiment of the inventive method a pressure in the gas phase of the pressure vessel is established in the pressure range of 1.5-5 bar. This larger pressure range will partly compensate for the reduction of precoat surface exposed to the gas phase, and ensures similar production capacity as of filtered alkaline filtrate. Further, more preferably a pressure in the gas phase of the pressure vessel is established in the pressure range of 2.0-3.5 bar. This is higher than the pressure established in old filters with drop leg, where a pressure differential of about 1 bar is established with a drop leg of 10 meter, and also higher than the pressure established in conventional pressurized disc filters for causticization liquors.

In a further preferred embodiment of the inventive method a pressure drop in the range of 0.3- 2 bar is established over the filter media and associated precoat; and more preferably a pressure drop in the range of 0.5-1.8 bar is established over the filter media and associated precoat.

In a further preferred embodiment, also the level of causticization liquor can be established, such that less than 35 % of the total surface of the filter media is exposed to the gas phase in the pressurized vessel, and in an extreme implementation the level of causticization liquor is established such that less than 30 % of the total surface of the filter media is exposed to the gas phase in the pressurized vessel. By reduction to 40 % exposure the pressure losses through the precoat can be reduced by as much as 50 %, and by reduction to 35 % exposure the pressure losses through the precoat can be reduced by as much as over 60 %, and the extreme reduction in exposure to 30 % the pressure losses through the precoat can be reduced even more.

Further, the invention also relates to a specific inventive design of a pressurized disc filter for causticization liquors containing lime mud particles, enabling implementation of the inventive method. The pressurized disc filter comprises a pressure vessel fed with causticization liquor containing lime mud particles; a rotating hollow shaft with filter discs arranged on a rotating shaft having a filter media on said filter discs and a hollow interior of said discs in communication with the rotating hollow shaft via filter disc outlets; a storage tank connected to the interior of the hollow shaft receiving alkaline filtrate; a compressor for pressurizing the interior of the pressure vessel, lime mud chutes for collecting lime mud being scraped off by doctor blades from said filter media; and wherein a precoat of lime mud is established on said filter media by arranging a distance between the edge of the doctor blade and the surface of the filter media, and is characterized in that the pressurized disc filter includes means for establishing a level of causticization liquor containing lime mud particles above the upper surface of the rotating hollow shaft and below the doctor blades, and in that the level of causticization liquor is established such that less than 40 %, but more than 10 %, of the total surface of the filter media is exposed to the pressurized gas phase in the pressure vessel.

Brief Description of the Drawings

In the following schematic drawings details are numbered alike in the figures, and details identified and numbered in one figure may not be numbered in other figures in order to simplify figures, wherein:

Fig. la shows a cross-sectional side view of a conventional disc filter;

Fig. lb shows a cross-sectional end view of the disc filter as seen in section A-A in Fig. l a, with conventional components identified;

Fig. lc illustrates how the doctor blade scrapes off a part of the lime mud layer, thereby leaving a precoat;

Fig. 2a shows a cross-sectional side view of a disc filter according to the invention;

Fig, 2b shows a cross-sectional end view of the disc filter as seen in section A-A in Fig. 2a; and Fig. 3a shows a cross-sectional side view of a disc filter according to the invention with an alternative liquor level control.

Detailed Description of the Invention

In Fig. la conventional pressurized disc filter is disclosed, also seen through the section A-A in Fig. lb; and in Fig. l c is a detail of a doctor blade shown. The disc filter is used to filter out fine lime particles in a causticization liquor.

The disc filter comprises a pressure vessel 1, which is fed with causticization liquor CL containing lime mud particles; a rotating hollow shaft 1 1 with filter discs 12 arranged on said rotating hollow shaft 11 and having a filter media on said filter discs and a hollow interior of said filter discs 12 in communication with the rotating hollow shaft 11 via filter disc outlets 13; a storage tank 40 connected to the interior of the rotating hollow shaft 1 1 and receiving alkaline filtrate Four; a compressor 50 for pressurizing the interior of the pressure vessel 1 ; lime mud chutes 28 for collecting lime mud being scraped off by doctor blades 30 from said filter media; and wherein a precoat of lime mud PC is established on said filter media by arranging a distance between the edge of the doctor blade 30 and the surface of the filter media. A lower portion of the pressure vessel 1 constitutes a vat or vessel 31.

In Fig. la only one lime mud chute 28 is disclosed, but it should be understood that there is at least one lime mud chute located close to all sides of the discs, i.e. in all nine lime mud chutes 28 in Fig. la, i.e. one at each end of the outermost discs and in total seven lime mud chutes 28 located between two discs, as in the example shown. Also, the causticization liquor CL can be fed to the vessel or vat 31 by a multitude of nozzles located one between each pair of filter discs. These nozzles can be of a so called mammoth-pump type, that includes an ejector that sucks up particles on the bottom of the vessel or vat 31 as the added causticization liquor CL is injected. The nozzles may also be arranged for improved agitation in the vat, such as the nozzle arrangement shown in WO2017007417. The disc filter 1 includes at least one disc 12. The disc 12 is similar to those described in the prior art in that lime mud particles are removed from the causticization liquor by forcing the liquid through filtering discs 12 when the discs 12 are in the lower part of the vat 31, while the lime mud particles accumulate on the filter media. The liquid is forced through the discs 12 by creating a pressure differential between the interior of the fi lter housing 10 and the outlet. The pressurization is obtained by a compressor 30. The filter media on the discs 12 can be a perforated material, such as metal, and covered with a fine mesh cloth, such as a polypropylene cloth. The filter material is generally referred to as the filter cloth. Each disc 12 is divided into disc sectors (not shown), which are individually covered with a filter cloth. The discs 12 are positioned within the housing 10 that holds the caustici ation liquor in the bottom part, i.e. in the vat 31, at a certain liquid level LiqL and the discs are mounted firmly to the rotation shaft 11. The level of the causticization liquor can vary, but is typically approximately close to or below the center of the rotation shaft 11. As the discs 12 rotate (the direction of rotation being indicated with ROT) each sector fully enters the slurry and an amount of lime mud particles are deposited on the surface of each disc sector. The lime mud particles are then removed from each sector, to separate the lime mud particles from the causticization liquor, thereby obtaining a clear alkaline filtrate on the inside of the filter sectors.

When filtering green or white liquor slurries in the recovery island of a pulp mill, the filter cloth itself is not sufficient to reach a low particle content in the filtrate. The cloth is rather used to enable and facilitate the forming of a precoat layer of fine particles on the cloth, and after establishment of a precoat with sufficient thickness, effective filtration can begin, thereby obtaining a filtrate with low particle content. When lime mud particles build up on this precoat, a scraper mechanism is used to remove the particles accumulated. The scraper mechanism comprises a doctor blade 30 that removes the cake layer from the precoat PC layer, as shown in Fig. l c. As each disc sector passes through the slurry and emerges from the slurry, additional solid material is deposited on the surface of the disc sector. As the disc sectors pass the doctor blade 30, the cake layer formed on top of the precoat layer is removed. The removed solid material falls by action of gravity into a chute 28 arranged immediately below the doctor blade 30,

Conventionally, first a precoat is established with a starting thickness of about 1 ,0-3,0 cm, and thereafter additional layers of lime mud particles on top of the precoat are scraped off by the doctor blade. As filtering proceeds, the outermost layer of the precoat will be blinded by smaller lime mud particles, which is seen as a reduction of clear filtrate flow, and then the doctor blade is advanced towards the filter cloth at a small incremental adjustment being a fraction of the starting precoat thickness. This advancement can be in the order of 1-3 mm and will scrape off the outer layer of the precoat that has become blocked or blinded. When the original precoat finally has been consumed, an entirely new precoat is formed with a starting thickness of about 1.0-3.0 cm. The disc filter can also include wash showers and a precoat renewal high pressure jet system, for example the SQUIRT™ system sold by Valine! In Fig. lb is disclosed how a SQUIRT™ system can include a high-pressure nozzle 80, which slowly can be moved radially on a rail 81, and this high-pressure jet can dislodge a strip of precoat from the filter media as well as provide for a thorough filter cloth cleaning. The advantage with this SQUIRT™ system is that the entire precoat can be renewed, as it becomes blocked over time, and the filtering process does not need to be halted for precoat renewal. In Fig. 2a a pressurized disc filter 1 according to the present invention is illustrated, with same basic design as the filter shown in Fig. l a, and also seen through the section A- A in Fig. 2b. The disc filter 1 is used to filter out fine lime particles in a causticization liquor. Three design differences emerge. Firstly, the liquor level LiqL is raised to a level well above the rotating shaft 11, and secondly, as a consequence, also the inlet of the lime mud chute 28 is raised to a position above the liquor level LiqL, and includes also a third difference in a liquor level control.

In this first embodiment of the invention, the liquor level LiqL is controlled and maintained at the desired height in the vat 31 by a level sensor 70s, emitting a control signal X depending upon the current liquor level. The control signal X can then control the degree of opening of an inlet valve 70v, which supplies causticization liquor to be filtered.

In Fig. 3a, an alternative level control is disclosed with a passive overflow pipe 70d, having a funnel with an inlet arranged at the maximum allowable height of the liquor level I. qL. The flow from the overflow pipe 70d can be led to the feeding pipe for new causticization liquor. This solution can also include a sensor in the overflow pipe 70d, which similarly control the degree of opening of an inlet valve 70v, which supplies causticization liquor to be filtered.

Fig. 2b discloses a complementary design difference in the squirt function that may be considered in order to reduce pressure losses through the precoat. In this alternative a squirt rail 81 is located above the liquor level LiqL, but the high-pressure nozzle 80 is located on a swan neck protruding down into the liquor volume in the vat. The high-pressure jet may dislodge a strip of precoat from the filter media as well as provide for a thorough filter cloth cleaning below the liquor level. The advantage here is that the dislodged strip quickly becomes refilled, at least in part by new unblocked lime mud particles before this strip ascends into the pressurized gas phase. Still, the advantage with this alternative squirt system is that the entire precoat may be renewed, as it becomes blocked over time, and the filtering process does not need to be halted for precoat renewal.

The increase of liquor level also requires improved shaft seals 15 at the end of the pressure vessel 1 , Normally conventional adjustable sealing boxes can be used, where the sealing force can be adjusted as the sealing material wears down. Alternatively, mechanical seals can be used which instead have additional sealing water applied on the machined sealing surfaces, with a controlled leakage flow into the vat.

It is to be understood that the present invention has been described by way of illustration and not limitation. The invention may be modified in a number of ways, such as implementation in a first stage of a two-stage pressurized white liquor disc filter, implementation in a pressurized lime mud disc filter of one or two stage design, where residual alkali is washed out from lime mud obtained from white liquor filters preceding the lime mud wash, and located ahead of the lime kiln; and/or implementation in pressurized green liquor disc filter, where typically the first precoat is built up using causticizat on liquor, because the precoat from lime mud particles obtains a far better fiiterabiiity compared to building up a precoat of green liquor grits.