阅读说明：本技术 白液制备中使用沉降罐澄清绿液 (Clarifying green liquor by using settling tank in white liquor preparation ) 是由 张子成 J·约翰逊 M·英格尔曼 S·郭 于 2018-09-20 设计创作，主要内容包括：本发明涉及一种在沉降罐中澄清原绿液的方法。根据本发明,沉降罐中分离的一部分渣滓再循环回到原绿液的进料流中,并且优选在使渣滓经过至少一个涡流发生器(30,31)之后,所述涡流发生器可以将较大的渣滓颗粒分散成较小的渣滓颗粒,从而在渣滓颗粒上产生更大的总表面,改善沉降罐中的沉降速率。在一个优选的实施方式中,在将絮凝剂添加到原绿液中之前,将再循环的渣滓添加到原绿液的物流中,并混合再循环渣滓。(The invention relates to a method for clarifying raw green liquor in a settling tank. According to the invention, a portion of the bagasse separated in the settling tank is recycled back into the feed stream of raw green liquor, and preferably after passing the bagasse through at least one vortex generator (30, 31) which can disperse larger bagasse particles into smaller bagasse particles, thereby creating a larger total surface on the bagasse particles, improving the settling rate in the settling tank. In a preferred embodiment, recycled bagasse is added to a stream of raw green liquor and mixed before the flocculant is added to the raw green liquor.)

1. A method for clarifying raw green liquor, in which raw green liquor is fed into a settling tank through a mixing chamber having a number of lower outlets which achieve a uniform distribution of the raw green liquor at a low flow rate in the lower half of the settling tank by means of a rotary distributor sweeping the cross-section of the settling tank, which settling tank is provided with an underflow outlet for settled dregs at the bottom of the settling tank and an overflow outlet for clarified green liquor in the upper part of the settling tank, characterized in that a part of the dregs fed out from the underflow outlet is recirculated back to the feed of raw green liquor and that before reaching the mixing chamber, the part of the dregs fed out from the underflow outlet is passed through at least one vortex generator, dispersing some of the dregs into smaller parts to increase the total exposed area of the dregs.

2. A method of clarifying raw green liquor according to claim 1, characterized in that the portion of dregs sent from the underflow outlet is passed through at least two vortex generators and where at least one vortex generator is a pump (30, 31).

3. A method of clarifying raw green liquor according to claim 2, characterized in that the portion of dregs sent from the underflow outlet is passed through at least one centrifugal pump having a shredding effect on the pumped flow of dregs.

4. A method for clarifying raw green liquor according to claim 1, characterized in that the volume ratio of the recycled portion of dregs to raw green liquor is 5-30%.

5. The process of clarifying raw green liquor according to claim 1, where a flocculant is added to the flow of raw green liquor.

6. A process for clarifying raw green liquor according to claim 5, characterized in that a flocculating agent is added to the flow of raw green liquor after passing through at least one vortex generator and before being sent to the mixing chamber of the settling tank.

7. A method of clarifying raw green liquor according to claim 1, characterized in that a flocculating agent is added to the flow of raw green liquor and recycled dregs in the mixing chamber under the influence of a rotating turbo-mixer (32).

8. A process for clarifying raw green liquor according to claim 7, characterized in that the total amount of flocculating agent added to the total flow of raw green liquor and recycled bagasse fraction is in the range of 1-5 ppm.

9. The process of clarifying raw green liquor according to claim 8, wherein said flocculant is a polyamine or polyacrylamide type polymer.

Technical Field

The invention relates to the clarification of green liquor using a settling tank.

Background

Several solutions have been proposed for clarifying green liquor in pulp mill recovery islands. In the following sections, the most common prior art is briefly described, and more details of the various processes are known from Chemical Pulping ("Chemical Pulping"), volume 6B, chapter 14/white liquor preparation, ISBN952-5216-06-3 (volume 6).

Conventional methods of green liquor purification utilize settling techniques that utilize the density difference between the dregs and the green liquor to settle heavy particles in the form of sediments to the bottom of large settling tanks. The overflow obtained from these settling tanks is a clear green liquor, while the underflow (from the bottom of the tank) contains settled dregs content. However, such settling tanks need to be large and typically integrate from about twelve hours of green liquor storage capacity.

These settling tanks have been further improved by the addition of various flocculants to increase the settling rate, often with the use of polymeric flocculants when it is desired to increase the production rate of clarified green liquor or to allow for the use of cheaper smaller settling tanks in new mill projects.

Another method of green liquor purification uses filters, although drum filters are also used, disc filters are commonly used. Filtration techniques require filters that are much more expensive than standard settling tanks, but generally result in better clarity of the green liquor than standard settling tanks. By applying a pressure difference over the filter surface, the dregs can be separated from the raw green liquor (raw green liquor), and the filter surface needs a precoat with better defined lime mud particles to obtain sufficient filterability.

Another method of green liquor purification is to use a candle-type cross-flow filter, wherein a steady high flow of raw green liquor is washed over a tubular filter element and a cake of slag is formed on the candle filter element until the pressure drop over the filter cake becomes excessive. This results in an effective filtration cycle of about one hour, eventually ending with blowback, releasing the cake with dregs formed on the candle filter. To maintain a high volumetric flow rate over the filter surface, a large amount of the underflow (i.e., unfiltered green liquor) is typically recycled back to the inlet.

Disclosure of Invention

In the following sections, the following terms are used: the term "raw green liquor" means raw green liquor obtained from a dissolver in which the smelt from the recovery boiler is dissolved in process water and may be temporarily stored in an equalization tank for controlling a uniform flow rate for subsequent processing steps; the term "polymeric flocculant" means any kind of polymeric flocculant used to increase the sedimentation rate, which may be, for example, polyamine [ PY ] or polyacrylamide [ PC ], added to the settling tank in ppm small doses. These flocculants are sold by the company Tamarindus (Kurita) under the trade names KURIVERTER CP-350 and KURIIFLOCK PA-322 or PA-322K.

The general object of the present invention is to obtain an improved method for increasing the settling rate in a settling tank for clarifying raw green liquor obtained from a smelt dissolving tank during recovery in a pulp mill.

The process for clarifying raw green liquor of the present invention is suitable for feeding raw green liquor into a settling tank through a mixing chamber having a plurality of lower outlets which achieve a uniform distribution of raw green liquor at a low flow rate in the lower half of the settling tank through a rotating distributor sweeping the cross section of the settling tank. In these settling tanks, it is of utmost importance that the inflow of raw green liquor does not cause an agitation effect in the upper layer of clarified green liquor. The upper layer of clarified green liquor rises upwards due to the difference in density between the green liquor and the dregs, which have a higher density and thus settle to the bottom. The bottom of the settling tank is also provided with an underflow outlet for settled dregs and an overflow outlet for clarified green liquor at the upper part of the settling tank. In principle, the present invention is an improvement over prior art settling tanks in which a portion of the bagasse that is sent out of the underflow outlet is recycled back into the feed of raw green liquor.

In previous operations like settling ponds, a portion of the clarified green liquor has been recycled, but tests have surprisingly shown that recycling of clarified green liquor actually has a negative effect on dross settling and should be avoided, whereas recycling of dross from the underflow outlet actually has a positive effect on the rate of settling.

In a preferred embodiment of the method of the invention, the portion of the dross output from the underflow outlet may pass through at least one vortex generator that is capable of dispersing some of the dross into smaller portions before it reaches the mixing chamber, thus increasing the total exposed area of the dross. This further increases the settling rate. Such vortex generators may have different designs, but should increase shear forces as the dross flows through. The vortex generators may be centrifugal pumps, but may also be stationary mixers that increase shear forces by deflecting the fluid sharply, or by passing the fluid through a narrow restriction, thereby causing a sudden pressure drop across the restriction.

In another preferred embodiment of the method of the present invention, the portion of the bagasse that is output from the underflow outlet can be passed through at least two vortex generators, and wherein at least one vortex generator is a pump. This repeated swirling action may improve the effect of most of the larger dross particles having the opportunity to break up into smaller dross particles, which may further increase the settling rate.

In a further preferred embodiment of the method of the invention, the portion of the dross output from the underflow outlet may be passed through at least one centrifugal pump having a shredding effect on the pumped flow of dross. The shredding action may be obtained by a mechanical cutting action between an edge in a pumping impeller of the centrifugal pump and a cutting edge of the centrifugal pump housing.

In a preferred embodiment of the process of the invention, the volume ratio of the fraction of dregs recycled with respect to the volume of the raw green liquor may be between 5 and 30%.

In a further development of the process according to the invention, it is also possible to add a flocculating agent to the raw green liquor. The dual effect of the dreg recirculation and the addition of flocculant greatly improves the settling effect compared to adding only flocculant. Preferably, the flocculating agent is added to the raw green stream after passing through the at least one vortex generator and before being fed to the mixing chamber in the settling tank.

In another variation of the inventive process, the flocculant may be added to the flow of green liquor and recycled bagasse in the mixing chamber under the influence of a rotating turbine mixer. The recycled bagasse may then be subjected to a total of three mixing actions, and after the bagasse is thoroughly mixed and dispersed into smaller bagasse particles, the flocculant may be subjected to at least one mixing action in a mixing chamber.

In a preferred embodiment, the total amount of flocculant added to the total flow of the recycled portion of the raw green liquor and dregs is in the range of 1-5 ppm. The flocculant is preferably a polymer of the polyamine or polyacrylamide type, and is preferably similar to the polymer flocculant sold under the trade name Kurita PA-322K.