CA1054416A - Method and apparatus for liberating the latent properties of a mechanical pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method and apparatus for liberating the latent properties of a mechanical pulp is disclosed. The method may be used either as a laboratory technique for sample pre-paration for further pulp testing or for treatment of pulp in a mill after refining and before further processing.
"Latency" is released by recirculating pulp from a pool thereof through a centrifugal pump at a rate of at least twice a minute for the period required to release at least 70% of the latent properties of the pulps ("Latency").
This requires at least one minute and up to 10 minutes depending on the rate of circulation on of the pulp.

Description

1~544~6 FIELD OF T~E INVENTION

The present invention relates to method and appa-tus for relaasing latent properties of ("latency"~ of mechan-ical pulps. More particularly the present invention relates to a method for releasing "latency" from mechanical pulps that may be used in the laboratory or in the plant.
PRIOR ART
In the early stages of manufacturing mechanical pulps from chips it was observed that to develop the inherent strength of the produced pulp a treatment by mild agitation, at low consistency and elevated temperature was required.
It has been postulated that fiber nodules formed during the refining (or grinding) process are frozen in distorted shape (twisted, kinked and curled) when diluted with cold water and that these nodules which are composed of c~llulose, hemi-cellulose and lignin must be elastically strai~htened by heating the pulp at the temperature above 60-C where the lignin begins to soften to develop substantially fully the properties of the pulp.
This phenomena of "latency" in mechanical pulps has been further investigated and laboratory methods for testing mechanical pulps by first liberating the ("latency") prop erties of the pulp have been proposed, however, neither CPPA nor TAPPI have suggested a standard laboratory proce-dure for "latency" release. An accepted "standard" method is to dilute the stock with boiling water at a consistency of 1.5~ and maint:ain the stock at an elevated temperature for 15 minutes followed by disintegrating the sample in a British disintegrator. Because of the relatively long time required in this "standard" method the temperature of the stock drops and inconsistent results have been observed

- 2 -~544~6 especially when treating thermo-mechanical pulp sa~ples of relatively low freeness.
In Commercial refining systems including thermo-mechanical and high consistency pulpiny it is conventional 5- practice to provide a stock chest having a capacity to pro-vide a residence for the pulps of at least 15 minutes and in many cases 30 minutes to an hour to release the "latency"
of the pulp before it reaches the paper machine. It is well recognized that the various cleaning equipments enroute to paper machine aid in liberating the latent properties of these pulps but nonetheless hiyh consistency refining systems and thermo-mechanical pulping installations in-variably maintain a large stock chest to provide the re-quired residence line to release the latent properties of the pulp.

BRIEF DESCRIPTION OF THE INVENTION

The present invention permits the release of "latency" from pulps in much less time than previously thought possible~
Broadly the present invention relates to a new laboratory procedure and equipm~nt for release of ~latency for the preparation of pulp samples for testing and to a new system for inplant release of "latency" from pulps after they have been produced.
Broadly the present invPn~ion relates to the method and apparatus for the release of "latency" from mechanical pulps comprising forming a pool of said pulp in a container, withdrawing pulp from said container to a cen- -trifical pump, pumping said stock through said centrifical 4~6 pump and returning it to ~aid container at least twice a minute for a period of time requi.red to remove the desired amount of latency from the pulp and in any event for at lea5t one minute and less than ten minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, objects and advantages will be evident from the following detailed description of the pre-ferred embodiments of the present invention taken in conjunc-tion with the accompanying drawings in which Figure 1 is a schematic illustration partly in section of a laboratory device for carrying out the present invention.
Figure 2 is a schematic illustration of a pulping installation incorporating the present invention.
Figure 3 is a plot of the time reguired to reach a constant freeness for different rates of recirculation.
Figure 4 is a plot of time to release "latency"
versus recirculation rate.

DESCRIPTION OF THE PREFERRED 3~:MBODIMENTS

The lcib apparatus 10 as illustrated in Fiyure 1 comprises a stand 12 on which is mounted the various ele-ments of the equipment. The apparatus 10 consists of a cylindrical sample container 14 connected via piping 16 to the inlet of a pump 18 and via Iine 22 from the outlet o~
18 back to the top of the container 140 The outlet lS and inlet lines 22 are arranged to withdraw and to reinject the sample tangentially from and to the cylindrical container 14. .

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The container 14 preferably is provided with a baffle 24 and a removable lid 26 the latter being provided with a steam release aperture 28.
The sample may be withdrawn from the device through the pipe 30 controlled by valve 32.
Preferably the lab dav:ice will be provided with an on-off switch 34, a jogging switch 36 and an automatic timer mechanism 38 to control the operation of the motor 20.
All the test results reported herein were generated utilizing the above described apparatus or a pilot plant recirculation system similar to the laboratory apparatus but treating larger quantities of pulp.
Figure 2 illustrates the present invention as applied to the commercial installation composed of one of more refiners 40 each of which is connected via a discharge chute 42 to a stock chest 44. The stock chest in turn is connected via piping 46 to a centrifugal pump 48 which ; -circulates the stock through a pipe 50 and a return pipe 52 back to the stock chest 44. A take off line 54 is 20 provided for directing the stock to further processing. The ~ -lines 52 and 54 are provided with sui~able valves 56 and 58 ~ -respectively to control the flow of pulp.
In the arrangement shown in Figure 2 the chips are introduced through the inlet 60 and fed via a screw 62 into the rsfiner between the discs 64 and the disc 66 driven by shaft 68. The refined pulp is ejected as above indicated through the piping 42 into the Qtock chest 44. Any suitable means may be provided to drive a disc 66 via the shaft 68.
The above system uses a single disc refiner but obviously a double disc refiner (both discs rotating) may be used to produce the pulp.

1~54~6 In accordance with the method of the present invantion as applied in the laboratory apparatus stock is introduced into the container 14 at a temperature of 80 C
to 86- C and pump 18 is activatecl to circulate the stock through the pipe 16 pump 18 and back to the tank 14 via line 22. In Table 1 various circulation rates are compared with the standard method of releasing "latency" and these results have also been plotted in Figure 3.
The time to remove latency is based on the Canadian standard freeness since it has previously been established in the art that the freeness is a very good indication of whether the "latency" of the pulp has been released, i.e. the latent properties of the pulp are avail-able, thus through out the disclosure the freeness has been taken as an indication of "latency" release.
Referring to Figure 3 and 4 and Table 1 it will be apparent that at very low recirculation rates (wherein the total stock in the container 14 is recirculated only once) the total time to remove latency is equal to the standard method.
Similarly from run 6 it will be noted even when an agitator as conventionally used was inserted and the stock was recircu-lated only once through the pump the time required to release "latency" remained the same as for the standard method (15 ~ i minutes). When the pump rate was doubled to 7 the time required to release "latency" decreased significantly to 10 minutes and when this rate was doubled to 4 the latency was removed in three minutes. However, in the laboratory appa- -ratus, it is preferred to operate at recirculation rates per minute of over 5, 10 to be in the regime indicated by , the runs no. 4 and 5 where the recirculation rate was 35 and 86 respectively so that the latency may be completely - -';' - 6 ~

105~
released within a period of two minutes (there is little point in incxeasing recirculation rate beyond that required to eliminate the latency in the required period of time).
It is noted that in the standard method where agitation was used the time required to remove latency was lS minutes whereas with tAe instant invention where recirculation of the material through a centrifical pump is used the time to remove the latency is drastically reduced.
It is not completely evident why recirculation through the pump results in the reduced time required to remove latancyO
It may be that the shear forces applied to the hot pulp suspension in the pump and as it is drawn and/or discharged through the piping of the system tends to pull the fibers or entrain the fibers and thus straighten them and reduce their kinked or curled formation.
The stock must be of pumpable consistency and pre-ferably will be at a consistency of between 2 to 3%. The time to release "latency" increases as the consistency is increased.
It is important that the temperature be maintained relatively high, preferably above 80-C and between 80 and 86-C so that the co~ponents of the stock are suffiaiently soft and pliable that the latency may easily be removed.
If the stock is too cool latency is very difficult to remove.
Table 2 provides a comparison of pulp and handsheet properties using the standard method of latency removal and the method of the present invention, i.e. a recirculation rate of stock for a time of two minutes.

1~4416 ABLE II
COMPARISON OF PULP AND HANDSHEET PROPERTIES

Standard, DomtarStandard Domtar Property Method Method Method Method Percent Debris * (%) 5.0 5.1 3.10 3.60 Fibre Fractionation R-14 11.6 14.4 21.7 24.6 P-14/R-2819.6 19.4 21.4 21.7 P-28/R-4819.6 19.5 17.6 18.3 P-48/R-10012.9 13.6 9.4 9.9 P-100/R-200 708 7.6 5.1 5.1 L-Factor 50.8 53.3 60.7 64.6 CS Freeness - 163 289 265 Handsheet Bulk (cc/g) 3.18 2.93 3.20 3034 F' Burst Factor XPA/(g/mg2) 1.6 1.7 1.8 1.6 Tear Fac~or MN/~g/m ~ 7.0 7.0 9-0 8.6 Tensile B.L.-km 3.37 3.53 3.76 3.41 Elongation (%) 1.51 ~.64 1.74 1.67 * Retained in ~ PFI mini-shive Analyzer equipped with a O.~8 mm slit.

It will be apparent from Table 2 that the results of the standard method of removal of latency and the method of the present invention are very comparable.
In applying the present invention to the commercial situation the sl:ock must be pumped through piping 46, pump 48 and pipes 50 and 52 at a rate sufficiently high that the latency will be removed within tha requisite period of time.
~f it is desired to reduce the size of the s~ock chest to provide only say a 4 minute residence time the recirculation '. .

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rate should be sufficiently high that the stock in the stock chest passes through the pump at least 4 times per minute.
The stock issuiny from the refiners generally at a high temperature (i.e. over 100C), and the dilution water is over 80 C so that the stock in the chest at about 2 - 3% consistency is a temperature of about 86C.
In a mill "latency" is also released in processing equipment enroute to the paper machine and therefore it is not essential to release all ~he "latency" at the stock chest 44. It has been found that if all less 70% of the "latencyU has been released at the stock chest the paper produced will be adequate.
Modifications may be made without departing from the spirit of the invention as defined in the appended claims.

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Claims (3)

1. A method for releasing latent properties of mechanical pulp comprising providing a pool of mechanical pulp at a pumpable consistency and a temperature of at least 80°C, recirculating said mechanical pulp from and to said pool by means of a centrifugal pump, said pump recirculating said pulp at least four times a minute for a period of time required to release substantially all the latent properties of said mechanical pulp.

2. A method as defined in Claim 1 wherein said pulp is recirculated at a rate of five times per minute for a period of between 2 and 5 minutes.

3. A method as defined in Claim 1 wherein said pulp re-circulated at a rate of over 10 times a minute for a period of between 1 - 5 minutes.