WO2002039108A1

WO2002039108A1 - A method for turbidity and conductivity measurements online in fibre suspensions

Info

Publication number: WO2002039108A1
Application number: PCT/SE2001/002453
Authority: WO
Inventors: Sylvain Renaud; Bertil Olsson
Original assignee: BTG Eclepens SA; BTG Kalle Inventing AB
Current assignee: BTG Eclepens SA; BTG Kalle Inventing AB
Priority date: 2000-11-08
Filing date: 2001-11-08
Publication date: 2002-05-16
Anticipated expiration: 2003-05-08
Also published as: SE0004093D0; AU2002214453A1

Abstract

The invention concerns a method for measuring the turbidity and conductivity in a filtrate from sample, taken from a pulp suspension. One sample taken from the original pulp suspension is used for the formation of a pulp foundation, which is build up under mainly gravitational conditions, after which a filtering is made with dropping fall of pressure over the pulp foundation to determine the turbidity and/or the conductivity of the sample.

Description

A method for turbidity and conductivity measurements online in fibre suspensions

The present invention relates to a method for on-line measurement of the turbidity in filtrates from samples extracted from pulp suspension so as to permit a fixative or coagulating control in order to reduce the process variation which is due to the variation of the quality of the filtrates which in a paper mill arises in response to the type of pulp and the water quality as caused by the time of the year, by chemical activation or by the load of the suspension.

Turbidity measurement in a pulp filtrate is previously known within the pulp and paper industry. However, this measurement is always made on a laboratory level, since there does not exist any filtrate sampling device which can deliver a filtrate free from colloidal material, i. e. small particles . The filtrate sample extractors now available do always extract the filtrate under processed pressure. Very small particles of filling material or other materials will always be included in the filtrate because the conduit pressure force them out through the sampling device during the first seconds of the filtration process.

Also in the laboratories small fibres consisting of filling particles can be mixed with the- collected filtrate because during its extraction vacuum is applied below the paper filter. This procedure yields unreliable results because of a certain undesired effect combining filter paper and filter cake. This means that a sample containing a considerable amount of colloidal material in the filtrate formed on the filter paper will contain a substantially less of that material than a sample containing a smaller amount of colloidal material . The inference of this is that it is very difficult to make a comparison between different filtrates stemming from different pulp types or from a pulp exhibiting different consistences .

Filtrates from pulp suspensions are generally used for measurement of turbidity, conductivity, pH and cationic load. Turbidity and cationic load measurements are often made in order to measure the (fixative and coagulating) , efficiency of chemicals. Coagulating or fixative chemicals are generally used to change the supply of colloidal substances from anionic to neutral so that they are joined creating bigger particles which will be extracted from the system together with the fibres .

The object of the present invention is to provide a new method admitting collection of filtrates under essentially atmospheric pressure and in such a manner that there is obtained a filtrate substantially without any small particles. This filtrate will now contain only colloidal substances (particles which have at least one dimension within a range from 10 nm to the micro interval) and which have not been neutralised by a chemical reaction and can move freely through a pulp bed. The characteristics of the invention are set out in the subsequent patent claims .

Thanks to the invention there has now been provided a method for turbidity and conductivity measurement of the type mentioned above which method does in an excellent manner satisfy the purposes and which, in addition thereto, can be carried out in a cheap and convenient way. When the method according to the invention is carried out a pulp suspension yields a filtrate which can be repeated and is representative and which is not affected by pressure, flow or temperature variations and likewise not by the particle dimension. Use is made of a so called double filtration process in which the volume and the time of each filtrate is measured accurately in order to guarantee that the result of turbidity and/or conductivity measurements will be repeated.

By means of the method according to the invention there is produced a filtrate by filtration through a well-defined filter cake which has been created by the pulp suspension the filtrate of which is to be measured. This measurement can be made in connection with measurement of the dehydration properties of a pulp sample as based on our Swedish patent SE 436602.

A preferred embodiment of the invention will now be described below.

According to that preferred, exemplifying embodiment there is carried out measurement of the turbidity in filtrates from samples extracted from a pulp suspension. The basic principle according to the invention is that at least two filtrates are derived from the same sample in order to eliminate the fine fraction which first leaves the sample as determined by the filter opening area and other mechanical disturbence sources.

During the first filtration there is formed a pulp bed on a filter unit constitued by the created filter cake. This filter cake is composed of fine and big fibres (and also a filling agent if such a substance is present in the suspension) which will attract charged substances adhering to and remaining in the cake. The weak, anionic substances will pass through the bed and be collected in a separate chamber for the purpose of turbidity and/or conductivity measurements. According to the invention the sample extracted from the pulp suspension is filtrated at least twice under essentially atmospheric pressure. The purpose of this is to make sure that as a result of the first filtration there will continously be built up a pulp bed in the shape of a cake consisting of fine and big fibres and, when this is applicable, filling agents, after a predetermined time interval or after achievement of a definite volume from the first sample so that there is obtained a first filtrate containing the fine fraction. When the first filtrate has been removed out from the measurement chamber used at least one subsequent filtration is carried out in order to create a filtrate substantially comprising colloidal material and which is collected in the measurement chamber for determination of the turbidity and/or the conductivity of the sample.

Consequently, with the aid of the method according to the invention, there can be obtained a filtrate which has been filtered through a well-defined filter cake built up by the suspension to be measured explained above.

The device or apparatus used in this connection comprises a sample extraction chamber and a measurement chamber. In the measurement chamber the turbidity and the conductivity of the filtrate is determined. A suspension sample is collected in the sampling chamber at the start of the filtration extraction procedure resulting in that a filter cake is continously created during the filtration on the filter felt forming part of the device. The exact volume is checked with a special instrument also measuring the filtration speed. When a predetermined volume of the pulp sample has in the shape of a filtrate passed down to the measurement chamber the filtrate is removed and, at the same time, a cleaning operation is initiated in order to remove all small particles which might have got jammed in the openings of the device during the executed starting filtration process. After this another filtrate is collected for use during the turbidity and/or the conductivity measurement proper.

Claims

1. A method for measuring the turbidity and the conductivity in filtrates from samples taken out from a pulp suspension, characterized in that in a, during a first step, from the pulp suspension proper extracted sample there is, at essentially atmospheric pressure, created a pulp bed in the shape of a cake consisting of fine and big fibres and, as the case may be, of fillers, the material in the fibre cake from the original slurry being created by gravimetric conditions, whereupon a filtration is carried out through the pulp bed with a sinking pressure drop over it permitting determination of the turbidity and/or the conductivity of the sample .

2. A method as claimed in claim 1, characterized in that the sample taken from the pulp suspension is subjected to a double filtration in order to produce two filtrates, only one of those being subsequently used in the determination of the turbidity and/or the conductivity.

3. A method as claimed in claim 1, characterized in that the second and subsequent filtrate is collected in the measuring chamber through which the first filtrate does, during a predetermined time period, pass during the formation of the pulp bed.