Analytical Instrumentation & Environmental Monitoring


White water pH in the pulp and paper industry

March 2000 Analytical Instrumentation & Environmental Monitoring

The drainage from wet pulp stock is called white water,regardless of its colour or what stage of the process it came from. A pulp and paper plant can use up to 25000 gallons of fresh water to produce a ton of paper, so it is imperative that plants recycle as much white water as possible. Controlling pH is an important factor in the reclamation of white water.

Background

During the paper making process, pulp stock is continuously sprayed onto the moving woven mesh brass or bonze cloth screen at the rear section of the headbox. This forms a pulp stock sheet consisting of over 99% water by weight. The water falls through the wire screen and into drainage trays as the continuous paper sheet is pulled along. Next, the sheet travels through a section of suction boxes to physically extract more paper. The sheet then moves onto a couch roll which prepares it to be lifted off the wire screen.

At this point, the paper sheet is barely strong enough to support its own weight (roughly 80-85% water) and is transferred to a felt. The felt takes the continuous paper sheet through a series of presses, leaving the sheet with 71 to 74% water concentration. The remaining water is removed by evaporation through heating in the drying section of the paper machine. The sheet is then rolled for storage and cut.

Plantwide, the single largest source for white water is the paper machine. As much as 25-30% of the original pulp stock is recovered as white water. Depending on where the white water is collected and its concentration of paper fibres, white water is considered rich or lean. The rich white water, accumulated in the drainage trays, suction boxes and the couch roll pit, contains the highest percentage of paper fibres. This rich white water is used as make-up water for the beaters and stock dilution prior to entering the headbox.

The excess rich white water and lean white water (contains a smaller percentage of paper fibres) collected at a device called a save-all which removes the fibres and fillers. This reclaimed white water is used for applications such as wire screen showers, felt sprays and seal water for vacuum pumps.

Reclaimed white water is always mixed with fresh water since reuse of undiluted white water can cause problems. These include build-up of suspended solids (felt plugging), accumulation of dissolved solids (slime, foam and scaling) and increased retention of thermal energy (temperature).

Application

Recycling white water is critical in minimising production costs. It is more economical to reclaim the fibres and fillers from the white water than it is to replace and process them at the waste treatment plant. The pH of the reclaimed white water must be controlled to specific values for each of its many uses. For example, white water is used for the beaters and headbox has a high ratio of filler to fibre. Consequently, the pH must be adjusted to precipitate the filler and other nonfibrous suspended matter.

There are many different ways a save-all can clarify water but one method commonly used is to control the pH to improve flocculation. This method optimises the removal of filler and fibre debris from the water.

The pH of the save-all effluent must also be controlled to reduce corrosion if it is to be used as seal water for the vacuum pumps.

Summary

The key to a pulp and paper plant's success is reducing costs without compromising production and quality. Reclaiming white water is one of the best ways to do this. Roughly 30% of pulp stock ends up as white water. Treatment of the white water is necessary for reuse in other areas of the plant. pH control is the most efficient method to treat reclaimed white water.

Instrumentation

A number of different sensors can be used to monitor the pH of white water. Selection criteria include cost, convenience, mounting style and personal preference.

* System 1 (insertion mounting):

- Model 692P3F5A7N pH transmitter (accepts differential sensor input).

- Model 6070P0 insertion pH sensor.

- MH718 insertion hardware (316SS).

* System 2 (flow-through mounting):

- Model 692P3F5A7N pH transmitter (accepts

differential sensor input).

- Model 6028P0 pH sensor (LCP).

- MH376 flow-through mounting hardware (PVC).


Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Optimised fuel blending
May 2019, Yokogawa South Africa , Analytical Instrumentation & Environmental Monitoring
This risk has led the refiners to give more than the norms specify, resulting in a giveaway on quality and profit. The use of near infrared (NIR) technology has helped refiners get real-time data to ensure ...

Read more...
Inline analytical technology for liquor process control
May 2019, Morton Controls , Analytical Instrumentation & Environmental Monitoring
One of the most important processes in the pulp and paper industry is the alkaline sulphate process, where lignin is separated from the fibrous material. Recycling of the chemicals used is vital for environmental ...

Read more...
Gas monitors
Africa Automation Fair 2019 Preview , Analytical Instrumentation & Environmental Monitoring
RTS Africa Technologies is exhibiting LaserGas gas monitors from Neo Monitors.    These are ideal for accurate measurement of specific components in gas mixtures such as flue gases. The NEO LaserGas monitor ...

Read more...
Spectrum analysers for RF signals
Africa Automation Fair 2019 Preview, Accutronics , Analytical Instrumentation & Environmental Monitoring
Accutronics is exhibiting the SignalShark real-time spectrum analysers for the detection, analysis, classification and localisation of RF Signals. These are on show at Stand D06. Features include a frequency ...

Read more...
In-situ gas analyser
Africa Automation Fair 2019 Preview, Microsep , Analytical Instrumentation & Environmental Monitoring
Mettler Toledo Ingold’s GPro 500 TDL system is ideal for tough and challenging gas measurement applications and is designed for in-situ petrochemical processes and safety applications. TDL gas analysers ...

Read more...
Process filtration solutions
Africa Automation Fair 2019 Preview , Analytical Instrumentation & Environmental Monitoring
National Separations is showcasing its range of liquid and gas filtration systems supplied by Amazon Filters at Stand B10. Amazon Filters is a global supplier of a comprehensive range of microfiltration ...

Read more...
A huge range of process instrumentation
Africa Automation Fair 2019 Preview, Danntech , Analytical Instrumentation & Environmental Monitoring
Danntech Process Instrumentation’s recently updated Vodex-200A for measuring the velocity of detonation of explosives is on show at Stand F03. Danntech has more than 80 products in local production, with ...

Read more...
A leader in process measurement
Africa Automation Fair 2019 Preview, PREI Instrumentation , Analytical Instrumentation & Environmental Monitoring
PREI Instrumentation is a leading supplier and technical support specialist for on-line, portable and laboratory analysers for the potable, waste and industrial water markets, as well as rotating machinery ...

Read more...
Instruments for the ­monitoring and control of pH
Africa Automation Fair 2019 Preview, Hanna Instruments , Analytical Instrumentation & Environmental Monitoring
Hanna Instruments, a specialist in analytical instrumentation, has on show at Stand C10 a range of instruments for the monitoring and control of pH in water conditioning and industrial effluent applications. ...

Read more...
Online analytical systems for the water industry
Africa Automation Fair 2019 Preview, Swan Instruments , Analytical Instrumentation & Environmental Monitoring
Swan is a global supplier of online analytical systems for the water industry. Visit Stand A08 to find out more about Swan’s product portfolio. This ranges from ultrapure water for pharmaceutical and ...

Read more...