Cleaning pH sensors in pulp and paper processes: cleaning pH sensors is not a complicated process, but it does require specific solutions. Most of those solutions are right in your mill.

Removing foulants that adhere to pH sensors requires patience--and sometimes elbow grease. Choosing the proper cleaning solution can ease this task. Table 1 shows recommended cleaning solutions for a number of applications. Always use proper safety equipment in any cleaning process.

Place the sensor into the solution for at least 5 minutes so that the cleaning agent can penetrate the foulant. Double cleaning the sensor is always a good idea. Between dips in the cleaning solution, brush the electrode with a soft toothbrush to loosen and remove contaminants. After cleaning a sensor in an acid or base, wash the sensor with a solvent such as isopropyl alcohol or methanol to remove any greases or organic residue. Follow this by rinsing the sensor with mill water or dipping it in a salt (KC1) solution.

CLEANING BLEACH PLANT PH SENSORS

A kraft mill bleach plant generally has all the cleaning solutions right in the pulp stock itself. Simply use a sample of pulp from a high pH process to clean the sensors measuring in low pH applications, and use low pH pulp to clean sensors in high pH applications.

Here is an example: To clean sensors in Cl[O.sub.2] or [Cl.sub.2] stages, simply use hot stock from the high pH Eop bleaching stage--that is, a caustic solution around 170[degrees]F. Put about 24 gallons of the hot caustic stock in a bucket, remove the sensor from the low pH (Cl[O.sub.2]) service and put it in the hot stock. Let it sit in the sample for 5-10 min. Remove the sensor; brush the glass with a soft toothbrush, and then thoroughly rinse the sensor with mill water. To clean sensors in basic bleaching applications (> 8 pH), simply use a sample of hot stock from the D0 or D1 stage of Cl[O.sub.2] bleaching and follow the same procedure as above.

SPECIAL CLEANER

For removing stubborn silica or sulfate scales, a 2% hydrofluoric (HF) acid solution is useful. Dip the sensor in the acid for" about 5 rain, then remove the sensor and rinse it. Let the sensor dry in the air for a few minutes. A white powder will appear on the glass. Remove this powder with a soft toothbrush or cloth. Thick scales may take two applications. DO NOT let the sensor sit in the acid too long. Be careful, HF dissolves glass and can injure humans!

CALIBRATING THE SENSOR AFTER CLEANING

Calibration should not take place right after cleaning. Put the sensor back in the process for at least 15 min so that it can re-acclimatize to process chemistry prior to calibration. If the sensor cannot easily be put back in the process, piece it in mill water or a salt solution.

Most experts will not recommend buffer (two point) calibrations for final calibration of pulp and paper mill sensors. Buffers are good for checking sensor reaction time and initial set up. However, final calibration should be a one-point calibration made with the sensor in the process.

To use buffers to check reaction time, place the sensor in the first buffer and let it come to a stable reading (usually in 2 min). The stable reading should be within 1.0 pH units of the actual buffer value.

Next, put the sensor in the second buffer. The sensor should react to 80% of the change in buffer values in 30 seconds or less. If the sensor achieves a T80 in 30 seconds or less, the sensor is fine. Re-install it in the process, wait 20 min, and then do a one-point calibration by comparing the sensor's pH to a sample measured at process temperature (using a portable sensor with temperature compensation).

To clean ...              Use:

General foulants          3%-5 % hydrochloric acid
                          High pressure water jet (< 100 psi)

Oils and greases          Isopropyl alcohol (rubbing alcohol)
                          Methanol
                          Other solvent known to cut the specific
                          grease
                          High pressure warm water jet (< 100 psi)

Scales etc. from medium   5%-10% hydrochloric acid
to high pH solutions      3%-7% sulfuric acid
                          Vanasol industrial toilet howl cleaner
                          (mix of strong hydrochloric acid and
                          phosphoric acid)

Scales etc. from low      5%-10 % warm caustic (>130F)
pH solutions              5%-10% hydrochloric acid
                          2%-3% hydrofluoric acid
                          (CAUTION: see article text)

Sulfates and carbonates   5%-10% hydrochloric acid
                          Vanasol industrial toilet bowl cleaner (mix
                          of strong hydrochloric acid and phosphoric
                          acid) Combination of sodium metabisulfite &
                          sodium hydrosulfite

Silica or tenacious       2%-3% hydrofluoric acid
scales                    (CAUTION: see article text)

Table 1: Recommended cleaning solutions.

Don Spriggs is senior applications engineer for ABB Instrumentation Division, Carson City, Nevada, USA. For additional pH and calibration information, contact him at don.spriggs@ us.abb.com and request technical papers for pulp and paper pH.