pH sensor cleaning during pulping and papermaking

Abstract: The commonly used solutions and cleaning methods for pH sensor cleaning are discussed, and the calibration after cleaning is discussed.

pH sensors are widely used in the pulp and paper industry. In the process of use, scaling of the pulp on the sensor is unavoidable. This will significantly reduce the accuracy of the sensor and will not be conducive to production. How can we make the sensor commonly used? We have studied the method of cleaning the pH sensor.

1 commonly used cleaning solution and ordinary cleaning method

Since the pH sensor is extremely delicate, removing the dirt stuck to the pH sensor requires sufficient patience, and an appropriate safety device is used during the cleaning process. Selecting the appropriate cleaning solution will make this job easier. Table 1 lists the cleaning solutions used in some cases.

When cleaning, immerse the sensor in the cleaning solution for at least 5 minutes so that the cleaning medium can seep into the dirt. Two washings of the sensor is an effective method. The sensor is first soaked in an acid or alkaline solution. During soaking, the sensor's electrodes are brushed with a soft toothbrush to remove the dirt. Then choose a solvent wash. For example, the grease or organic residue can be selected from one of four methods listed in Table 1 Oil or Fat to remove dirt. Finally, the sensor should be rinsed and soaked in fresh water or salt solution (KCl).

Table 1 Applicable conditions of cleaning solution used under different conditions The used solution is usually washed with 3%~5% hydrochloric acid and high-pressure water (<689.476kPa).
Oil or fat isopropyl alcohol (wear-resistant ethanol)
Methanol, other solution that can remove special fats PH sensor working in alkaline conditions W = 5% to 10% hydrochloric acid solution
W=3%~7% sulfuric acid mixture of concentrated hydrochloric acid and phosphoric acid PH sensor working under acidic conditions W=5%~10% hot alkaline solution (>56°C)
W = 5% to 10% hydrochloric acid
W=2%~3% hydrofluoric acid (reference text)
Sulfate and carbonate W=5%~10% hydrochloric acid Concentrated hydrochloric acid and phosphoric acid mixed solution Mixture of sodium sulfite and sodium dithionite Silica or viscous material W=2%~% hydrofluoric acid (Refer to text)

2 bleach plant pH sensor cleaning

In the kraft bleaching plant, the pulp contains various cleaning solutions. Alkaline slurry can be used to clean the sensor used to measure low pH, and acidic slurry is used to clean the sensor used to measure high pH.

To give a simple example: If you clean the ClO3 or Cl2 drift sensor, use a slurry with a pH greater than 7 in the Eop drift section, that is, an alkaline solution around 80°C. Put 10~20L hot alkaline pulp into the bucket, remove the sensor and immerse it in the pulp, soak it for 5~10min, remove it, brush the mirror with a soft toothbrush, and rinse thoroughly with water. If you are cleaning a sensor that operates in alkaline conditions, use the hot slurry from the Do or Dl section of the Cl2 drift section to clean as described above.

3 special cleaning agent

To remove difficult-to-treat silica or sulfate, a solution of W=2% hydrofluoric acid is often used. Immerse the sensor in the acid solution for 5 minutes, then remove and rinse. Then place the sensor in the air for a few minutes. After naturally drying, white powder will appear on the sight glass. Use a soft toothbrush or a soft cloth to remove the powder. If the dirt is too thick, wash it twice. The sensor cannot stay in acid for too long and hydrofluoric acid can corrode the glass. Be careful during the cleaning process to prevent hydrofluoric acid injury.

4 sensor calibration

The calibration work cannot be performed immediately after cleaning. It should be checked after the sensor has been loaded into the system for at least 15 minutes to allow the sensor to adapt to the chemical conditions of the system before calibration. If the sensor is not easily installed in the system, it can be checked in clear water or saline solution.

Most experts do not agree with the use of buffer (two-point) calibration for the final calibration work of the pulp mill or paper mill sensors. The buffering agent is suitable for checking the reaction time and initial setting of the sensor, and the final calibration should be completed in the system, and one point verification should be used.

When using buffers to check the reaction time, first place the sensor in the first set of buffers so that it has a stable reading (generally after 2 min), and the stable reading should not exceed one unit of the actual buffer pH; then Put the sensor in the second set of buffer and the sensor should reach 80% of the buffer pH within 30 seconds or less. If the sensor reaches 80% of the target value within 30s or less, this sensor is proven to be accurate. Finally, at the use temperature, the sensor is reinstalled in the system. After 20 minutes, the pH value measured by the sensor is compared with the pH value of the sample, ie a one-point calibration (movable sensor with temperature compensation). Qualified after the test can be put into use.

To sum up, if you use an appropriate solution, the cleaning of the pH sensor is not complicated and many cleaning solutions can be found in your factory. May your sensor be accurate forever!