Products & Technique Management Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Abstract: In this paper,the online measurement of the alkalinity of caustic washing liquid by the conductivity method was discussed,and a working curve of the conductivity vs.alkalinity was established,for which the correlation coefficient is 0.997 9 (n=7).The influence of the temperature of the caustic washing liquid and the presence of coexisting ions,such as iron and oil,on the accuracy of the conductivity method was discussed.The temperature is compensated for by establishing the correlation between conductivity and temperature.When the iron concentration is ≤1 000 mg/L,and the oil concentration is ≤1 000 mg/L,the deviation in the results obtained using the conductivity and titration methods is ≤2.5 g/L.The t-test results show that there is no systematic devia-tion between the conductivity and titration methods.The conductivity method has the advantages of a fast response and good real-time performance,which meet the requirements for the online determination of the alkalinity of caustic washing liquid.

Key words: alkalinity of caustic washing liquid; conductivity method; online detection

1 Introduction

In the cold-rolling process,there is typically a residual emulsion on the surface of the steel belt after pickling or cold rolling.In addition,the steel belt is usually coated with antirust oil to prevent corrosion.The oil on the surface of the steel belt can cause product defects,including annealing,hot zinc plating,electro-galvanization,tin plating,and chromium electroplating,which must be removed by degreasing.

The modern cold-rolling process involves a con-tinuous and high-speed production line.To remove oil from the surface of the steel belt,the degreasing process generally consists of caustic washing,brushing,electrolytic cleaning,and washing with water in sequence.NaOH,Na2CO3,Na2SiO3,and additives are the main components of lye,and their concentrations are referred to as the alkalinity,which directly affects the outcome of the lye washing pro-cess.Acid-base titration is typically used to measure the alkalinity in an alkali solution,but the titration process is characterized by hysteresis,and therefore does not meet the requirements of continuous and high-speed production.The conductivity method is widely used in environmental monitoring,chemical engineering,microelectronics,biopharmaceuticals,food processing, electric analytical chemistry,metal-lurgy,and other fields.When the temperature and pressure in the solution remain constant,there is a linear relationship between the conductivity and the solution concentration[1].Therefore,based on the correlation between conductivity and alkalinity,the alkalinity can be measured online and the conduc-tivity signal is transmitted to the PLC to auto-matically adjust the alkalinity to realize closed-loop control and intelligent production.

2 Test section

2.1 Major reagents and apparatus

The main reagents and instruments used in the experiment include a Mettler conductivity meter,a platinum-plated bright electrode,a Wantong 905 titrimeter,a pH complex electrode,a conductivity standard solution 1 413 μS/cm (25 ℃),and a 0.2-mol/L NaOH standard solution.

2.2 Test method

2.2.1 Conductivity test method

A solution with low alkalinityC1…Cn-1was obtained by diluting lye with a known alkalinityCto 100 mL.To obtain a solution with high alkalinityCn…Cn+m,a certain amount of lye was added to the original lye to stabilize the volume to 100 mL.The test solution was then heated to the process control temperature of the unit,and its conductivity was measured under constant agitation.

2.2.2 Alkalinity measurement

Five mL of test solution was poured into a 250-mL beaker and titrated with 0.2-mol/L NaOH standard solution with a composite electrode under constant agitation.The dynamic titration method was adopted.The pH range was set from 7 to 9,with a threshold value of 10 as the end point.The end point standard is determined based on the maximum pH value obtained in the titration process.

3 Results and discussion

3.1 Working curve of conductivity vs.alkalinity

Using the conductivity detection method,the conductivity was obtained with respect to the concentration of lye and the process control tem-perature.The working curve of conductivity vs.alkalinity is plotted in Fig.1,and the calibration details are provided in Table 1.

Table 1 Calibration of conductivity and alkalinity (65 ℃)

Fig.1 Calibration of conductivity vs. alkalinity (65 ℃)

3.2 Effect of temperature on conductivity

The conductivity analysis is based on the fact that the conductivity of a solution is proportional to its concentration of ions,which enables quantitative analysis[2].Because the temperature directly affects the conductivity of a solution,it is necessary to compensate for the temperature.The theoretical formula for conductivity temperature compensation is as follows:

γt=γ0[1+β(t-t0)]

(1)

where,γtandγ0are the conductivity values when the temperature istandt0,respectively;andβis the temperature coefficient of conductivity.

Because the temperature coefficient of a measured solution is based on a number of complex factors,the temperature coefficients of different solutions and different concentrations of the same solution are not the same.As such,fundamentally,it is practically impossible to obtain an accurate temperature compensation[3].The experimental results show that there is a certain correlation between conductivity and temperature (for details,refer to Table 2).The effect of temperature on conductivity can be reduced by correcting the linear regression equation for the temperature and conductivity.

Table 2 Relationship between conductivity and solution temperature

3.3 Influence of coexisting components in caustic washing liquid on accuracy of the conductivity

3.3.1 Effect of Fe ions on conductivity in caustic washing liquid

The Fe ions in caustic washing liquid are mainly from the residual Fe ions on the surface of the steel belt and the Fe ions produced by electrolytic caustic washing.In caustic washing liquid,some Fe ions react with oil in a certain temperature range (50-75 ℃) to form a fatty acid iron,called iron soap,and some Fe ions form ferric hydroxide in the caustic washing medium.

The results show that with increases in the number of Fe ions,the deviation in the conductivity measure-ment increases (for details,refer to Table 3).How-ever,as the concentration of Fe ions in caustic washing liquid is less than 1 000 mg/L,the accuracy of the conductivity measurement is acceptable.

Table 3 Effect of Fe ions on the conductivity accuracy (65 ℃)

3.3.2 Effect of oil in caustic washing liquid on measurement of conductivity

The objective of the caustic washing process is to remove the oil from the surface of the steel belt.In the course of lye washing,some of the oil forms iron soap with the Fe ions,which turns the caustic washing liquid black.Some of the oil also exists in free form,and its presence increases the viscosity of the solution and affects the rate of ion migration, the detection of conductivity[4],and the effect of caustic washing.

The results show that the concentration of oil in caustic washing liquid is less than 1 000 mg/L (for details,refer to Table 4),so the effect of oil on the accuracy of the results obtained by the conductivity method is acceptable.

3.3.3 Influence of component concentration in caustic washing liquid on electrode measurement

As the amounts of ferric hydroxide,iron soap,and oil in caustic washing liquid increase,the caustic washing liquid becomes turbid and viscous,and these components adhere to the surface of the electrode.These adhered components reduce the effective measurement area of the electrode and the response speed of the conductivity.The measurement accuracy of the conductivity method is then affected by the conductivity of the iron hydroxyl compound and iron soap in the caustic washing liquid.The experimental results show that the measurement accuracy of the conductivity method can be improved by filtering (for details,refer to Table 5).

Table 4 Effect of oil content on the accuracy of conductivity measurement (65 ℃)

Table 5 Comparison of alkalinity measurements before and after filtration

3.3.4 Other factors

When measuring the conductivity of caustic washing liquids with different components,such as NaOH,Na2CO3,or Na2SiO3,different models are used.The accuracy of a conductivity measurement can also be affected by the presence of different additives in the same caustic washing liquid.In addition,the conductivity is actually proportional to the activity of the solution,so even if the same components are used,a difference in alkalinity will affect the conductivity measurement accuracy.Therefore,a conductivity and alkalinity measure-ment model must be established for each working tank.

3.4 Comparison of measurement accuracies of conductivity and titration methods

A series of different samples were tested using the conductivity and titration methods,respectively.In thet-test[5],t=1.00,which is less thant(0.95,19)=1.96 (for details,refer to Table 6).No systematic error was identified in either method.

Table 6 Comparison of conductivity and titration methods (65 ℃) g/L

4 Conclusions

In this paper,an online method for determining the alkalinity of caustic washing liquid using the conductivity method,a method for determining the factors influencing conductivity,the range of their applications,and the measurement accuracy,was discussed.The conductivity method has the advan-tages of providing fast measurement,good real-time performance,and the use of simple measurement equipment,which make it very suitable for achieving intelligent online measurement and con-trol in industrial production.