Sodium carbonate softening method was optimized

Optimization of sodium carbonate softening treatment of converter gas washing water exploration and practiceQI Na1，HeFang1，Ling Ping2, Wang Feng3(QINa,LiangPing.Hubei Haili Chemical Technology Co., Ltd. Hubeiwuhan, 430000 china ;HeFang,WangFeng. Hunan Valin Xiangtan Iron and Steel Co., Ltd. (XISC)hunanxiangtan 411100 china)

Abstract: At present, the major chemistry treatmeng way of washing water of converter smoke is sodiumcarbonate softening, but in different phases of steel making proceduce,the water quality vary with the changeof the components and amount of converter gas.This article mainly study the optimization of the sodium carbonate softening methods to reduce the general hardness in different situations.It’s decided by the relatis of P andM-P basicity to select the watetreatmedicine .The amount of pharmacy isalso determined by the circulation flow rate and the general hardness.

Keyword: Washing water of converter smoke; calcium Hardness; magnesium Hardness;

0 Introduction

The flue gas washing water of converter is the product of direct contact between flue gas and water in the process of cooling and dust removal by wet method. Its formation process can be expressed as follows:

In the process of steelmaking, the gas volume and composition of each smelting stage of the converter are different, and the quality of the waste water from the purified flue gas also changes. Generally, there is more smoke during the blowing process, and the corresponding suspended matter mass concentration in the wastewater is higher, generally 2000-5000mg/l. The PH value in the wastewater is also determined by the smelting method itself, and the water temperature of the wastewater also changes greatly, and the general temperature rise is 10-20℃, so the turbid ring water of the converter has the characteristics of high temperature and high turbidity. According to the characteristics of turbid water in converter, the main purpose of water treatment is to reduce temperature, turbid and scale formation of high-temperature components. From the current water treatment technology, the water treatment of converter flue gas washing water has gone through the following stages:

The first stage  -- adding efficient scale inhibitor and dispersant to avoid hard scale formation;

The second stage -- adding high molecular flocculant -- adding high efficiency scale inhibition and dispersant,

In the third stage, a chemical treatment process based on flocculation ---- sodium carbonate softening method and adding efficient scale inhibition and dispersant was formed, which made the water treatment of the converter turbidity ring water system mature. In the long-term operation process, the sodium carbonate softening method reduces the scaling rate, and the scaling situation of the equipment is well controlled in the long-term operation process, but because the characteristics of the converter gas washing water itself are largely affected by the auxiliary materials in the production process, the sodium carbonate softening method is limited in the specific water environment.

1 Main factors affecting the stable operation of sodium carbonate softening method

The premise of water quality stability is according to the production process and water quality conditions, according to local conditions, the right medicine. Due to the dynamic balance in the water quality treatment process, affected by many factors, when the balance is broken, the water quality can not meet the requirements, the following factors affecting sodium carbonate softening method are analyzed:

1.1 Active ash as auxiliary material for steelmaking slag making. The content of silicon in hot metal is the main factor to determine the amount of active ash, the content of silicon is high, the amount of active ash is high. When oxygen is blown, part of the lime is blown out without contact with the liquid steel, so the properties of active ash should be large particles, and should not contain powder. Mainly because active ash contains a lot of alkaline substances such as CaO, it is an important factor restricting the balance of hardness.

1.2 Acidic substances formed in the blowing process: such as CO2, SiO2, etc. CO2 accounts for about 17% to 22%(volume fraction) of the composition of the converter flue gas. In the middle of blowing, the water temperature is the highest, the smoke volume is the largest, and CO2, SiO2 and so on are mainly formed in the early stage of blowing.

1.3 Other accessories: light burnt dolomite, fluorite, magnesium ball, etc.

1.4 On-site water treatment management and drug administration. This paper mainly carries out experimental analysis and detailed discussion on the influencing factors 1.1 and 1.2.

2. Field experiment and discussion

2.1 Magnesium hard high

There are three 30t converter in a steel mill, Q=266--340 m3/h, and the water capacity is about 800 m3. The main characteristics of turbid ring water in converter are: low pH, hard calcium and hard magnesium coexist, and hard magnesium is on the high side.

2.1.1 Index control requirements: SS<100 mg/L, hardness <50mg/L;

2.1.2 Water Quality: 8.0

Hydration features: hardness in 100-130 mg/L, magnesium hard 15-30mg /L;

2.1.3 Duration: 10 months;

2.1.4 Water quality inspection items: pH, total hard, calcium hard, magnesium hard, P-alkali, M-P alkali;

2.1.5 Conventional sodium carbonate softening hardness method: add sodium carbonate. Disadvantages: Although the calcium hardness has decreased to a certain extent, the magnesium hardness is almost unchanged. The following is the data after adding sodium carbonate;

2.1.7 Solution: a: Add alkali tablets or powdered lime to neutralize excess HCO3- and make the system reach a new balance. b: Most of the water replenishment is HCO3- and magnesium is hard and low, so appropriate drainage can be considered. 2.1.6 According to the daily inspection data, the reasons are as follows: the amount of lime added during smelting is less, the ore selected for slagging contains higher magnesium compounds, and the acidic ore used in the steelmaking process makes the pH of the system low.

2.1.8 Water treatment debugging data

(1) : Adding powdered lime method

Results and discussion: After adding lime milk, pH increased significantly, calcium hardness and magnesium hardness decreased at the same time, lime milk contained OH-, neutralized a large amount of HCO3- in the water, and CO32- was formed. At the same time, it reacts with Ca2+ in the system and Ca2+ introduced by itself to precipitate CaCO3, and OH- and Mg2+ to precipitate Mg(OH)2. The turbidity of the water is significantly reduced, and the high pH is favorable for reducing the hardness, but the pH should not be too high. Disadvantages: The dosage is large, generally need more than a group of lime dosing equipment, and dosing equipment is easy to jam, time consuming and manpower when dosing agents. This method can only be maintained for a period of time if acid components are constantly introduced into the flue gas.

(2) : Adding tablet alkali method

Results and discussion: After addition, pH increased obviously, calcium hardness and magnesium hardness decreased at the same time, alkali tablets contained OH-, neutralized a large amount of HCO3- in the water, and CO32- was formed. At the same time, it reacts with Ca2+ and Mg2+ in the system to precipitate. The turbidity of the water is significantly reduced, and the high pH is favorable for reducing the hardness, but the pH should not be too high. Disadvantages: large dosage, high drug cost, time consuming and manpower when adding drugs. This method can only be maintained for a period of time if acid components are constantly introduced into the flue gas. Results and discussion: After addition, pH increased obviously, calcium hardness and magnesium hardness decreased at the same time, alkali tablets contained OH-, neutralized a large amount of HCO3- in the water, and CO32- was formed. At the same time, it reacts with Ca2+ and Mg2+ in the system to precipitate. The turbidity of the water is significantly reduced, and the high pH is favorable for reducing the hardness, but the pH should not be too high.

(3) : drainage method：

Results and discussion: pH changed little before and after drainage, calcium hardness increased slightly after drainage, and magnesium hardness decreased. The total hardness can be reduced by adding sodium carbonate after proper drainage. The specific displacement should be determined by experiments according to the field conditions. Excessive drainage will affect the concentration of the agent, making the hardness difficult to qualify.

Advantages: save manpower, no need to add additional dosing equipment;

Disadvantages: Drainage can pollute the surrounding environment.

2.2 Calcium hard high

There are four 30 t converter in a steel mill, Q=780 m3/h, and the water capacity is about 1500m3. The main characteristics of the turbid ring water in the converter are: high pH, high hardness, calcium hardness up to several hundred mg/L, and magnesium hardness is almost 0.

2.2.1 Index control requirements: SS<100 mg/L; Hardness <50mg/L.

2.2.2 Water quality characteristics: pH is about 12, hardness is 200-450mg /L, most of which hardness is calcium hard. Hydration features: Hardness is about 240mg/L.

2.2.3 Duration: The situation of calcium hardening is not continuous, ranging from 10-20 days per month.

2.2.4 Water quality inspection items: pH, total hard, calcium hard, magnesium hard, P-base, M-P base.

2.2.5 Conventional softening hardness method: adding sodium carbonate. Disadvantages: Although calcium hardness has decreased to a certain extent, the effect is not ideal.

The following is the data after adding sodium carbonate

2.2.6 According to the daily inspection data, the reasons are as follows: the amount of lime added during smelting is too large, and the active lime contains more dust. When blowing, the powdery active ash rises with the flue gas, and the balance between CaO and CO2 is destroyed. After reacting with the washing water, the apparent phenomenon is that there is a layer of gray powder floating on the water surface. Usually, the water quality test shows that P>M-P, M-P is very low, and pH is high, CO32- and OH- coexist, most of them are OH-, and the balance between Ca2+, Mg2+ and CO32- and OH- is destroyed.

2.2.7 Solution: a: Add HCl and sodium carbonate to neutralize excessive OH- and make the system reach a new balance. b: Add sodium bicarbonate. c: Charge CO2 into the system.

2.2.8 Water treatment debugging data

(1) : Sodium carbonate was added after adding an appropriate amount of HCL. Although the laboratory test proved its feasibility, the commissioning failed because the rate of adding acid was not enough to neutralize a large amount of OH- in the incoming water during the field commissioning.

(2) : Adding sodium bicarbonate. The following are laboratory data.

Results and discussion: When the small test is enlarged, this condition is more effective in reducing the hardness, and the effect time is faster. Field debugging proves that the method is feasible. HCO3- can neutralize the excess OH- and introduce CO32- to react with Ca2+ to precipitate. However, it should be noted that sodium bicarbonate should not be added too much. 3 Summary of results and discussion: If the small test is enlarged, this situation is more effective in reducing the hardness, and the effect time is faster. Field debugging proves that the method is feasible. HCO3- can neutralize the excess OH- and introduce CO32- to react with Ca2+ to precipitate. However, it should be noted that sodium bicarbonate should not be added too much.

3 Summary

Converter gas washing water is a complex system, which is difficult to deal with circulating water. The use of sodium carbonate softening method to treat the quality of the dust removal water of the converter flue gas, the key is to control the balance of hardness and alkalinity in the water, the pH value of the converter turbidity ring water can be monitored in the process of production practice, and the critical value is determined according to the specific situation of the site

(1) Under normal circumstances, the addition of sodium carbonate can well control the balance of hardness and alkalinity, so that the water system runs smoothly.

(2) When the pH is less than 9, it is easy to have high magnesium hardness, and the effect of auxiliary adding sodium carbonate after proper drainage is more obvious, but it is not appropriate to overflow after adding sodium carbonate, so as to avoid the loss of agents. Or add lime or alkali tablets, the amount must be controlled within a reasonable range.

(3) When the pH is very high or close to 12, calcium is easy to appear very hard phenomenon, at this time, the method of rapidly reducing the hardness is to add sodium bicarbonate, neutralize the excessive OH- in the water, and generate CO32-, at the same time, the amount must be controlled within a reasonable range. The experiment and the actual operation results show that the hardness can be effectively reduced by different methods combined with sodium carbonate softening method for different water quality of flue gas dusting water of converter.

reference

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〔2〕Recirculating cooling water treatment manual. Tianjin Science and Technology Press, 1991.

〔3〕Technical improvement and effect analysis of circulating water treatment for dust removal in 3*90t converter, 2006.

〔4〕Discussion and practice on the optimization of sodium carbonate softening method for the treatment of converter flue gas washing water, Energy Saving Water Treatment, 2013.