JP2015168875A - Water quality management method for skid pipe cooling water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water quality management method for skid pipe cooling water capable of preventing surely inexpensively corrosion and occlusion of a skid pipe of a furnace of a hot rolling plant.SOLUTION: Provided is the water quality management method for circulating mixture waste water in which direct waste water which is discarded from a hot rolling plant and subjected to waste water treatment and indirect waste water which is discarded from the hot rolling plant are mixed, for using it as coolant water for a skid pipe of a furnace, in which lime Ca(OH)is injected to the direct waste water in an upstream side of the waste water treatment step of the direct waste water, and a Langelier index L1 in a skid pipe inlet temperature T1 and a skid pipe outlet temperature T2 is controlled in a range of -1 to 0.

Description

  The present invention relates to a water quality management method for skid pipe cooling water in a heating furnace that heats a steel material (steel piece) subjected to hot rolling.

  The hot rolling process in the steel manufacturing process is performed by inserting a steel material (steel piece) such as slab produced by continuous casting or ingot-bundling into a continuous heating furnace, and several hundred to thousands of degrees Celsius After reheating to a predetermined temperature, the film is extracted, and is pressed and thinned with a pair or a plurality of pairs of rotating rolls in a high temperature state, or rolled into a predetermined shape.

  The above-mentioned continuous heating furnace is broadly divided into a pusher type that loads and extracts all steel pieces in the furnace by pushing them from the charging side with a pusher, and the hearth is divided into a moving hearth and a fixed hearth, There is a walking beam type that conveys a steel piece by repeating a rectangular movement of moving hearth up → forward → down → reverse, in recent years there is no generation of scratches, small temperature irregularities (skid marks), and The latter, which is excellent in productivity, has become the mainstream.

  By the way, in the said heating furnace, a steel piece is mounted on a skid and is normally heated to 1100 degreeC or more by the case, 1300 degreeC or more depending on the case. The skid has a structure in which a heat-resistant skid button or skid rail that is in direct contact with the steel slab is installed on the steel skid pipe, and protects the skid button and the like from the high-temperature atmosphere in the furnace. In order to secure the strength that can resist the load of the heavy steel slab, the outer periphery of the skid pipe is covered with a refractory heat insulating material and cooled by flowing cooling water into the skid pipe.

  However, when cooling water is allowed to flow through the skid pipe, there is a problem that an oxidation reaction or the like occurs between dissolved oxygen in the cooling water and the inner surface of the steel skid pipe, and corrosion progresses over time. In particular, in the case of an open circulation system in which cooling water is drained and repeatedly used, oxygen in the atmosphere is absorbed by the cooling water, so that the above-described oxidation reaction and accompanying corrosion are further promoted. Further, in the case of the circulating cooling water, chlorine ions, sulfate ions and the like in the cooling water are concentrated as the water evaporates, so that corrosion is further promoted.

  As the above corrosion progresses, the skid pipe is perforated, causing water leakage, causing significant energy loss and reducing productivity. In addition, rust (rust) caused by corrosion may lead to a decrease in thermal conductivity, or the inner diameter of the skid pipe may be reduced, resulting in a decrease in cooling water volume and a decrease in cooling capacity. Or cause the skid pipe to become blocked. As a result, deterioration of skids, bricks in the furnace, etc. is promoted, so that the damage on the equipment is extremely large.

  Several countermeasures have been proposed for the above problems. For example, a highly corrosion-resistant material is used for the skid pipe, the inner surface of the skid pipe is lined, or an organic phosphoric acid-based or chlorine-based chemical (anticorrosive) is injected into the cooling water. A method for improving water quality, a method for suppressing scale generation by applying a magnetic force (for example, see Patent Document 1), and the like are known. However, the method of using a highly corrosion-resistant material or applying a lining has a high material cost, and in the case of a lining, the deterioration due to heat is large and the effect does not last long. In addition, the method of injecting a drug (anticorrosive) has a high drug cost and often cannot measure the drug component continuously, so it is difficult to stably manage the drug at an appropriate concentration. Further, the method of applying a magnetic force has a problem that a sufficient effect cannot be obtained by itself.

  In addition, when the calcium hardness and pH in the cooling water are high, if the temperature of the cooling water rises due to heat exchange during cooling, carbonate ions and calcium ions in the cooling water are saturated and precipitated as calcium carbonate on the inner surface of the pipe. It has been. This precipitation of calcium carbonate has the effect of preventing corrosion by forming a corrosion-resistant coating if it is within the proper range, but if it is excessively deposited and deposited, it may cause heat transfer failure or piping as well as corrosion products. Cause the problem of blocking.

Therefore, for the problem of pipe corrosion and blockage due to the quality of the cooling water as described above, the “Langelia index”, which is an index indicating the corrosiveness and calcium carbonate precipitation of the cooling water, is used. A method for water quality management has been proposed. Here, the Langeria index LI is a difference between an actual pH value of water and a theoretical pH value (pHs: a pH value in an equilibrium state where calcium carbonate in water is neither dissolved nor precipitated). As the absolute value is larger and the absolute value is larger, calcium carbonate is more likely to precipitate and corrosion protection is more likely to occur. On the other hand, as LI is negative and the absolute value is larger, the corrosion tendency is stronger. From the pH of water, calcium ion concentration, total alkalinity (also referred to as M alkalinity) and the amount of soluble substance (used for correction value calculation), it is obtained by the following equation (1).
Langeria Index (LI) at 25 ° C. = pH−pHs = pH−8.313 + log [Ca ²⁺ ] + log [A] −S (1)
Where: pH: actual pH value of water
pHs: pH value when in equilibrium
log [Ca ²⁺ ]: Logarithm of calcium ion concentration
log [A]; logarithm of total alkalinity (M alkalinity)
S: Correction value

  As a method for managing the quality of cooling water flowing through a skid pipe using the Langeria index, for example, Patent Document 2 discloses a temperature measurement result obtained by measuring the temperature of cooling water in a skid pipe of a heating furnace. M alkalinity and pH of the cooling water are calculated based on the calculation result, a Langelia index in the skid pipe is calculated using the calculation results of the M alkalinity and the pH, and an alkaline agent is calculated based on the calculation result of the Langeria index A method for cooling a skid pipe of a heating furnace that determines the input amount has been proposed.

Japanese Patent Laid-Open No. 10-066978 JP 2009-074116 A

  By the way, it is common to circulate and use the water drained from the wastewater used in the hot rolling plant as the cooling water for the skid pipe (hereinafter, the wastewater is treated by being recycled as described above. Water is also called “circulating water”). The waste water discharged from the hot rolling factory is indirect waste water that does not come into direct contact with the product or heat source, such as the above-mentioned skid pipe of the heating furnace or the cooling water of the heat exchanger, and is hardly contaminated. Like water used for cooling descalers and rolling mills, steel sheets, etc., it comes into direct contact with products, etc., and is divided into direct system wastewater contaminated with scale (iron oxide) and oil. In general, the waste water is treated in a separate facility or cooled and used separately. In this case, in order to remove scales and pollutants, wastewater treatment of indirect system wastewater is treated with heavy chemical treatment such as injection of coagulant and chemicals and precipitation. Is often not done or partially processed. In the wastewater treatment of indirect system wastewater, an anticorrosive agent may be injected to prevent corrosion.

  However, when the number of wastewater treatment systems and water supply systems increases, the equipment cost increases, and water loss due to operation and running costs increase. Therefore, after the wastewater treatment to remove pollutants and the like is performed on the direct wastewater discharged from the hot rolling factory, the indirect wastewater discharged from the hot rolling factory is also discharged into this treated direct wastewater. In some cases, the mixed waste water is mixed as it is without being circulated and used as a direct system water supply (direct contact with a product or the like) and an indirect system water supply (non-contact with a product or the like). However, when using the above mixed waste water as feed water, the equipment costs etc. can be kept low, but indirect water feed such as cooling water for skid pipes, for example, contains pollutants derived from direct waste water, There is a problem that this may promote corrosion and blockage of the pipe. Moreover, when a chemical | medical agent is thrown in in the wastewater treatment process of a direct system waste_water | drain in order to remove the said pollutant, depending on the component of a chemical | medical agent, there exists a possibility that it may have a bad influence on steel products. In addition, it is not practical to remodel equipment so that direct wastewater and indirect wastewater are separated and treated for wastewater, which requires a large amount of capital investment.

From this point of view, the technology disclosed in Patent Document 2 shows that in the above technology, the cooling water discharged from the skid pipe is drained alone, and the direct and indirect drainage is mixed and circulated into the cooling water. Therefore, it cannot be applied as it is.
Moreover, in the Example of patent document 2, although sodium hydroxide NaOH is used as an alkaline agent, NaOH is expensive and has a problem in terms of wastewater treatment cost. Further, even though NaOH is effective for adjusting the pH, it is not effective for adjusting the calcium ion concentration (Ca hardness) which is the main element of the above-described equation (1) for calculating the Langeria index LI. Further, when the cooling water of the heating furnace passes through the heating furnace, the temperature rises due to the heat of the heating furnace, so that there is a temperature difference between the entry side and the exit side. Since the Langelia index changes depending on the temperature, even if the technique of Patent Document 2 in which the temperature of the cooling water is measured only at one point in the skid pipe, a sufficient effect cannot be obtained. There is also a problem.

  The present invention has been made in view of the above-mentioned problems in the prior art, and its purpose is to be discharged from a hot rolling factory and discharged directly from the hot rolling factory into direct system wastewater that has been subjected to wastewater treatment. When the wastewater treated mixed wastewater mixed with indirect wastewater (hereinafter simply referred to as “mixed wastewater”) is circulated and used as the cooling water for the heating furnace skid pipe, corrosion and blockage of the skid pipe must be ensured. The object is to propose a water quality control method for skid pipe cooling water that is effective in preventing the cost reduction.

The inventors have intensively studied to solve the above problems. As a result, in the case of using mixed wastewater mixed with indirect wastewater in the direct wastewater treated with wastewater treatment and circulating it into the cooling water of the skid pipe, corrosion products such as corrosion of the skid pipe, corrosion products such as rust, and Ca system In order to prevent clogging of piping due to deposits, etc., slaked lime Ca (OH) ₂ is injected into the direct drainage upstream from the settling basin in the direct drainage treatment process, and the cooling water skid pipe inlet side It has been found that it is effective to control the Langeria index LI to be in the range of −1 to 0 at any of the temperature and the outlet temperature, and the present invention has been developed.

That is, the present invention circulates mixed waste water mixed with indirect waste water discharged from a hot rolling factory to direct system waste water discharged from a hot rolling factory and subjected to waste water treatment, and a skid pipe of a heating furnace. Quality control method when used for cooling water of the above-mentioned, wherein slaked lime Ca (OH) ₂ is injected into the direct system waste water upstream of the direct waste water treatment process, and the cooling water skid pipe inlet side A water quality management method for skid pipe cooling water is proposed, characterized in that the Langelia index LI at temperature T1 and skid pipe outlet temperature T2 is controlled in the range of -1 to 0.

  The water quality management method for skid pipe cooling water according to the present invention is characterized in that the injection of the slaked lime is performed upstream of the settling basin in the wastewater treatment process for direct wastewater.

  In addition, the water quality management method for skid pipe cooling water according to the present invention provides the Langerian index of the cooling water on the skid pipe inlet side when the temperature difference ΔT between the skid pipe inlet side temperature T1 and the skid pipe outlet side temperature T2 is used. The LI is controlled in a range of (−1 + 0.016ΔT) to 0.

  Further, the water quality management method of the skid pipe cooling water according to the present invention provides the Langerian index of the cooling water at the outlet side of the skid pipe when the temperature difference ΔT between the skid pipe inlet side temperature T1 and the skid pipe outlet side temperature T2 of the cooling water is set. The LI is controlled in the range of −1 to −0.016ΔT.

According to the present invention, the mixed wastewater obtained by mixing the direct wastewater discharged from the hot rolling mill and the wastewater treatment with the indirect wastewater discharged from the hot rolling factory is circulated to the cooling water of the skid pipe. When using it, the langeria index LI of skid pipe cooling water is in the proper range by injecting slaked lime Ca (OH) ₂ that is inexpensive and harmless to the product upstream of the wastewater treatment process of direct system wastewater Therefore, it is possible to prevent corrosion and blockage of the skid pipe easily and inexpensively without requiring special capital investment.

It is a figure explaining the flow of the circulating water in the case of circulating the mixed waste water which mixed the indirect system waste water with the direct system waste water which performed the waste water treatment, and using it for the cooling water of a skid pipe. It is a graph which shows the time-dependent change of pH of the aqueous solution after slaked lime injection | pouring. It is a graph which shows the influence which the Langeria index LI has on corrosion of steel. It is a figure which shows the example which applied the water quality management method of this invention to the flow chart of the circulating water shown in FIG.

  Fig. 1 shows the cooling water of a skid pipe by circulating a mixed wastewater mixed with indirect wastewater discharged from a hot rolling mill in combination with direct wastewater discharged from a hot rolling mill and subjected to wastewater treatment. It explains the flow of circulating water when used in the above. In FIG. 1, the direct water supply 1 a is supplied from an elevated water tank 2 to a rolling facility 3 such as a hot rolling mill and used as cooling water for a rolling mill, cooling water for a hot run table, cooling water for a coiler, etc. The direct wastewater 1a 'is sent to the direct wastewater treatment facility 4 for removing pollutants. In the wastewater treatment facility 4, after the flocculant and chemicals are poured into the direct wastewater 1 a ′ in the mixing basin 4 a and stirred and mixed, the pollutants are aggregated in the agglomeration basin 4 b and further contaminated in the sedimentation basin 4 c The substance is precipitated and removed to be cleaned and sent to the joint well 4d.

As the flocculant, polyaluminum chloride (Al ₂ (OH) _n Cl _6-n ] _m : PAC) or aluminum sulfate (Al ₂ (SO ₄ ) ₃ · 16H ₂ O: sulfate band) is generally used. However, they can also be suitably used in the wastewater treatment step. Moreover, as a chemical | medical agent added in the mixing basin 4a, although there exists a polymer flocculent, it is preferable to determine the use possibility, after confirming the influence which acts on a product.

  On the other hand, the indirect system water supply 1b is supplied from the elevated water tank 2 to the heating furnace 5 in the hot rolling mill, used as cooling water for the skid pipe 6, and then discharged to become indirect system drainage 1b '. Since this indirect drainage 1b ′ is not contaminated, it is sent to the joint well 4d as it is and mixed with the direct drainage 1a ′ after the above-described cleaning. In addition, the amount of indirect system waste water 1b 'is smaller than the amount of direct system waste water 1a', and when it is circulated, it exhibits the same characteristics as the direct system water distribution 1a '.

  Next, the mixed wastewater 1 ′ obtained by mixing the direct wastewater 1a ′ after the wastewater treatment with the indirect wastewater 1b ′ is sent to the cooling tower 7, cooled to a temperature of about 30 ° C., and then sent to the reservoir 8. Thereafter, the water is pumped into the elevated water tank 2 by the pumping pump 9 and supplied again to the hot rolling mill as the feed water 1.

By the way, it is important for the cooling water used for cooling the skid pipe of the heating furnace to control the Langeria index LI defined by the above-described equation (1) within an appropriate range.
Various methods for calculating the Langeria index LI have been proposed. In the present invention, the following formula (2) can be used from the actual pH value of water, water temperature, concentration of evaporation residue, calcium hardness, and total alkalinity. The Nodel method that can be found in
Langeria index LI = pH−pHs = pH− (pHs = 9.3 + (A value + B value) − (C value + D value)) (2)
Where pH is the actual pH value of water
pHs: Theoretical pH value
A value: Value determined by the concentration of evaporation residue
B value: Value determined by the water temperature
C value: Value determined by calcium hardness
D value: a value determined by the total alkalinity (M alkalinity), and the above A value to D value can be obtained from Table 1.

  In order to obtain the Langeeria index LI of the cooling water from the above equation (2), the actual pH value of the cooling water, the concentration of the evaporation residue, the temperature, the calcium hardness and the total alkalinity (M alkalinity) may be measured. I understand that. However, in the present invention, the temperature, pH and calcium hardness of the cooling water are mainly measured for the reasons described below.

  First, as can be seen from Table 1, the A value determined by the concentration of the evaporation residue in the equation (2) has little influence on the Langeria index LI. Therefore, the A value can be made constant by obtaining in advance the concentration of the evaporation residue in the circulating water system to be controlled.

  Next, according to the knowledge of the inventors, the D value determined by the total alkalinity (M alkalinity) in the formula (2) varies depending on the series of circulating water, but between the C value determined by the calcium hardness. I know there is a good correlation. Therefore, if a relational expression between the C value and the D value of the circulating water system to be controlled is obtained in advance, the D value can be estimated from the C value.

  Accordingly, the Langeria index LI defined by the above equation (2) of the skid pipe cooling water can be obtained by calculating the concentration of the evaporation residue and the relational expression between the C value and the D value in advance. It can obtain | require by measuring calcium hardness.

  Next, the water quality management method of the present invention for controlling the Langeria index LI of the skid pipe cooling water will be described. As described above, the Langeria index LI of the cooling water can be obtained from the pH value, temperature, and calcium hardness of the cooling water. In other words, the Langeria index LI of the cooling water is determined by the pH value, temperature, and calcium hardness of the cooling water. Can be controlled. However, the temperature of the cooling water (especially the outlet temperature) varies depending on the operating condition of the heating furnace, and thus cannot be adjusted in the wastewater treatment process.

Moreover, it is not preferable to use the pH value of the cooling water for controlling the Langeria index LI for the following reason.
FIG. 2 shows five types of aqueous solutions having different pH values by adding slaked lime Ca (H) ₂ to indirect wastewater having a pH of 8.0, and stirring with a magnetic stirrer at 0.2 m / s. The result of having measured the time-dependent change of pH of aqueous solution is shown. From this figure, it can be seen that even if Ca (H) ₂ was added to change the pH, the pH returned to the original pH (8.0) after about 4 days. This is thought to be due to neutralization by absorbing CO ₂ in the air. From this result, the method of controlling the Langeria index LI by adjusting the pH is not preferable in terms of the LI control effect and stability when applied to the cooling water of a skid pipe that is circulated and used in an open system.
Therefore, in the present invention, the Langerial index LI of the skid pipe cooling water is controlled mainly by adjusting the calcium hardness.

The calcium hardness of the skid pipe cooling water is adjusted on the upstream side of the direct wastewater treatment facility 4 in the flow chart shown in FIG. 1, specifically, on the coagulation basin 4b and / or the mixing pond upstream of the settling basin 4c. 4a, preferably by adding a Ca-containing material.
Here, slaked lime Ca (OH) ₂ is used as the Ca-containing substance in the present invention. The reason is that as the Ca-containing substance that can be used to adjust the calcium hardness, in addition to the slaked lime Ca _{(OH) 2,} there are quick lime CaO and the like calcium carbonate CaCO _3, if calcium hydroxide Ca _{(OH) 2} This is because slaked lime Ca (OH) ₂ is most suitable from the viewpoints of harmlessness even when it comes into contact with the product, availability, safety during storage, and raw material costs.

The reason why slaked lime Ca (OH) ₂ is injected upstream of the direct wastewater treatment facility 4 is that the mixing basin 4a of the direct wastewater treatment facility 4 is usually mixed with a flocculant. Since the agitation equipment is provided, the existing equipment can be used as it is, and insoluble substances contained in the slaked lime can be removed in the coagulation basin 4b and the sedimentation basin 4c installed after the mixing basin 4a. This is because even low quality (inexpensive) slaked lime with many impurities can be used.

Next, the control range of the Langeria index LI of the skid pipe cooling water will be described.
FIG. 3 shows A to F6 kinds of circulating water collected from different ring water systems in the steelworks shown in Table 2, and G to which the Langeria index was adjusted by adding slaked lime Ca (OH) ₂ to the circulating water C. The experimental result which measured the corrosion rate of the steel plate using 3 types of circulating water of I (total 9 types) as a test liquid is shown. In the above experiment, circulating waters A to I shown in Table 2 were put into a beaker and maintained at a temperature of 50 ° C., the cold-rolled steel plate SPCC was immersed for 7 days, and the corrosion rate was determined from the mass difference before and after the test. . In addition, the said test liquid was replenished or drained, and it managed so that it might become the water balance of an actual machine. Moreover, it stirred at 0.2 m / s with the magnetic stirrer during the test period.

  From Table 2 and FIG. 3, it can be seen that the corrosion of the skid pipe can be greatly reduced by controlling the Langeria index LI to −1 or more by injection of slaked lime. However, if it exceeds 0, calcium carbonate may precipitate on the inner surface of the skid pipe and the skid pipe may be blocked. Therefore, in the present invention, the Langeria index LI of the cooling water of the skid pipe of the heating furnace is controlled in the range of −1 to 0. A more preferred Langeria index LI is in the range of −0.5 to 0.

  However, since the B value in the equation (2) is a value that varies depending on the water temperature as described above, the Langeria index LI also varies depending on the temperature. Therefore, in order to reliably prevent the corrosion and blockage of the skid pipe, all the temperatures of the cooling water flowing through the skid pipe, that is, the cooling water skid pipe inlet side temperature T1 and the outlet side temperature T2 are set. , T1 and T2 must be controlled so that the Langeria index LI is within a range of −1 to 0. Therefore, in the present invention, it is necessary to measure the temperature of the cooling water at two places on the entrance side and the exit side of the skid pipe.

  However, if there is no significant variation in the difference ΔT between the skid pipe entry side temperature T1 and the exit side temperature T2, the temperature measurement on either the skid pipe entry side or the exit side can be omitted. For example, when only the measurement of the skid pipe entry side temperature T1 and the Langeria index LI at all temperatures of the cooling water flowing in the skid pipe is controlled to be in the range of −1 to 0, when passing through the skid pipe Therefore, it is necessary to control the Langeria index LI at the skid pipe entry temperature T1 within the range of (−1 + 0.016ΔT) to 0. On the other hand, when the Langeria index LI at all temperatures of the cooling water flowing in the skid pipe is controlled to be in the range of −1 to 0 by only measuring the skid pipe outlet side temperature T2, it passes through the skid pipe. It is necessary to control the Langeria index LI at the outlet temperature T2 of the skid pipe in the range of −1 to −0.016ΔT in consideration of the temperature rise ΔT at that time.

Next, the water quality management method for skid pipe cooling water according to the present invention, that is, the Langeria index LI control method for skid pipe cooling water will be described.
As described above, in order to obtain the Langeria index LI, it is not always necessary to measure all of the A to D values used in the equation (2), and the A value determined by the concentration of the evaporation residue is the evaporation residue. Since the influence on the A value is small, the A value can be fixed by measuring the evaporation residue in advance. In addition, the D value determined by the total alkalinity (M alkalinity) can be obtained from the C-D value relational expression measured in advance by measuring the C value. Therefore, in the present invention, the data necessary for calculating the Langeria index LI of the skid pipe cooling water is the pH value and calcium hardness of the direct system waste water after slaked lime injection and the temperature of the cooling water in the skid pipe. Here, the pH, water temperature, and calcium hardness measuring apparatus need not be special ones, and may be commercially available. For example, a calcium ion electrode meter manufactured by HORIBA Co., Ltd. : D53, electrode: 6583-10C) and the like, and can be calculated from a calibration curve prepared in advance.

FIG. 4 shows an example in which the water quality management method of the present invention is applied to the flow chart of circulating water shown in FIG. As described above, the calcium hardness is adjusted on the upstream side of the wastewater treatment facility 4 for direct wastewater, specifically, in the mixing basin 4a and / or the coagulation basin 4b upstream of the settling basin 4c. ) ₂ is directly injected into the system waste water, and the injection amount is calculated as follows.

First, the pH and calcium hardness of the direct waste water after slaked lime injection is measured and the cooling water temperature T1 on the skid pipe inlet side and the outlet are measured at the cohesive basin 4b of the direct waste water treatment facility 4 shown in FIG. The cooling water temperature T2 on the side is measured, and the estimated value of the Langeria index LI at the cooling water temperature T1 on the inlet side of the skid pipe and the cooling water temperature T2 on the outlet side is calculated from these measured values. To do.
Next, the estimated Langeria index LI at the calculated temperature T1 and temperature T2 is compared with the control target Langeria index LI (−1 to 0), and both of the estimated Langeria index LI at the temperature T1 and the temperature T2 are the control target. The calcium hardness required to be within the range is calculated, and the injection amount of slaked lime is calculated from the difference between this required calcium hardness and the actual measured value of the calcium hardness measured after the aggregation pond 4b.

  Here, FIG. 4 illustrates an example of controlling the Langeria index LI based on the measurement results of the cooling water temperature T1 on the skid pipe inlet side and the cooling water temperature T2 on the outlet side, but there is a large variation in ΔT. If not, the Langeria index LI may be controlled based on the measurement result of the cooling water temperature T1 on the skid pipe inlet side or the cooling water temperature T2 on the outlet side, for example, based on the skid pipe inlet temperature T1. When controlling the Langeria index LI, the Langeria index LI at the skid pipe inlet side temperature T1 is controlled within the range of (−1 + 0.016ΔT) to 0, and conversely, the Langeria index LI is controlled based on the skid pipe outlet side temperature T2. In this case, the Langeria index LI at the skid pipe outlet temperature T2 may be controlled in the range of −1 to −0.016ΔT.

The direct wastewater discharged from the hot rolling mill shown in Fig. 1 and treated with wastewater is mixed with the indirect wastewater discharged from the hot rolling mill to produce mixed wastewater, which is used as a skid for the heating furnace. A process experiment in which the water quality management method of the present invention shown in FIG. 4 was applied to a wastewater treatment system of a hot rolling factory that circulates and uses as cooling water for pipes was performed for 6 months.
In the above-described process experiment, the Langelier index LI of the cooling water in the skid pipe is determined based on the cooling water temperature T1 and the outlet cooling water temperature T2 on the skid pipe entry side, and the pH of the direct drainage in the coagulation pond after the slaked lime is charged. And the calcium hardness is calculated, and from these measured values and the formula (2), the Langeria index LI at the temperatures T1 and T2 is calculated, and at any temperature of the T1 and T2, the Langelia index is −1 to 0. The amount of slaked lime in which the difference between the calculated value of the calcium hardness and the measured value of the calcium hardness is zero was added on the inlet side of the mixing pond.

Next, after completion of the above-described process experiment, the skid pipe was removed, and the corrosion rate on the inner surface of the skid pipe, the presence or absence of clogging, and the presence or absence of precipitation of CaCO ₃ were investigated, and the results were compared with those before and after applying the present invention. It was shown to. In addition, the corrosion rate shown in Table 3 is obtained by converting the change in the thickness of the skid pipe for 6 months into the corrosion rate per year.
From Table 3, it was confirmed that the corrosion rate can be significantly reduced by applying the present invention without causing the generation of rust on the inner surface of the skid pipe and the precipitation of CaCO ₃ .

  The technology of the present invention is a method for cooling a skid pipe of a heating furnace using mixed wastewater mixed with direct wastewater discharged from a hot rolling factory and subjected to wastewater treatment and indirect wastewater discharged from the hot rolling factory. It can be applied as a water quality management method not only when used by circulating in water but also when directly treating wastewater from direct and indirect wastewater and using it as cooling water for skid pipes.

1: Water supply 1 ′: Mixed wastewater 1a: Direct system water supply 1a ′: Direct system drainage 1b: Indirect system water supply 1b ′: Indirect system drainage 2: Elevated water tank 3: Rolling equipment 4: Direct system wastewater treatment equipment 4a: Mixing basin 4b: Coagulation basin 4c: Sedimentation basin 4d: Junction well 5: Heating furnace 6: Skid pipe 7: Cooling tower 8: Reservoir 9: Pumping pump

Claims (4)

Circulating mixed wastewater mixed with indirect wastewater discharged from the hot rolling mill and direct wastewater discharged from the hot rolling mill and used as cooling water for the heating furnace skid pipe A water quality management method when
The slaked lime Ca (OH) ₂ is injected into the direct wastewater upstream of the direct wastewater treatment process, and the Langeria index LI at the skid pipe inlet side temperature T1 and the skid pipe outlet side temperature T2 is − A water quality management method for skid pipe cooling water, wherein the water quality is controlled within a range of 1 to 0. The water quality management method for skid pipe cooling water according to claim 1, wherein the injection of the slaked lime is performed upstream of the settling basin in the wastewater treatment step of direct system wastewater. When the temperature difference ΔT between the cooling water skid pipe inlet side temperature T1 and the skid pipe outlet side temperature T2 is set, the Langeria index LI of the cooling water inlet side cooling water is controlled in the range of (−1 + 0.016ΔT) to 0. The water quality management method for skid pipe cooling water according to claim 1 or 2. When the temperature difference ΔT between the cooling water skid pipe inlet side temperature T1 and the skid pipe outlet side temperature T2 is set, the Languelian index LI of the cooling water on the skid pipe outlet side is controlled in the range of −1 to −0.016 ΔT. The water quality management method for skid pipe cooling water according to claim 1 or 2.

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