Zedong Chemical's High-Hardness and High-Alkalinity Circulating Water Treatment

Zedong Chemical's High-Hardness and High-Alkalinity Circulating Water Treatment

Zedong Chemical is located in the western Hubei region. Its circulating water system uses groundwater as make-up water, and the water quality is poor, which is specifically reflected in the high total alkalinity (799mg/L) and total hardness (473mg/L). This makes it extremely prone to scaling in the circulating water system, and long-term operation will cause harm to the equipment. Combining the characteristics of such water quality, our company adopts the following three treatment measures as the focus of this plan:

Acid-adding treatment

By adding sulfuric acid to the circulating water to adjust the alkalinity and pH value, it can prevent scaling within the allowable concentration multiple range, so as to achieve the expected water treatment effect.

1.1 Calculation of acid dosage

According to the principle of acid-base neutralization, the theoretical acid dosage is equal to the reduction of alkalinity. If the alkalinity before adding acid is M_before and the controlled value of alkalinity is M_limit, then the difference in alkalinity of the circulating water before and after adding acid is:

△M = M_before - M_limit

If 98% sulfuric acid is used to adjust the pH value, the unit water consumption of the circulating water system is:

X = 49·△M/(50×0.98×1000) = △M/1000

Where: X - acid dosage of circulating water, g/m³

49 - mass of 1 [H+] mol H2SO4, g/[H+] mol H2SO4 (i.e., gram equivalent)

1.2 Automatic control of acid dosage

In general, the pH value of circulating water is highly correlated with alkalinity. Therefore, this plan is designed to automatically control the dosage of concentrated sulfuric acid through an online pH meter, which avoids the lag and error of manual dosage and reduces the personal injury that may be caused by improper manual operation.

Formula screening for high-alkalinity and high-hardness water quality

Water treatment agents that can meet the stable operation of circulating water under high-alkalinity and high-hardness conditions were selected through anti-scaling experiments. The experimental results are as follows:

From the experimental data in the above table, the following conclusions can be drawn:

HL-401A scale and corrosion inhibitor does not decompose at high temperatures and can maintain normal operation when the system temperature is as high as 90℃;

HL-401A scale and corrosion inhibitor still has good corrosion and scale inhibition performance when the conductivity is as high as 6000 us/cm;

HL-401A scale and corrosion inhibitor still has a good scale inhibition and dispersion effect when the calcium hardness in the system reaches 1500 mg/L;

HL-401A scale and corrosion inhibitor still has a good scale inhibition and dispersion effect when the alkalinity in the system reaches 2250 mg/L;

HL-401A scale and corrosion inhibitor has a good dispersion ability for slime.

Control of concentration multiple

During the operation of circulating water, it will be continuously concentrated, and the concentration of various harmful ions in the water will also continue to increase. When the concentration of these harmful ions exceeds the range that the water treatment agent can withstand, the system will collapse. Therefore, controlling the concentration multiple is also the focus of circulating water treatment.

3.1 Control range of concentration multiple

The concentration multiple has always been a problem that manufacturers pay great attention to. Because a low concentration multiple cannot achieve the effect of water saving, while a high concentration multiple cannot reflect the water treatment effect. Therefore, it is very important to control the concentration multiple within a reasonable range according to the agent formula. Any water treatment agent has a tolerance range for specific water quality. When the water quality exceeds this range, the water treatment agent will fail.

Through experiments, we can determine the most suitable range of concentration multiple:

Judging from different water quality stability indexes, when K > 3, the circulating water of the system has a tendency to scale, and when K > 4, it scales seriously. Therefore, it is recommended that the concentration multiple of the circulating water system be controlled between 3 and 3.5.

3.2 Blow-down control

In general, the circulating water system does not need to discharge sewage. When the concentration multiple reaches the limit value, it is necessary to reduce it through blow down. The amount of blow-down water can be calculated according to the following formula:

Qb = (Qe - Qw(N - 1)) / (N - 1)

Where: Qb - percentage of the minimum blow-down water amount in the circulating water amount, %

Qe - percentage of evaporation loss in the circulating water amount, %

Qw - percentage of wind blowing loss in the circulating water amount, %

N - concentration multiple.