Process for the production of sulphates from spent pickling liquors

Abstract

This invention discloses a method for producing sulfate from waste acid wastewater, relating to the field of industrial wastewater reuse technology. The method includes the following steps: raw material pretreatment: collecting waste acid wastewater generated from the washing process of non-ferrous metal smelting flue gas, and concentrating it to obtain concentrated acid water; slurrying and leaching: mixing materials containing metal oxides with the concentrated acid water in a certain proportion, and carrying out slurrying and leaching reactions; purification: purifying the leached solution to obtain a purified solution; sulfate preparation: treating the purified solution to obtain a solid sulfate product. This method directly uses waste acid to replace expensive purchased sulfuric acid, significantly reducing the production cost of sulfate. It is particularly suitable when sulfuric acid market prices are high. Conventionally, producing one ton of zinc sulfate consumes 0.43 tons of sulfuric acid, and producing one ton of manganese sulfate consumes 1.23 tons of sulfuric acid. Using this method, sulfuric acid is directly replaced, resulting in significant cost savings.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater reuse technology, and in particular to a method for producing sulfates from waste acid water. Background Technology

[0002] The smelting of non-ferrous metals such as zinc and copper generates large quantities of complex and highly acidic wastewater. Traditional treatment methods involve neutralization and precipitation followed by delivery to wastewater treatment plants, which is costly and does not effectively recover the acid and valuable metals. Furthermore, the production of zinc sulfate and manganese sulfate typically requires large quantities of purchased sulfuric acid as a leaching agent, making production costs significantly affected by fluctuations in sulfuric acid market prices.

[0003] While existing technologies include research on the treatment of smelting wastewater, most focus on its harmless treatment. There are no systematic reports of mature processes that directly use it as a raw material to replace sulfuric acid and achieve stable production of high-quality sulfates. Therefore, developing a method that can simultaneously solve the problem of acid wastewater treatment and reduce sulfate production costs has significant industrial application value. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for producing sulfates from waste acid water, relating to the field of industrial wastewater reuse technology.

[0005] To achieve this technical objective, the present invention adopts the following solution: The method for producing sulfates from waste acid water includes the following steps: Raw material pretreatment: Collect the waste acid water generated from the washing process of non-ferrous metal smelting flue gas, and obtain concentrated acid water through concentration treatment; Pulping and leaching: Materials containing metal oxides or carbonates are mixed with concentrated acid water in a certain proportion to carry out pulping and leaching reactions; Purification: The leaching solution is purified to obtain a purified solution; Preparation of sulfate: The purified solution is treated to obtain solid sulfate product.

[0006] Furthermore, the acidity of the waste acid water is 20-50 g / l; the acidity of the concentrated acid water is 100-200 g / l, and the impurity components include F 1.0~4.0 g / l, As 2.0~5.0 g / l, Cl 0.5~2 g / l, and Cd 0.05~0.2 g / l.

[0007] Furthermore, the non-ferrous metal smelting flue gas enters the scrubbing pipe and is washed, circulated and absorbed, and filtered to obtain concentrated acid water.

[0008] Furthermore, the metal element in materials containing metal oxides or carbonates refers to zinc or manganese.

[0009] Furthermore, the material containing metal oxides is stirred and pulped with concentrated acid water in a pulping tank to obtain pulped slag, which is then introduced into a leaching tank.

[0010] Furthermore, when the material containing metal oxides or carbonates is a material containing ZnO or ZnCO3, the pH value of the solution in the pulping tank is controlled at 5.2~5.4, the temperature of the leaching tank is controlled at 70~90℃, and concentrated acid water is added while stirring. When the pH value of the mixed solution in the leaching tank reaches 2.5, the addition of concentrated acid water is stopped, stirring continues and the pH value is monitored.

[0011] As the leaching reaction proceeds, when the pH value reaches 5.0, concentrated acid solution is added again until the pH value reaches 2.5-3.0. At this point, acid addition is stopped, and stirring continues while the pH is monitored.

[0012] Repeat this process of adjusting the acidity of the leaching tank solution until the pH value of the solution remains constant at 3.0–3.5 for more than half an hour, at which point the acid addition is complete.

[0013] Furthermore, when the material containing metal oxides or carbonates is a material containing MnO, Mn2O3 or MnCO3, the pH value of the solution in the pulping tank is controlled at 1.0-2.0, the temperature of the leaching tank is controlled at 65-85℃, and concentrated acid water is added while stirring. When the pH value of the mixed solution in the leaching tank reaches 0.5-1.5, the addition of concentrated acid water is stopped, stirring continues and the pH value is monitored.

[0014] As the leaching reaction proceeds, when the pH value is greater than 2.0, add concentrated acid water again until the pH value reaches 0.5~1.5. Then stop adding acid, continue stirring and monitor the pH.

[0015] Repeat this process of adjusting the acidity of the leaching tank solution until the pH value of the solution remains constant at 0.5~1.5 for more than half an hour, at which point the acid addition is complete.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Significantly reduced costs: By directly using waste acid to replace expensive purchased sulfuric acid, the production cost of sulfates is greatly reduced. This method is especially suitable when the sulfuric acid market price is high. Normally, producing one ton of zinc sulfate consumes 0.43 tons of sulfuric acid, and producing one ton of manganese sulfate consumes 1.23 tons of sulfuric acid. By using the method of this invention, sulfuric acid can be directly replaced, resulting in significant cost savings.

[0017] 2. Outstanding environmental benefits: It transforms difficult-to-treat hazardous waste (acidic wastewater) into production raw materials, realizing the resource utilization of wastewater, reducing the amount of wastewater to be treated and the consumption of neutralizing agents from the source, and reducing environmental risks and end-of-pipe treatment costs.

[0018] 3. Comprehensive resource recovery: During the concentration of waste acid water, dust is washed out from the gas, and then filtered to effectively recover valuable metal elements such as lead and silver carried in the waste acid water, thereby improving the metal recovery rate of the entire smelting system.

[0019] 4. Strong process feasibility: Based on the existing production of waste acid water (tens of thousands of tons / month) and historical production data (tens of thousands of tons have been successfully tested), the method of this invention is easy to integrate and apply on existing zinc sulfate and manganese sulfate production lines without the need for large-scale equipment modification.

[0020] 5. Stable product quality: Through source control and process adjustment, the key indicators of sulfate products produced using waste acid water all meet or exceed national standards, proving the reliability of the method. Detailed Implementation

[0021] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0022] This invention provides a method for producing sulfates from waste acid water, comprising the following steps: S1. Raw Material Pretreatment: Collect the waste acid water generated from the dynamic wave scrubbing process of non-ferrous metal smelting flue gas (mainly SO2), with an initial acidity of 20-50 g / l. Perform concentration pretreatment on the waste acid water to obtain concentrated acid water, which has an acidity increased to 100-200 g / l. The main impurities include F 1.0-4.0 g / l, As 2.0-5.0 g / l, Cl 0.5-2 g / l, and Cd 0.05-0.2 g / l.

[0023] The pretreatment process is as follows: Sulfur dioxide flue gas enters the scrubbing pipe at high speed (e.g., 7-8 Wm³ / h flow rate) from top to bottom. The scrubbing liquid (water) is injected counter-currently into the gas flow through nozzles from bottom to top. The gas and liquid phases collide at high speed in opposite directions. When the momentum of the gas and liquid phases reaches equilibrium, a high-speed turbulent foam zone is formed. The gas and liquid phases are in high-speed turbulent contact with a large contact surface area, and these contact surfaces are constantly and rapidly renewed, achieving a highly efficient scrubbing effect. The scrubbing liquid (with a certain acidity) that has absorbed the flue gas continues to be recycled as scrubbing liquid, repeatedly absorbing flue gas to achieve a concentration effect.

[0024] S2. Pulping and Leaching: Materials containing metal oxides or carbonates are mixed with concentrated acid water at a certain solid-liquid ratio to carry out pulping and leaching reactions.

[0025] S3. Purification: Perform routine purification treatment on the filtrate (such as displacement, impurity removal, etc.) to obtain purified liquid.

[0026] S4. Preparation of sulfate: The purified sulfate solution is concentrated, crystallized, and dried to obtain solid sulfate product.

[0027] Example 1: Method for producing zinc sulfate from waste acid water S1. Raw Material Pretreatment: Collect the waste acid water generated from the dynamic wave scrubbing process of non-ferrous metal smelting flue gas (mainly SO2), with an initial acidity of 20-50 g / l. Perform concentration pretreatment on the waste acid water to obtain concentrated acid water, which has an acidity increased to 100-200 g / l. The main impurities include F 1.0-4.0 g / l, As 2.0-5.0 g / l, Cl 0.5-2 g / l, and Cd 0.05-0.2 g / l.

[0028] S2-1, Add 15-17m to the pulping tank 3 Concentrate the acid solution, then add 8-10 tons of material containing zinc oxide or zinc carbonate (zinc content >35wt% or total zinc and lead content >50%) while stirring and adding water until the solution pH is adjusted to 5.2-5.4, resulting in a pulped residue, which is then transferred to a leaching tank. Repeat the above operation, adding acid and material again to the pulping tank for a second pulping process.

[0029] S2-2. Open the steam valve to heat the leaching tank, start the agitator and blower, and slowly add concentrated acid water while stirring. At the same time, check the pH value of the mixture in the leaching tank. When the pH value reaches 2.5, stop adding concentrated acid water and continue heating and stirring.

[0030] S2-3. Continue to monitor the pH value, checking it every 10 minutes.

[0031] S2-4. When the pH value reaches 5.0, add concentrated acid solution again until the pH value reaches 2.5-3.0. Then stop adding acid and continue heating and stirring. Continue to monitor the pH value every 10 minutes.

[0032] S2-5. Repeat this process to adjust the acidity of the leaching tank mixture until the pH value of the mixture remains constant at 3.0-3.5 for more than half an hour. Then confirm that the acid addition is complete.

[0033] S2-6. Open the inlet valve of the leaching tank and the outlet valve of the pulping tank. Start the pulping liquid transfer pump to pour the remaining pulping liquid in the pulping tank into the leaching tank. While pouring, check the pH value of the mixed liquid in the leaching tank. When the pH value reaches 5.2, stop the liquid transfer.

[0034] S2-7. Continue stirring for 10 minutes. If the pH value of the leaching tank mixture does not change, the leaching operation is complete. The leaching temperature is 70~90℃. If the pH value is less than 5.2, continue pouring the liquid until the pH value of the leaching tank mixture reaches 5.2.

[0035] S2-8. Start the filter press and send the leachate into the filter press for liquid-solid separation. Place the filter residue in the pit, add concentrated acid water at a ratio of 2 to 2.5:1, slurry it, and then transfer it into the acid leaching tank.

[0036] Add sulfuric acid to the acid leaching tank, with an initial acid concentration of 200–220 g / L. Heat and stir, maintaining a temperature above 85°C. Leach for 3–5 hours. During the process, check the acidity of the mixture in the acid leaching tank 2–3 times. If the acidity is lower than the target (80–120 g / L), continue adding sulfuric acid to ensure that the final acid content is 80–120 g / L.

[0037] S2-9. The acidic leaching residue produced by the acidic leaching tank is introduced into the water washing tank, and water from the storage tank is added at a ratio of (5-6):1. The tank is heated to 60-70℃ and the water washing time is 1-2 hours.

[0038] S2-10. After washing, the liquid is introduced into a filter press for liquid-solid separation. The filtrate is returned to the system (slurrying) and the filter residue is reused.

[0039] S3. Purification: Perform routine purification treatment on the filtrate (such as displacement, impurity removal, etc.) to obtain purified liquid.

[0040] By using forced-air oxidation and copper sulfate catalysis, ferrous ions in the leaching solution are oxidized to ferric ions, neutralized with zinc oxide, and hydrolyzed into ferric hydroxide under certain pH conditions. The ferric ions are then adsorbed and co-precipitated with arsenic and other compounds to form arsenic-iron slag, which is removed, providing a relatively pure cadmium-containing solution for the replacement process.

[0041] S3-1, the neutral filtrate (i.e., the filtrate separated by the S2-8 filter press) enters the purification tank, and the content of ferrous iron in the solution is tested. When the liquid level exceeds the bottom of the steam pipe, heating and stirring are started. At the same time, the blower is turned on to start the oxidation process. If the iron content of the neutral filtrate exceeds 3g / L, copper sulfate is added appropriately to accelerate the iron removal process.

[0042] S3-2. Check the ferrous iron content in the solution every 20-30 minutes and test the pH value with test paper. When the pH value is 3.0-4.0, add zinc oxide to adjust the pH value to 4.5-5.0.

[0043] During the iron removal process, air was continuously blown in, and the iron content was intermittently monitored and the acidity of the solution was adjusted until the iron was removed to below 0.1 g / L, with the endpoint pH=5.2.

[0044] S4. Preparation of sulfate: The purified sulfate solution is concentrated, crystallized, and dried to obtain solid sulfate product.

[0045] S4-1, Displacement: Zinc powder is used to displace cadmium in the solution in the form of a sponge. After being compressed into briquettes, the solution is transferred to the cadmium smelting process. The solution after displacement becomes the raw material for the production of zinc sulfate.

[0046] S4-1-1, First Replacement: Pour the purified filtrate into the replacement tank, start stirring, add an appropriate amount of sulfuric acid according to the cadmium content, and control the pH value to around 2.5. Slowly add zinc powder, while monitoring the cadmium content and pH changes. Add acid as needed if the acidity is insufficient. Continue until the cadmium removal is 0.5-1 g / L (if cadmium flocs are not being produced, remove cadmium to below 0.5 g / L).

[0047] S4-1-2, Secondary Replacement: Stop the primary replacement, clarify for 15 minutes, and pump the replaced liquid into the secondary replacement tank. Start the secondary replacement, add zinc powder, and stop the process when the pH reaches 5.0. Pump the slightly precipitated supernatant into a filter press for forced liquid-solid separation. Send the filter residue to the oxidation plant for return leaching. The filtrate is sent to the zinc sulfate production line.

[0048] S4-1-3, Secondary Purification: Pour the prescribed amount of liquid into the tank, add hydrogen peroxide, and measure the pH value of the solution. If the pH < 5.0, add a neutralizing agent to adjust it to 5.0–5.2. After iron removal is successful, pump the slag-liquid mixture into a filter press for liquid-solid separation. The filtrate is then transferred to the cadmium removal process, and the filter residue is returned to the power plant for recycling. After iron and cadmium removal, samples are taken for iron and cadmium analysis. Once the samples pass the analysis, stirring is stopped. After cadmium removal is successful, pump the slag-liquid mixture into a filter press for liquid-solid separation. The filtrate is then transferred to the evaporation process, and the filter residue is sent to a designated site for oxidation and recycling.

[0049] S4-2. Evaporation: Open the vacuum valve and the liquid storage valve. After storing the liquid according to the process requirements, open the steam valve to start evaporation. During the evaporation process, adhere to the technical requirements (pressure, vacuum level, cooling water pressure, etc.), ensuring the boiling liquid level is below the third sight glass. Take samples periodically to measure specific gravity during evaporation. At the end of evaporation, first close the liquid storage valve. At the endpoint, first close the steam valve, then the vacuum valve, and open the sampling valve to drain the liquid. After draining completely, close the drain valve, open the vacuum valve, open the water storage valve, and turn on the water pump. After storing the required amount of white water, evaporate for half an hour, then close the steam valve again, followed by the vacuum valve.

[0050] S4-3. Crystallization: The supersaturated zinc sulfate solution after evaporation is pumped into the crystallizer. The machine is started and stirred slowly. The solution is cooled by circulating cooling water and natural ventilation, causing the zinc sulfate to crystallize. Some impurities remain in the mother liquor.

[0051] S4-4 Dehydration: The crystallized zinc sulfate heptahydrate contains a certain amount of free water. It is dehydrated by centrifugation to reduce the moisture content and ensure that Zn > 22% so that the finished zinc sulfate can be obtained after the next step of airflow drying.

[0052] S4-5. Drying: The dehydrated zinc sulfate still contains about 2% moisture. It is dried by hot or cold air to further reduce the moisture content and meet the quality standards of the finished zinc sulfate. Then it is weighed and packaged.

[0053] The composition of the obtained zinc sulfate product is shown in the table below: Table 1 Chemical composition of zinc sulfate products Main content is expressed as Zn%. <![CDATA[Calculated on the basis of ZnSO4·7H2O]]> Insoluble matter % PH chloride% lead% iron% manganese% cadmium% chromium 22.06～23.56 ≥97 ≤0.02 — — ≤0.001 ≤0.003 — ≤0.001 — The waste acid water is industrial waste generated by the applicant's factory. Previously, it was sent to a wastewater treatment plant for processing, but now it is being reused using this method. In the past three years, approximately 49,000 tons of waste acid water have been used in the production of zinc sulfate, and the applicant has accumulated certain production experience and technical foundation (see Table 2).

[0054] years 23 years 24 years January to November 25th Usage amount t 20307 12797 15774 Example 2: Method for producing manganese sulfate from waste acid water S1. Raw Material Pretreatment: Collect the waste acid water generated from the dynamic wave scrubbing process of non-ferrous metal smelting flue gas (mainly SO2), with an initial acidity of 20-50 g / l. Perform concentration pretreatment on the waste acid water to obtain concentrated acid water, which has an acidity increased to 100-200 g / l. The main impurities include F 1.0-4.0 g / l, As 2.0-5.0 g / l, Cl 0.5-2 g / l, and Cd 0.05-0.2 g / l.

[0055] S2-1. Add concentrated acid water and material containing manganese oxide or manganese carbonate (manganese content in the material > 20%) to the pulping tank. The liquid-solid ratio of concentrated acid water to manganese oxide material is 4-6:1. Add water while stirring until the solution acidity pH is adjusted to 1.0-2.0 to obtain pulped residue, which is then introduced into the leaching tank.

[0056] S2-2. Open the steam valve to heat the leaching tank, start the agitator and blower, and slowly add concentrated acid water while stirring. At the same time, check the pH value of the mixed solution in the leaching tank. When the pH value reaches 0.5~1.5, stop adding concentrated acid water, continue heating and stirring, and maintain the leaching temperature at 65℃~85℃.

[0057] S2-3. Continue to monitor the pH value, checking it every 10 minutes.

[0058] S2-4. When the pH value is greater than 2.0, add concentrated acid solution again until the pH value reaches 0.5~1.5. Then stop adding acid and continue heating and stirring. Continue to monitor the pH value every 10 minutes.

[0059] S2-5. Repeat this process to adjust the acidity of the leaching tank solution until the pH value of the solution remains constant at 0.5~1.5 for more than half an hour. Then confirm that the acid addition is complete.

[0060] S2-6. Open the inlet valve of the leaching tank and the outlet valve of the pulping tank. Start the pulping liquid transfer pump and pour the remaining pulping liquid in the pulping tank into the leaching tank. While pouring, check the pH value of the mixed liquid in the leaching tank. When the pH value reaches 0.5~1.5, stop the liquid transfer.

[0061] S2-7. Continue stirring for 10 minutes. If the pH value of the leaching tank mixture does not change, the leaching operation is complete. The leaching temperature is 65~85℃. If the pH value is less than 1.5, continue pouring the liquid until the pH value of the leaching tank mixture reaches 1.5.

[0062] S2-8. Start the filter press and send the leachate into the filter press for liquid-solid separation. Wash the filter residue with water.

[0063] S2-10 After washing, the liquid is introduced into the filter press again for liquid-solid separation. The filtrate is returned to the system, and the filter residue is reused.

[0064] S3, Purification: S3-1. Pour the leachate into the iron removal tank. After the solution in the iron removal tank exceeds the steam pipe, start heating to 60℃~80℃. Adjust the steam valve to ensure the temperature of the iron removal tank.

[0065] S3-2. Add manganese oxide-containing material (pyrolusite) according to the iron content. The manganese oxide-containing material (pyrolusite) should be added at a uniform rate.

[0066] S3-3. Check the ferrous iron content every 30 to 60 minutes. Once the ferrous iron content reaches the specified value, slowly add limestone powder and adjust the pH value to approximately 4.0 to 4.5.

[0067] S3-4. Sampling and analysis of total iron in the liquid after pH adjustment for iron removal. When the specified value is reached, iron removal is completed. The iron removal slurry is pumped into a filter press for filtration to obtain iron removal liquid and iron removal slag. The iron removal slag is sold as iron slag.

[0068] S3-5. Pour the iron-removed liquid into the copper removal tank. After the solution in the copper removal tank exceeds the steam pipe, start heating to 60-70℃ and adjust the steam valve to ensure the temperature of the copper removal tank.

[0069] S3-6. Add manganese powder according to the copper content of the liquid after iron removal. The manganese powder should be added slowly and evenly. After the reaction is completed for 1 hour, take a sample to test the copper content of the copper removal liquid. After the copper content reaches the specified value, filter the liquid to obtain the copper removal liquid and copper removal slag. Adjust the pH value of the copper removal liquid and sell the copper removal slag as copper material.

[0070] S4. Preparation of Sulfate: The purified manganese sulfate solution is concentrated, crystallized, and dried to obtain solid manganese sulfate product. This preparation process is the existing technology for preparing solid manganese sulfate from manganese sulfate solution.

[0071] The final product, manganese sulfate, has the following composition: Table 3 Chemical composition of manganese sulfate products

[0072] Finally, it should be noted that the above-listed embodiments are merely preferred embodiments of the present invention. Of course, those skilled in the art can make modifications and variations to the present invention. If such modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should be considered as being within the protection scope of the present invention.

Claims

1. A method for producing sulfates from waste acid water, characterized in that, Includes the following steps: Raw material pretreatment: Collect the waste acid water generated from the washing process of non-ferrous metal smelting flue gas, and obtain concentrated acid water through concentration treatment; Pulping and leaching: Materials containing metal oxides or carbonates are mixed with concentrated acid water in a certain proportion to carry out pulping and leaching reactions; Purification: The leaching solution is purified to obtain a purified solution; Preparation of sulfate: The purified solution is treated to obtain solid sulfate product.

2. The method for producing sulfate from waste acid water according to claim 1, characterized in that, The acidity of the waste acid water is 20-50 g / l; the acidity of the concentrated acid water is 100-200 g / l, and the impurity components include F 1.0~4.0 g / l, As 2.0~5.0 g / l, Cl 0.5~2 g / l, and Cd 0.05~0.2 g / l.

3. The method for producing sulfate from waste acid water according to claim 1, characterized in that, Non-ferrous metal smelting fumes enter the scrubbing pipe, where they are scrubbed, circulated, and absorbed to obtain concentrated acid water.

4. The method for producing sulfate from waste acid water according to claim 1, characterized in that, In materials containing metal oxides or carbonates, the metal element refers to zinc or manganese.

5. The method for producing sulfate from waste acid water according to claim 4, characterized in that, Materials containing metal oxides or carbonates are stirred and pulped with concentrated acid water in a pulping tank to obtain pulped slurry, which is then introduced into a leaching tank.

6. The method for producing sulfate from waste acid water according to claim 5, characterized in that, When the material containing metal oxides is a material containing ZnO or ZnCO3, The pH value of the solution in the pulping tank is controlled at 5.2~5.4, and the temperature of the leaching tank is controlled at 70~90℃. Concentrated acid water is added while stirring. When the pH value of the mixed solution in the leaching tank reaches 2.5, the addition of concentrated acid water is stopped, stirring is continued and the pH value is monitored. As the leaching reaction proceeds, when the pH value reaches 5.0, concentrated acid solution is added again until the pH value reaches 2.5-3.

0. At this point, acid addition is stopped, and stirring continues while monitoring the pH. Repeat this process of adjusting the acidity of the leaching tank solution until the pH value of the solution remains constant at 3.0–3.5 for more than half an hour, at which point the acid addition is complete.

7. The method for producing sulfate from waste acid water according to claim 5, characterized in that, When the material containing metal oxides or carbonates is a material containing MnO, Mn2O3, or MnCO3, The pH value of the solution in the pulping tank is controlled at 1.0-2.0, and the temperature of the leaching tank is controlled at 65-85℃. Concentrated acid water is added while stirring. When the pH value of the mixed solution in the leaching tank reaches 0.5-1.5, the addition of concentrated acid water is stopped, stirring is continued and the pH value is monitored. As the leaching reaction proceeds, when the pH value is greater than 2.0, add concentrated acid water again until the pH value reaches 0.5~1.5, then stop adding acid, continue stirring and monitor the pH; Repeat this process of adjusting the acidity of the leaching tank solution until the pH value of the solution remains constant at 0.5~1.5 for more than half an hour, at which point the acid addition is complete.

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