Neodymium iron boron waste magnetic steel environment-friendly recycling method

Abstract

This application relates to the field of NdFeB waste recycling, specifically to an environmentally friendly recycling method for NdFeB waste magnets. The method includes the following steps: crushing and separation, combined reaction, roasting, purification, and precipitation. The combined reaction involves adding sulfuric acid solution to the magnetic material for an initial reaction, followed by adding ammonium persulfate for a further reaction, resulting in a mixture. Purification involves adjusting the pH of the aqueous extract to 3-3.7, waiting for complete precipitation, and then filtering to obtain a filtrate. Precipitation involves adding the filtrate to a sodium carbonate solution for reaction and filtration to obtain rare earth precipitates. This application utilizes the combination of sulfuric acid and ammonium persulfate to convert rare earth elements and iron elements in the magnetic material into rare earth sulfate and iron sulfate as much as possible. Roasting further enhances the conversion effect, and then, under pH 3-3.7 conditions, the Fe in the aqueous extract... 3+ It is converted into ferric hydroxide precipitate as much as possible, and finally rare earth ions are precipitated using sodium carbonate solution for recovery.

Description

Technical Field

[0001] This application relates to the field of neodymium iron boron waste recycling, specifically to an environmentally friendly recycling method for neodymium iron boron waste magnets. Background Technology

[0002] Neodymium iron boron (NdFeB) magnets, due to their high coercivity and high energy product, are currently the most cost-effective commercial magnetic material and are widely used in many fields related to people's livelihoods, such as home appliances, transportation, and medical care. With the rapid development of the sintered NdFeB permanent magnet material industry in recent years, rare earth resources have been consumed in large quantities. On the other hand, during the production and processing of sintered NdFeB magnets, from the initial raw materials to the final product, scrap materials are inevitably generated at every stage. Coupled with the large amount of scrapped NdFeB motors, electronic products, and other finished products, the amount of usable NdFeB scrap magnets each year is enormous.

[0003] Existing methods for recycling NdFeB waste magnets mainly include high-temperature smelting and chemical reagent recovery. High-temperature smelting consumes a large amount of energy, while chemical reagent recovery primarily uses hydrochloric acid and sulfuric acid to leach the NdFeB waste, then further recovers rare earth elements from the leachate. These methods are simple to operate, but generate large amounts of acidic wastewater, polluting the environment and incurring high treatment costs. Both methods place a significant burden on the environment and conflict with current environmentally friendly production practices. Summary of the Invention

[0004] To address the environmental impact of existing methods for recycling NdFeB waste magnets, this application provides an environmentally friendly recycling method for NdFeB waste magnets. This method utilizes the combination of sulfuric acid and ammonium persulfate, followed by roasting, to better transfer rare earth substances from the waste magnets into an aqueous leaching solution, facilitating precipitation and recycling.

[0005] Firstly, this application provides an environmentally friendly method for recycling neodymium iron boron waste magnets, employing the following technical solution: An environmentally friendly method for recycling waste neodymium iron boron magnets includes the following steps: Crushing and Separation: After crushing the NdFeB waste magnets, magnetic and non-magnetic materials are separated using magnetic separation technology, while retaining the magnetic materials; Complexation reaction: sulfuric acid solution is added to the magnetic material to carry out an initial reaction, and then ammonium persulfate is added to carry out a further reaction to obtain a mixture; Calcination: The mixture is calcined to obtain calcined sand, which is then soaked in water and filtered to obtain an aqueous extract. Purification: Adjust the pH of the aqueous extract to 3-3.7, wait for complete precipitation, filter, and obtain the filtrate; Precipitation: The filtrate is added to a sodium carbonate solution for reaction and filtration to obtain rare earth precipitate.

[0006] By employing the above technical solution, non-magnetic materials, mainly composed of metals such as copper and aluminum combined with oxygen, can be effectively removed by magnetic separation technology, which can remove copper, aluminum, and other metals, as well as some oxygen. Magnetic materials retain the majority of rare earth elements and iron, and some oxygen. In the initial reaction between the magnetic material and sulfuric acid solution, some iron oxides and rare earth oxides are converted into ferric sulfate and rare earth sulfate. Ammonium persulfate, possessing strong oxidizing properties, can remove Fe... 2+ Oxidized to Fe 3 + After the reaction and decomposition of ammonium persulfate, the resulting ammonium bisulfate has the effect of activating roasting, enhancing the decomposition of magnetic substances, and promoting the conversion of more rare earth oxides into rare earth sulfate.

[0007] During the water leaching of calcined sand, both ferric sulfate and rare earth sulfate enter the leachate. At a pH of 3-3.7, Fe3+ ions convert to ferric hydroxide precipitate, and the filtrate after filtration mainly contains rare earth sulfate. Sodium carbonate can be used to effectively precipitate and recover the rare earth elements. Adding sodium carbonate solution to the filtrate results in a precipitate with a fast settling rate and easy filtration. In contrast, adding sodium carbonate solution to the filtrate, or adding both filtrate and sodium carbonate solution simultaneously to the same container, results in a flocculent precipitate that is difficult to settle, has a high water content, and is more difficult to filter.

[0008] This application utilizes the combination of sulfuric acid and ammonium persulfate to convert rare earth elements and iron elements in magnetic materials into rare earth sulfate and ferric sulfate as much as possible. Calcination further enhances the conversion effect. Then, under pH 3-3.7 conditions, the Fe in the aqueous extract... 3+ It is converted into ferric hydroxide precipitate as much as possible, and finally rare earth ions are precipitated using sodium carbonate solution for recovery.

[0009] Preferably, in the coordination reaction step, the sulfuric acid solution contains 70-85 wt% sulfuric acid.

[0010] By adopting the above technical solution, if the sulfuric acid mass fraction is too low, its concentration will be low, which may lead to a reduced reaction rate and prolonged reaction time with magnetic materials, thereby reducing production efficiency. Simultaneously, low-concentration sulfuric acid may not completely dissolve rare earth elements and iron elements in the magnetic materials, resulting in decreased recovery efficiency. Sulfuric acid is a strong acid; if the mass fraction is too high, its corrosiveness and hazard will significantly increase. A high sulfuric acid mass fraction may also lead to a more vigorous reaction, making it difficult to precisely control reaction conditions and processes. This may result in the generation of large amounts of heat and gas during the reaction, increasing instability and uncertainty in the production process. Therefore, after extensive research and experimental verification, the applicant has ultimately determined that the sulfuric acid mass fraction in the sulfuric acid solution of this application is preferably as described above.

[0011] Preferably, in the coordination reaction step, the mass ratio of the magnetic material to the sulfuric acid solution is 1:3-6.

[0012] By employing the above technical solution, if the mass of the sulfuric acid solution is too low—that is, its concentration or quantity is insufficient to fully wet and react with the magnetic material—the rare earth elements and iron elements in the magnetic material may not be completely dissolved. This will lead to reduced reaction efficiency and affect the recovery rate of rare earth elements. If the mass of the sulfuric acid solution is too high—that is, its concentration or quantity far exceeds the actual need—over-reaction will occur, resulting in waste of sulfuric acid. This not only increases production costs but may also cause unnecessary trouble for subsequent processing steps. High-concentration sulfuric acid is highly corrosive and may corrode reaction equipment and pipelines. This will not only shorten the service life of the equipment but may also cause safety hazards such as leaks and explosions. Therefore, after extensive research and experimental verification, the applicant has finally determined that the mass ratio of the magnetic material to the sulfuric acid solution in this application is as described above.

[0013] Preferably, in the coordination reaction step, the mass ratio of the magnetic material to the ammonium persulfate is 1:0.3-0.6.

[0014] By adopting the above technical solution, if the quality of ammonium persulfate is too low, its oxidizing power may be insufficient to oxidize the Fe in the magnetic material. 2+ Completely oxidized to Fe 3+ Incomplete reactions can occur, affecting the recovery efficiency of rare earth elements and product quality in subsequent steps. Insufficient ammonium persulfate, used as an oxidant, can also reduce the overall oxidizing activity of the reaction system. This can affect the dissolution and transformation of rare earth elements in magnetic materials, thus lowering the recovery rate.

[0015] Excessive ammonium persulfate may trigger side reactions, such as decomposition to produce oxygen. These byproducts may interfere with the main reaction, affecting the recovery rate of rare earth elements and product quality. Ammonium persulfate is a strong oxidizing agent, and excessive use may increase safety risks during production. Accidental leakage or contact with flammable substances may cause serious accidents such as fires or explosions.

[0016] Preferably, in the reaction step, the magnetic material and activated carbon are mixed before sulfuric acid solution is added to carry out the initial reaction.

[0017] Preferably, in the coordination reaction step, the mass ratio of the sulfuric acid solution to the activated carbon is 1:0.1-0.3.

[0018] By employing the above technical solution, the reaction of magnetic materials with sulfuric acid and ammonium persulfate may generate waste gases such as sulfur dioxide, sulfur trioxide, and ammonia. These waste gases are not only harmful to the environment but may also adversely affect production equipment and operators. Adding activated carbon, with its strong adsorption capacity, can effectively capture these waste gases, thereby purifying the reaction environment and reducing pollutant emissions. The waste gases adsorbed by activated carbon are not permanently fixed but are released again during the subsequent roasting process. These waste gases recombine with moisture in the mixture to generate sulfuric acid (mostly) and ammonia (a small portion). During roasting, a small amount of sulfuric acid reacts with the ammonia to neutralize it, and the remaining sulfuric acid reacts with the remaining rare earth oxides in the magnetic materials, converting them into rare earth sulfate.

[0019] If the quality of activated carbon is too low, the adsorption surface area and adsorption sites it provides will be insufficient to effectively adsorb the waste gases generated during the reaction process, such as sulfur dioxide, sulfur trioxide, and ammonia. This will lead to an increase in the residual amount of waste gas in the reaction system, which may pollute the environment and also hinder the recovery and purification of rare earth elements in subsequent steps. If the quality of activated carbon is too high, that is, its usage far exceeds the actual needs, it will result in a waste of resources.

[0020] Preferably, in the roasting step, the roasting temperature range is 120-150℃.

[0021] By employing the above-mentioned technical solutions, the chemical reactions during the roasting process typically require a certain activation energy to initiate and proceed. When the temperature is too low, the molecular motion of the reactants slows down, the collision frequency decreases, and thus the chemical reaction rate drops significantly. This will affect the roasting efficiency and the product formation rate. Increasing the roasting temperature requires more energy input, thereby increasing production costs. Simultaneously, high-temperature roasting may also cause greater wear and corrosion to equipment, shortening its service life. High-temperature roasting may generate more pollutants such as waste gas and slag, placing greater pressure on the environment.

[0022] Preferably, the roasting time in the roasting step is 1-2 hours.

[0023] By employing the aforementioned technical solutions, the chemical reactions during the roasting process typically require a certain activation energy to initiate and proceed. When the roasting time is too short, the molecular motion of the reactants slows down, the collision frequency decreases, and thus the chemical reaction rate drops significantly. This will affect the roasting efficiency and the product formation rate. Increasing the roasting temperature requires more energy input, thereby increasing production costs. Simultaneously, extending the roasting time will increase energy consumption and costs. This includes not only the consumption of electricity and fuel but also increased equipment wear and maintenance costs. Prolonged roasting may generate more pollutants such as waste gas and residue, placing greater pressure on the environment.

[0024] Preferably, in the precipitation step, the sodium carbonate solution contains 2.5-3.5 wt% sodium carbonate.

[0025] By employing the above technical solution, using sodium carbonate as a precipitant, a low mass fraction means a reduced molar amount of sodium carbonate in the same volume of solution. During the precipitation reaction, incomplete precipitation may occur, affecting precipitation efficiency and product purity. Incomplete precipitation directly leads to a decrease in product yield. This not only increases production costs but may also affect subsequent processing steps and the application effect of the product. A high mass fraction of sodium carbonate means an increased molar amount of sodium carbonate in the same volume of solution. However, not all sodium carbonate participates in the precipitation reaction; therefore, an excessively high mass fraction leads to raw material waste and increased costs. Excessive use of sodium carbonate may increase the load on wastewater treatment, placing greater pressure on the environment. Simultaneously, unreacted sodium carbonate in wastewater may also have adverse effects on the ecosystem.

[0026] Preferably, in the precipitation step, the reaction time between the filtrate and the sodium carbonate solution is 2-4 hours.

[0027] By employing the above technical solution, the precipitation reaction is a step-by-step process that requires a certain amount of time to ensure sufficient contact and reaction between the reactants. If the reaction time is too short, the ions in the filtrate may not react fully with the sodium carbonate, resulting in incomplete precipitation. This will directly affect the purity and yield of the product, reducing its application value. Extending the reaction time will increase energy consumption and costs, and also reduce production efficiency.

[0028] In summary, this application has the following beneficial effects: 1. This application utilizes the combination of sulfuric acid and ammonium persulfate to convert rare earth elements and iron elements in magnetic materials into rare earth sulfate and ferric sulfate as much as possible. Calcination further enhances the conversion effect. Then, under pH 3-3.7 conditions, the Fe in the aqueous extract... 3+ It is converted into ferric hydroxide precipitate as much as possible, and finally rare earth ions are precipitated using sodium carbonate solution for recovery; 2. This application mixes activated carbon and magnetic materials and then reacts them with sulfuric acid solution to effectively absorb the waste gas generated during the reaction process. At the same time, the waste gas is released during the roasting process, which helps to convert the remaining rare earth oxides in the magnetic materials into rare earth sulfate. Detailed Implementation

[0029] The raw materials in this application include the following: Neodymium iron boron scrap magnets: recycled products.

[0030] Sulfuric acid: Commercially available product with CAS number 7664-93-9 is used; Ammonium persulfate: Commercially available product with CAS number 7727-54-0 is used; Water: Commercially available product with CAS number 7732-18-5; Sodium carbonate: Use commercially available product with CAS number 497-19-8; The present application will be further described in detail below with reference to embodiments and comparative examples.

[0031] Example 1 Crushing and Separation: After crushing the NdFeB waste magnets, magnetic and non-magnetic materials are separated using magnetic separation technology, while retaining the magnetic materials. The elemental composition of the magnetic materials is measured using inductively coupled plasma atomic emission spectrometry, and the contents are: Nd 11.3%, Pr 8.6%, Ce 7.3%, Fe 53.6%, O 17.8%, with the remainder being impurities.

[0032] Complexation reaction: Add 60g of sulfuric acid solution (85wt% sulfuric acid) to 10g of magnetic material and carry out the initial reaction for 30min, then add 6g of ammonium persulfate and carry out the further reaction for 25min to obtain a mixture; Calcination: The mixture is calcined to obtain calcined sand at a temperature of 150℃ for 2 hours. Then, the calcined sand is soaked in 700 mL of water for 1 hour and filtered to obtain an aqueous extract. Purification: Adjust the pH of the aqueous extract to 3.7 (pH can be between 3 and 3.7, but more ferric hydroxide will precipitate at pH 3.7). Sulfuric acid can be used as the adjusting agent. After the precipitation is complete, filter to obtain the filtrate. Precipitation: The filtrate was added to 3L of sodium carbonate solution (3.5wt% sodium carbonate) for reaction and filtration. The reaction time was 4h to obtain rare earth precipitate.

[0033] Examples 2-5 Examples 2-5 are based on the preparation method of Example 1, but the mass fraction of sulfuric acid is adjusted. The specific adjustments are shown in Table 1.

[0034] Comparative Example 1 Comparative Example 1 did not add ammonium persulfate in the coordination reaction step, and other conditions remained unchanged.

[0035] Comparative Example 2 Comparative Example 2: The mixture was directly soaked in water without roasting, and other conditions remained unchanged.

[0036] Table 1. Mass fraction and performance test results of sulfuric acid in Examples 1-5 and Comparative Examples 1-2.

[0037] Performance testing The samples from Examples 1-5 and Comparative Examples 1-2 were subjected to the following performance tests, and the test results are shown in Table 1.

[0038] The aqueous extract and the residual liquid after filtering rare earth precipitates were collected. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to measure the Nd, Pr, and Ce content in the aqueous extract and residual liquid, respectively, which were then used as the rare earth content. The rare earth leaching rate in the aqueous extract and the rare earth precipitation rate in the residual liquid were calculated using the following formulas: Rare earth leaching rate = [Rare earth content in the aqueous leaching solution / (10g * percentage of rare earth in the magnetic material)] * 100% Rare earth precipitation rate = (1 - rare earth content in residual liquid / rare earth content in water leaching solution) * 100%.

[0039] Referring to Table 1, comparing Examples 1-5 and Comparative Examples 1-2, it can be seen that the leaching rate of Examples 1-5 is higher than that of Comparative Examples 1-2, indicating that ammonium persulfate and calcination can effectively promote the decomposition of magnetic materials by sulfuric acid, converting rare earth oxides in magnetic materials into rare earth sulfate.

[0040] As the mass fraction of sulfuric acid increases, the leaching rate shows a trend of continuous increase followed by stabilization. This may be because as the mass fraction of sulfuric acid increases, the reaction rate with magnetic materials is enhanced, thereby extracting rare earth elements from the magnetic materials as much as possible and improving the rare earth leaching rate.

[0041] A comparison of Examples 1-5 revealed that Example 1 had the highest leaching rate, with a lower mass fraction of sulfuric acid than Example 5. Therefore, Example 1 is preferred.

[0042] Examples 6-9 Examples 6-9 are based on the preparation method of Example 1, but the amount of sulfuric acid solution added is adjusted. The specific adjustments are shown in Table 2.

[0043] The samples from Examples 6-9 were subjected to the performance tests described above, and the test results are shown in Table 2.

[0044] Table 2. Amount of sulfuric acid solution added and performance test results for Examples 1 and 6-9.

[0045] Referring to Table 2, a comparison of Examples 1 and 6-9 shows that as the amount of sulfuric acid solution added increases, the leaching rate shows a trend of continuous increase followed by stabilization. This may be because as the amount of sulfuric acid solution added increases, the reaction rate with the magnetic material is enhanced, thereby extracting as many rare earth elements as possible from the magnetic material and improving the rare earth leaching rate.

[0046] Examples 10-13 Examples 10-13 are based on the preparation method of Example 1, but the amount of ammonium persulfate added is adjusted. The specific adjustments are shown in Table 3.

[0047] The samples from Examples 10-13 were subjected to the performance tests described above, and the test results are shown in Table 3.

[0048] Table 3. Ammonium persulfate addition amount and performance test results in Examples 1 and 10-13.

[0049] Referring to Table 3, a comparison of Examples 1 and 10-13 shows that as the amount of ammonium persulfate added increases, the leaching rate shows an upward trend followed by a period of stabilization. This may be because as the amount of ammonium persulfate added increases, the rare earth elements in the magnetic material are continuously converted into the water leaching solution during the reaction and calcination processes, thereby improving the rare earth leaching rate.

[0050] Examples 14-17 Example 14 is based on the preparation method of Example 1. In the coordination reaction step, 10g of magnetic material and 18g of activated carbon are mixed and then 60g of sulfuric acid solution is added for the initial reaction, while the other conditions remain unchanged.

[0051] Examples 15-17 are based on the preparation method of Example 14, but the amount of activated carbon added is adjusted. The specific adjustments are shown in Table 4.

[0052] The samples from Examples 14-17 were subjected to the performance tests described above, and the test results are shown in Table 4.

[0053] Table 4. Activated carbon addition amount and performance test results for Examples 1 and 14-17.

[0054] Referring to Table 4, a comparison of Example 1 and Examples 14-17 shows that mixing magnetic materials with activated carbon and then reacting them with sulfuric acid solution is beneficial to improving the leaching rate of rare earth elements.

[0055] As the amount of activated carbon added increases, the leaching rate shows an upward trend followed by a stabilization. This may be because as the amount of activated carbon added increases, it absorbs the waste gases such as sulfur dioxide, sulfur trioxide, and ammonia generated during the reaction stage, and then gradually releases them during the roasting stage to form sulfuric acid. This further promotes the continuous conversion of rare earth elements in the magnetic material into the water leaching solution, thereby increasing the rare earth leaching rate.

[0056] Examples 18-23 Examples 18-23 are based on the preparation method of Example 1, with adjustments made to the calcination temperature and time. The specific adjustments are shown in Table 5.

[0057] The samples from Examples 18-23 were subjected to the performance tests described above, and the test results are shown in Table 5.

[0058] Table 5. Calcination temperature and time, and performance testing results for Examples 1 and 18-23.

[0059] Referring to Table 5, a comparison of Example 1 and Examples 18-23 shows that as the calcination temperature increases, the leaching rate shows an upward trend followed by a period of stabilization. This may be because as the calcination temperature increases, the molecular motion of the reactants accelerates and the collision frequency increases, leading to a continuous increase in the chemical reaction rate and thus improving the rare earth leaching rate.

[0060] As the roasting time increases, the leaching rate shows an upward trend followed by a stabilization. This may be because as the roasting time increases, the molecular motion of the reactants accelerates and the collision frequency increases, leading to a continuous increase in the chemical reaction rate and thus improving the rare earth leaching rate.

[0061] Examples 24-29 Examples 24-29 are based on the preparation method of Example 1, with adjustments made to the mass fraction of sodium carbonate and the reaction time between the filtrate and the sodium carbonate solution. The specific adjustments are shown in Table 6.

[0062] The samples from Examples 24-29 were subjected to the performance tests described above, and the test results are shown in Table 6.

[0063] Table 6 shows the mass fraction of sodium carbonate in Examples 1 and 24-29, as well as the reaction time and performance test results of the filtrate and sodium carbonate solution.

[0064] Referring to Table 6, a comparison of Example 1 and Examples 24-29 shows that as the mass fraction of sodium carbonate increases, the precipitation rate shows an upward trend followed by a stabilization. This may be because as the mass fraction of sodium carbonate increases, the precipitation reaction becomes more and more complete, thereby increasing the rare earth precipitation rate.

[0065] As the reaction time between the filtrate and the sodium carbonate solution increases, the precipitation rate shows an upward trend followed by a stabilization. This may be because as the reaction time between the filtrate and the sodium carbonate solution increases, the precipitation reaction becomes more and more complete, thereby increasing the rare earth precipitation rate.

[0066] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A neodymium iron boron waste magnetic steel environmentally friendly recycling method, characterized in that, Includes the following steps: Crushing and Separation: After crushing the NdFeB waste magnets, magnetic and non-magnetic materials are separated using magnetic separation technology, while retaining the magnetic materials; Complexation reaction: sulfuric acid solution is added to the magnetic material to carry out an initial reaction, and then ammonium persulfate is added to carry out a further reaction to obtain a mixture; Calcination: The mixture is calcined to obtain calcined sand, which is then soaked in water and filtered to obtain an aqueous extract. Purification: Adjust the pH of the aqueous extract to 3-3.7, wait for complete precipitation, filter, and obtain the filtrate; Precipitation: The filtrate is added to a sodium carbonate solution for reaction and filtration to obtain rare earth precipitate.

2. The NdFeB waste magnet steel environmental protection recycling method according to claim 1, characterized in that: In the coordination reaction step, the sulfuric acid solution contains 70-85 wt% sulfuric acid.

3. The environmentally friendly recycling method for NdFeB waste magnets according to claim 2, characterized in that: In the coordination reaction step, the mass ratio of the magnetic material to the sulfuric acid solution is 1:3-6.

4. The environmentally friendly recycling method for neodymium iron boron waste magnets according to claim 3, characterized in that: In the coordination reaction step, the mass ratio of the magnetic material to the ammonium persulfate is 1:0.3-0.

6.

5. The environmentally friendly recycling method for NdFeB waste magnets according to claim 3, characterized in that: In the reaction step, the magnetic material and activated carbon are mixed and then sulfuric acid solution is added to carry out the initial reaction.

6. The environmentally friendly recycling method for NdFeB waste magnets according to claim 5, characterized in that: In the coordination reaction step, the mass ratio of the sulfuric acid solution to the activated carbon is 1:0.1-0.

3.

7. The environmentally friendly recycling method for NdFeB waste magnets according to claim 1, characterized in that: In the roasting step, the roasting temperature range is 120-150℃.

8. The environmentally friendly recycling method for neodymium iron boron waste magnets according to claim 7, characterized in that: In the roasting step, the roasting time is 1-2 hours.

9. The environmentally friendly recycling method for neodymium iron boron waste magnets according to claim 1, characterized in that: In the precipitation step, the sodium carbonate solution contains 2.5-3.5 wt% sodium carbonate.

10. The environmentally friendly recycling method for NdFeB waste magnets according to claim 9, characterized in that: In the precipitation step, the reaction time between the filtrate and the sodium carbonate solution is 2-4 hours.

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