Method for improving the yield of fluosilicic acid using wet-process phosphoric acid concentration circulating water

Abstract

This invention discloses a method for using circulating water from a wet-process phosphoric acid concentration system to improve the yield of fluorosilicic acid. The method includes the following steps: First, circulating water from a 58% ω(P₂O₅) concentrated phosphoric acid production system is pumped into a cooling system; the cooled circulating water is then pumped into the first and second fluorine absorption tanks of a 48% ω(P₂O₅) concentrated phosphoric acid production system. Second, the circulating water from the 48% ω(P₂O₅) concentrated phosphoric acid production system is used as makeup water for the 58% ω(P₂O₅) concentrated phosphoric acid production system. Finally, process water is pumped into the 48% ω(P₂O₅) concentrated phosphoric acid production system to replenish the water level. The method provided by this invention can effectively recover fluorine resources from the circulating water in the concentration system, improve the yield of fluorosilicic acid, reduce silica gel deposition in the 48% ω(P₂O₅) concentrated phosphoric acid production system, reduce cleaning workload, and continuously refresh the circulating water to reduce the corrosion rate of equipment and pipelines by HF. Furthermore, it can reduce the amount of circulating water discharged and the amount of calcium hydroxide used.

Description

Technical Field

[0001] This invention relates to the field of resource recovery and recycling technology in the wet-process phosphoric acid production process, specifically to a method for using concentrated circulating water from the wet-process phosphoric acid production process to improve the yield of fluorosilicic acid. Background Technology

[0002] Fluorine resources are a vital strategic resource for my country, with over 90% of fluorine being associated with phosphate rock, making it the most crucial source of fluorine after fluorite, accounting for 1.5% to 4.0% of the total mass of phosphate rock. In the wet-process phosphoric acid production, fluorine in phosphate rock is partially converted into gaseous products such as HF and SiF4 during the extraction stage, entering the tail gas. After washing, it is primarily enriched in the washing liquid as fluorosilicic acid, becoming a key carrier for fluorine resource recovery. Fluorosilicic acid is not only a crucial intermediate in the production of anhydrous hydrogen fluoride but is also used to prepare various fluorosilicates, synthesize cryolite, aluminum fluoride, and other high-value products. Therefore, efficient recovery of fluorosilicic acid is essential for ensuring fluorine resource supply and enhancing the value of the phosphate chemical industry.

[0003] However, fluorosilicic acid has poor thermal stability. During the concentration and purification stage, it is prone to decomposition if the temperature is not properly controlled. Simultaneously, the silicon component in the solution easily precipitates silica gel, causing blockages in equipment pipelines and severely affecting the long-term stable operation of the system. Constrained by these technical pain points, a large amount of fluorine is lost with phosphogypsum solid waste or production wastewater, wasting valuable strategic resources and increasing the environmental pressure on solid waste disposal and wastewater treatment. This has become a key factor restricting the green transformation and high-quality development of the phosphate chemical industry.

[0004] Currently, the wet-process phosphoric acid concentration stage commonly employs vacuum flash evaporation. During this process, fluorides in the phosphoric acid escape with the secondary steam and are typically recovered via a two-stage countercurrent absorption tower: the first stage uses circulating fluorosilicic acid solution to absorb most of the fluoride, and the second stage uses process water to capture residual fluoride. This process relies heavily on circulating water to maintain the system vacuum and absorption temperature. Although this process achieves basic fluoride recovery, there is still room for improvement in overall recovery efficiency, and system blockage issues persist.

[0005] The industry is also committed to optimizing fluorine absorption processes. For example, Chinese utility model patent CN205235702U uses a high-tower, multi-layer spraying method, and Chinese invention patent CN101474523B uses multi-stage pipeline series washing, aiming to improve the gas-liquid contact efficiency and fluorine recovery rate within a single unit. However, these improvements mainly focus on enhancing the performance of a single absorption unit or process, failing to consider the flow and distribution of fluorine resources from the overall perspective of the entire phosphoric acid production system.

[0006] In particular, there is an inherent contradiction in the fluorine recovery efficiency of systems producing products of different concentrations (such as ω(P2O5)48% and ω(P2O5)58% concentrated phosphoric acid). During the production process, the system temperature for ω(P2O5)58% concentrated phosphoric acid is between 85 and 95°C, meaning that the thermal decomposition rate of fluorosilicic acid is higher within this temperature range. Furthermore, because the ω(P2O5)48% concentrated phosphoric acid is filtered before being used in the ω(P2O5)58% concentrated phosphoric acid production system, the SiO2 content in the ω(P2O5)58% concentrated phosphoric acid production system is significantly lower than that in the ω(P2O5)48% concentrated phosphoric acid production system. This results in HF in the ω(P2O5)58% concentrated phosphoric acid production system not being effectively converted to H2SiF6 and instead entering the circulating water with the tail gas, leading to a higher HF content in the circulating water of the ω(P2O5)58% concentrated phosphoric acid production system and a waste of fluorine resources. Conversely, the system temperature for producing ω(P2O5)48% concentrated phosphoric acid is relatively low, and the SiO2 content is relatively abundant, which is more conducive to the formation and stable existence of fluorosilicic acid. However, the fluorine resources in its circulating water system are not fully utilized.

[0007] In summary, existing technologies suffer from the following core problems: traditional fluorine recovery processes are limited to localized optimization within a single system, failing to address the imbalance in the distribution and form of fluorine resources across production systems caused by variations in product concentration and process parameters. Specifically, in high-concentration phosphoric acid systems, the high temperature and low silicon conditions lead to the loss of fluorine as volatile HF, while the silicon-rich conditions of low-concentration phosphoric acid systems are not utilized for the co-conversion of this HF. This has become a key technological bottleneck restricting further improvements in the overall fluorine resource recovery rate of the wet-process phosphoric acid industry. Summary of the Invention

[0008] The purpose of this invention is to provide a method for improving the yield of fluorosilicic acid using circulating water from wet-process phosphoric acid concentration. This method effectively recovers HF from the circulating water, reacts it with SiO2 in the fluoride absorption tower to generate fluorosilicic acid, and ensures that the circulating water quality meets production requirements. It can increase the yield of fluorosilicic acid, reduce silica gel deposition in the ω(P2O5)48% concentrated phosphoric acid fluoride absorption system, ensure production operation, and reduce cleaning work.

[0009] The technical solution of this invention: A method for improving the yield of fluorosilicic acid by using wet-process phosphoric acid concentration and circulating water, comprising two concentrated phosphoric acid production systems with different concentrations: a first concentrated phosphoric acid production system and a second concentrated phosphoric acid production system. The circulating water from the first concentrated phosphoric acid production system is introduced into the fluoride absorption tank of the second concentrated phosphoric acid production system to recover fluorine; at the same time, the circulating water from the second concentrated phosphoric acid production system is returned to the circulating water system of the first concentrated phosphoric acid production system, and process water is added to the second concentrated phosphoric acid production system to achieve water balance. The phosphoric acid concentration of the first concentrated phosphoric acid production system is higher than that of the second concentrated phosphoric acid production system, and the operating temperature of the first concentrated phosphoric acid production system is higher than that of the second concentrated phosphoric acid production system.

[0010] Furthermore, the first concentrated phosphoric acid production system is a concentrated phosphoric acid production system with ω(P2O5) 58%, and the second concentrated phosphoric acid production system is a concentrated phosphoric acid production system with ω(P2O5) 48%; the method specifically includes the following steps: Step 1: After the circulating water in the ω(P2O5) 58% concentrated phosphoric acid production system is cooled by the cooling system, it is transported to the fluoride absorption tank of the ω(P2O5) 48% concentrated phosphoric acid production system. Step 2: Transfer the circulating water from the ω(P2O5)48% concentrated phosphoric acid production system to the circulating water system of the ω(P2O5)58% concentrated phosphoric acid production system; Step 3: Replenish the circulating water system of the ω(P2O5) 48% concentrated phosphoric acid production system with process water to maintain its water balance. Furthermore, the raw material dilute phosphoric acid used in the second concentrated phosphoric acid production system is phosphoric acid with an ω(P₂O₅) content of 26%, and the mass content of fluorine in it is 1.9% to 2.2%. Furthermore, the concentrated phosphoric acid produced by the ω(P2O5)48% concentrated phosphoric acid production system contains 0.8% to 1.2% fluorine by mass.

[0011] Furthermore, in the circulating water of the ω(P2O5)48% concentrated phosphoric acid production system, the mass content of HF is 0.1% to 0.3%.

[0012] Furthermore, in the circulating water of the ω(P2O5)58% concentrated phosphoric acid production system, the mass content of HF is 0.5% to 0.7%.

[0013] Furthermore, in the ω(P2O5)48% concentrated phosphoric acid production system, the flash chamber operates at a temperature of 75°C to 88°C and a vacuum degree of -0.075MPa to -0.080MPa.

[0014] Furthermore, the cooling system controls the temperature of the circulating water from the ω(P2O5)58% concentrated phosphoric acid production system to between 30°C and 40°C.

[0015] Furthermore, the fluoride absorption system of the ω(P2O5)48% concentrated phosphoric acid production system includes a first fluoride absorption tank and a second fluoride absorption tank connected in series, and the total rate of replenishing circulating water into the first fluoride absorption tank and / or the second fluoride absorption tank is controlled at 40 m. 3 / h to 50 m 3 / h.

[0016] The beneficial effects of this invention are as follows: This invention introduces the circulating water from the ω(P2O5) 58% concentrated phosphoric acid production system into the first and second fluorine absorption tanks of the ω(P2O5) 48% concentrated phosphoric acid production system, where it reacts with excess SiO2 in the absorption tower to generate fluorosilicic acid, thereby increasing the yield of fluorosilicic acid and reducing silica gel deposition in the fluorine absorption system. Simultaneously, circulating water containing HF from the ω(P2O5) 48% concentrated phosphoric acid production system is input into the ω(P2O5) 58% concentrated phosphoric acid production system's circulating water for enrichment and recovery, and process water is used to replenish the circulating water level of the ω(P2O5) 48% concentrated phosphoric acid production system. This process not only effectively recovers fluorine resources but also ensures the quality of circulating water to maintain normal system operation and saves on circulating water treatment costs. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flow chart of the wet-process phosphoric acid concentration process; Figure 2 This is a flowchart of the method for improving the yield of fluorosilicic acid according to the present invention; Figure reference numerals: 1-Graphite heat exchanger, 2-Flash chamber, 3-Acid circulation tank, 4-First fluorine absorption tower, 5-First fluorine absorption tank, 6-Second fluorine absorption tower, 7-Second fluorine absorption tank, 8-Condenser, 9-Hot water tank, 10-Cooling system. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should not be construed that the scope of the subject matter of the present invention is limited to the following embodiments. All modifications, substitutions and alterations made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention are included within the scope of the present invention.

[0020] This invention provides a method for synergistically improving the yield of fluorosilicic acid and ensuring stable system operation by establishing a directional exchange of circulating water between two phosphoric acid concentration systems with different concentrations.

[0021] Reference Figure 1 This is a flow chart of the wet-process phosphoric acid concentration process. The specific process is as follows: The phosphoric acid solution first enters flash chamber 2 and is simultaneously transported from acid circulation tank 3 to graphite heat exchanger 1 by a circulation pump. In graphite heat exchanger 1, the solution is heated to a superheated state. The heated superheated acid solution is then sent to vacuum flash chamber 2. Under the high vacuum maintained in flash chamber 2, the acid pressure drops sharply, and some of the water rapidly boils and evaporates, i.e., flash evaporation.

[0022] The concentrated phosphoric acid solution after flash evaporation drips back into acid circulation tank 3, mixes with newly added dilute phosphoric acid, and continues to participate in the circulation heating. This cycle continues until the acid solution reaches the target concentration. Finally, the product phosphoric acid, having reached the required concentration, overflows from acid circulation tank 3 or is pumped out of the system by the product acid pump.

[0023] Fluorine Recovery: The large amount of fluorine-containing vapor generated by flash evaporation is discharged from the top of flash chamber 2 and first enters the first fluorine absorption tower 4. Here, the fluorosilicic acid circulating liquid from the first fluorine absorption tank 5 performs countercurrent spray washing on the vapor, absorbing about 70%-80% of the fluorine, thus concentrating the fluorosilicic acid solution. The fluorine-containing tail gas that is not completely absorbed enters the second fluorine absorption tower 6. Here, a second countercurrent spray is performed using dilute fluorosilicic acid liquid or process water from the second fluorine absorption tank 7 to ensure that the vast majority of residual fluorine is captured.

[0024] The absorbent solution with increased concentration in the second fluorine absorption tank 7 overflows and replenishes the first fluorine absorption tank 5. When the concentration of fluorosilicic acid in the first fluorine absorption tank 5 reaches the specified value, a portion can be output as a by-product, fluorosilicic acid.

[0025] The exhaust gas, after two stages of absorption, enters the condenser 8 from the top of the second fluorine absorption tower 6. Circulating cooling water flows through the condenser 8, condensing most of the water vapor. This process generates and maintains the vacuum required by the system. Non-condensable gases exit from the top of the condenser 8. The circulating cooling water, which absorbs heat and heats up in the condenser 8, flows by gravity to the hot water tank 9. The high-temperature circulating water is pumped to the cooling system and cooled to a suitable temperature. The cooled circulating water can be returned to the second fluorine absorption tank 7 as makeup water or directly supplied to the condenser 8 for recycling, thus forming a complete circulating water system.

[0026] Reference Figure 2 This is the overall process route of the present invention. The implementation of the present invention is based on... Figure 1 The invention comprises two independent yet interconnected phosphoric acid concentration production lines: one producing concentrated phosphoric acid with ω(P2O5) of 48% (hereinafter referred to as the 48% system), and the other producing concentrated phosphoric acid with ω(P2O5) of 58% (hereinafter referred to as the 58% system). The core innovation of this invention lies in introducing the circulating water from the 58% system into the fluoride absorption tank of the 48% system, while simultaneously diverting a portion of the circulating water from the 48% system back to the 58% system and supplementing it with process water to maintain balance, thereby forming a cross-system optimized fluoride resource recovery loop. The process includes the following steps: Step 1: Extraction and cooling of high-HF concentration circulating water. A portion of the high-temperature circulating water is drawn from the 58% system (whose circulating water is rich in HF due to high temperature and low silica conditions) circulating water system (e.g., the outlet of hot water tank 9) and pumped to a separate cooling system 10. By controlling the flow rate of the cooling medium, the temperature of this circulating water is precisely reduced to approximately 35°C. The purpose of this step is to lower the water temperature to the optimal range for the fluoride absorption reaction without changing its HF content, avoiding high temperatures affecting subsequent absorption efficiency or low temperatures causing premature silica gel precipitation.

[0027] Step 2: Introduce circulating water into the silicon-rich system reaction. The cooled, HF-rich circulating water from Step 1 is introduced at a rate of 40-50 m³ / h. 3 A stable flow rate of [flow rate] / h is supplied to the first fluorine absorption tank 5 and the second fluorine absorption tank 7 of the 48% system. Due to the characteristics of its raw materials and processes, the 48% system has a relatively high SiO2 content in both the gas and liquid phases. When the HF-rich circulating water enters the absorption tank as a makeup liquid and mixes with the absorbent containing a large amount of SiO2 components that is circulated and sprayed from the first fluorine absorption tower 4 and the first fluorine absorption tower 6, a core chemical reaction occurs: 6HF + SiO2 = H2SiF6 + 2H2O. This step efficiently converts the fluorine present in dissolved state in the 58% system into fluorosilicic acid products within the 48% system, directly increasing the overall yield of fluorosilicic acid. At the same time, the external HF consumes the excess SiO2 in the system, significantly reducing the equipment and pipeline blockage problems caused by silica gel precipitation.

[0028] Step 3: Water Balance Maintenance and HF-Enriched Water Recirculation. To maintain the total circulating water balance of the 58% system, an equal amount of water from the 48% system circulating water system (containing a small amount of incompletely converted HF) is drawn out and returned to the 58% system circulating water system. The HF contained in this returned water is further enriched in the high-temperature circulating water of the 58% system. This step completes the material loop and realizes the exchange of water resources between the two systems.

[0029] Step 4: The system water level is finally balanced. Since steps 2 and 3 resulted in a net output of 48% of the system's circulating water volume, it is necessary to replenish the 48% system's circulating water system with fresh process water to replenish and stabilize its design water level, ensuring the stable operation of the entire concentration and vacuum system.

[0030] Example 1: This example was implemented in a phosphoric acid plant that has two concentration production lines with 48% and 58% concentrations.

[0031] Raw materials and initial system conditions: The 48% system uses dilute phosphoric acid with ω(P2O5) of 26% and a fluorine content of 1.9% as raw material. The operating temperature of flash chamber 2 in this system is controlled at 79℃, and the vacuum degree is controlled at -0.078 MPa. The 58% system uses concentrated phosphoric acid produced by the 48% system as raw material, and its flash chamber operating temperature is controlled at 89℃, and the vacuum degree is controlled at -0.083 MPa.

[0032] according to Figure 2 The process route involves cooling 58% of the system circulating water (HF content 0.5%) to 35℃, and then... (The sentence is incomplete and requires more context to translate accurately.) 3 At a rate of / h, the first fluoride absorption tank 5 and the second fluoride absorption tank of the 48% system are introduced. At the same time, an equal amount of circulating water (HF content 0.2%) from the 48% system is returned to the 58% system, and sufficient process water is added to the 48% system.

[0033] After implementation of this embodiment, the fluoride content in the concentrated phosphoric acid produced by the 48% system stabilized at 1.0%, and the yield of fluorosilicic acid increased from 70% in the traditional process to 75%, a relative improvement of 7.1%. Due to the reduced silica gel precipitation within the 48% system, the system cleaning frequency decreased, and the annual effective operating time increased by 220 hours. Because the circulating water is utilized internally, the amount discharged is reduced, saving approximately 450 tons of calcium hydroxide used for neutralization treatment annually.

[0034] Example 2: Raw materials and initial system conditions: The 48% system uses dilute phosphoric acid with a slightly higher fluorine content (ω(P2O5) 26%, fluorine content 2.0%). Its flash chamber 2 temperature is controlled at 83℃, and the vacuum degree is -0.079 MPa. The 58% system flash chamber temperature is controlled at 92℃, and the vacuum degree is -0.085 MPa; its circulating water has an initial HF content of 0.6%.

[0035] After cooling 58% of the system's high-temperature circulating water, it was cooled to 48 m 3 A higher rate of / h is introduced into the 48% system absorption tank. Other material exchange steps are the same as in Example 1.

[0036] Under these conditions, the yield of fluorosilicic acid is further increased to 77%, a 10.0% improvement over the traditional benchmark (70%). System operational stability is improved, with an additional 240 hours of annual uptime. Calcium hydroxide consumption is reduced by 500 tons per year.

[0037] Comparative Example 1: To highlight the advantages of this invention, a conventional process is shown as a comparative example. The 48% and 58% systems operate completely independently. The fluoride absorption process of each system relies solely on its own circulation: process water is added to the second fluoride absorption tank of each system, and the dilute absorbent overflows to the first fluoride absorption tank 5 for concentration, and is output after reaching the required concentration (e.g., 10%). The two systems are only connected in terms of product acid (48% acid is used as raw material for the 58% system), and there is no coupling in the circulating water system. Sometimes, to increase the concentration, the dilute fluorosilicic acid from the 58% system is sent to the 48% system for further concentration, but this is a discontinuous material transfer and does not change the system's water balance or fluoride speciation distribution.

[0038] In 58% of the system, the high temperature causes thermal decomposition of fluorosilicic acid (H₂SiF₆⇋SiF₄↑ + 2HF↑), and insufficient SiO₂ in the system results in a large amount of fluorine dissolving in its circulating water as HF, which cannot be converted and is ultimately lost through drainage or evaporation. This leads to the overall fluorosilicic acid yield remaining at a low level of around 70% for a long time. In 48% of the system, the relative excess of SiO₂ easily generates silica gel, requiring frequent shutdowns for cleaning. In 58% of the system, the abundant HF in the circulating water is not utilized, and direct discharge requires a large amount of neutralizing agent (such as calcium hydroxide), increasing environmental costs.

[0039] The above provides a detailed description of the method for improving the yield of fluorosilicic acid by using wet-process phosphoric acid concentration and circulating water. Specific examples have been used to illustrate the structure and working principle of the invention. The descriptions of the embodiments are merely for the purpose of helping to understand the method and core ideas of the invention. It should be noted that those skilled in the art can make various improvements and modifications to the invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims.

Claims

1. A method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid, characterized in that: This includes two concentrated phosphoric acid production systems with different concentrations: a primary concentrated phosphoric acid production system and a secondary concentrated phosphoric acid production system. The circulating water from the first concentrated phosphoric acid production system is introduced into the fluoride absorption tank of the second concentrated phosphoric acid production system to recover fluorine; at the same time, the circulating water from the second concentrated phosphoric acid production system is returned to the circulating water system of the first concentrated phosphoric acid production system, and process water is added to the second concentrated phosphoric acid production system to achieve water balance. The phosphoric acid concentration of the first concentrated phosphoric acid production system is higher than that of the second concentrated phosphoric acid production system, and the operating temperature of the first concentrated phosphoric acid production system is higher than that of the second concentrated phosphoric acid production system.

2. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 1, characterized in that: The first concentrated phosphoric acid production system is a concentrated phosphoric acid production system with ω(P2O5) 58%, and the second concentrated phosphoric acid production system is a concentrated phosphoric acid production system with ω(P2O5) 48%; the method specifically includes the following steps: Step 1: After the circulating water in the ω(P2O5) 58% concentrated phosphoric acid production system is cooled by the cooling system, it is transported to the fluoride absorption tank of the ω(P2O5) 48% concentrated phosphoric acid production system. Step 2: Transfer the circulating water from the ω(P2O5)48% concentrated phosphoric acid production system to the circulating water system of the ω(P2O5)58% concentrated phosphoric acid production system; Step 3: Add process water to the circulating water system of the ω(P2O5)48% concentrated phosphoric acid production system to maintain its water balance.

3. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 1 or 2, characterized in that: The raw material dilute phosphoric acid used in the second concentrated phosphoric acid production system is phosphoric acid with ω(P2O5) 26% and a fluorine content of 1.9% to 2.2% by mass.

4. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 2, characterized in that: The phosphoric acid produced by the ω(P2O5)48% concentrated phosphoric acid production system contains 0.8% to 1.2% fluorine by mass.

5. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 4, characterized in that: The circulating water in the ω(P2O5)48% concentrated phosphoric acid production system contains HF at a mass content of 0.1% to 0.3%.

6. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 2, characterized in that: The circulating water in the ω(P2O5) 58% concentrated phosphoric acid production system contains HF at a mass content of 0.5% to 0.7%.

7. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 2, characterized in that: In the ω(P2O5)48% concentrated phosphoric acid production system, the flash chamber operates at a temperature of 75°C to 88°C and a vacuum degree of -0.075MPa to -0.080MPa.

8. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 2, characterized in that: The cooling system controls the temperature of the circulating water from the ω(P2O5)58% concentrated phosphoric acid production system to be between 30°C and 40°C.

9. The method for using wet-process phosphoric acid concentration circulating water to improve the yield of fluorosilicic acid according to claim 2, characterized in that: The fluoride absorption system of the ω(P2O5)48% concentrated phosphoric acid production system includes a first fluoride absorption tank and a second fluoride absorption tank connected in series. The total rate of replenishing circulating water into the first fluoride absorption tank and / or the second fluoride absorption tank is controlled at 40 m / s. 3 / h to 50m 3 / h.

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