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Water-Phosphorus Nexus for Wet-process Phosphoric Acid Production Hang Ma, Xiao Feng, and Chun Deng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05399 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Industrial & Engineering Chemistry Research

Water-Phosphorus Nexus for Wet-process Phosphoric Acid Production Hang Maa, Xiao Fenga*, Chun Dengb* a

School of Chemical Engineering & Technology, Xi’an Jiaotong University, Xi’an 710049, China

b

State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, China

Abstract: The water-phosphorus nexus problem for wet-process phosphoric acid production is firstly addressed in this paper. A systematic methodology for water system optimization and water-phosphorus nexus analysis is proposed. Based on the preliminary process flowsheet and water flowrate balance, the potential water sources and sinks as well as the key component can be extracted. The mathematical model for the water system optimization integrated with water flowrate balance is presented. The process flowsheet can be improved according to the optimized water system. The flowrate of freshwater is reduced from 1803.98 t/h (preliminary design) to 160.98 t/h (improved design). The utilization efficiencies of phosphorus element (calculated as P2O5) are calculated for the preliminary and improved designs. Because the process for wet-process phosphoric acid production is mostly aqueous phase, the utilization efficiency of phosphorus element is increased from 94.22% (preliminary design) to 98.76% (improved design) due to the reuse and recycling water stream with phosphorus element. The water minimization and phosphorus recovery can be achieved simultaneously. The additional annualized profit for the improved design reaches 248.8×106 CNY/a, which is a great benefit for the production plant. Keywords: water-phosphorus nexus; phosphorusrecovery; water minimization; mathematical programming; optimization *

Corresponding authors. E-mail addresses: [email protected] (Xiao Feng) and [email protected] (Chun Deng).

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1. Introduction China is rich in phosphorus resources. At present, the proven amount of phosphate ore resources is 21.449 billion tons, ranking the top in the world. In 2014, China's actual output of phosphate ore reached 120 million tons, accounting for 54.7% of the global output, ranking the first in the world.1 All phosphate fertilizer and wet-process phosphorous chemical products need to be manufactured from wet-process phosphoric acid. In 2015, the capacity of wet-process phosphoric acid production (calculated as P2O5) in China was 21.7 million t/year, and the capacity of phosphate fertilizer production (calculated as P2O5) was 23.7 million t/year, accounting for 40% of global production capacity and ranking the first in the world.2 The basic production route of wet-process phosphoric acid is to decompose phosphate ore by sulfuric acid, and then to produce the final phosphoric acid through the filtration, concentration and other treatment processes. A large amount of water and steam is consumed in the process of wet-process phosphoric acid production. In addition, certain quantity of phosphorus substance will be discharged from the water effluent of production processing of wet-process phosphoric acid. Therefore, if the effluent water rich in phosphorus is discharged directly, it will lead to waste of phosphorus resources and serious environmental pollution. The wet-process phosphoric acid production plant under study locates in Kunming City, Yunnan province, China, near the Dianchi Lake. To control the pollution in Dianchi Lake, the phosphorus content in the discharged water from surrounding factories must be less than 0.5 ppm required via the local government according to the Chinese standard.3 Due to the strict legislation of wastewater discharge, depletion of water and phosphorus resources, Chinese phosphate fertilizer and phosphorus chemical enterprises are seeking for more efficient utilization ways of water and phosphorus resources. The industrial water management and reclamation is very important and necessary for the sustainable development of manufacture industries. Enormous pinch techniques and mathematical optimization approaches were proposed for the synthesis of water network for wastewater minimization. Typical reviews on water pinch analysis,4 methods of water network design,5 and books on process integration for resource conservation6 and sustainable design7 can be available for good reference. On the basis of the concept of synthesis of mass exchange networks,8 Wang and Smith9 firstly 2


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proposed the limiting composite curve and water supply line to locate the flowrate targets (i.e. minimum flowrate of freshwater, regenerated water) and it is considered as the pioneering work of the water pinch technique. The mass problem table as a good alternative technique for flowrate targeting is proposed by Castro et al..10 Considerable improvements, i.e. improved problem table as well as limiting composite curve and water supply line for regeneration recycling11 and regeneration re-use,12 zero liquid discharge,13 have been addressed. Numerous graphical and/or tabular water pinch techniques were proposed, i.e. Water Surplus Diagram,14 Material Recovery Pinch Diagram developed separately by El-Halwagi15 and Prakash and Shenoy,16 Water Cascade Analysis,17 Source Composite Curve,18 Composite Table Algorithm and improved limiting composite curve19 and its extension improved problem table for water network with multiple resources20 and total water network.21 The state-of-art of the pinch technique for water system is addressed by Foo.4 However, it has certain difficulties for pinch techniques to handle the synthesis of water networks with multiple contaminants, and the annualized cost as the objective. In 1980s, Takama and his collaborators22, 23 firstly proposed a superstructure for water system optimization which consists water-using processes and water-regeneration/treatment units and developed a non-linear programming model. Kuo and Smith24 divided the water system into three parts, a water-using sub-system, regeneration sub-system for reuse and/or recycling and effluent treatment system. Numerous work has been conducted on the optimization of water-using sub-systems where direct reuse was the only option,25 system with regeneration reuse26 that the regenerated water cannot be recycled back to the original units that generate the wastewater, regeneration recycling,27 effluent treatment systems28 and total water networks29 in which three parts were addressed simultaneously. Property integration which provides more wide consideration on water quality (i.e. pH, conductivity, COD, hardness, tocity and color) was novelly proposed via El-Halwagi and his co-workers.30 Considerable work has been conducted on the synthesis of property-based water network, such as, direct reuse/recycling and treatment processes,31 in-plant32 and interplant33 interception reuse/recycling, batch processes,34 combined batch and continuous processes,35 economic and environmental objectives.36 The optimization of practical industrial water networks has been addressed widely, such as catalyst factory,37 chemical plant,38 refinery,39 semiconductor factory,40 papermaking factory,41 3



polyvinyl chloride plant,42 coal-based chemical complex.43 However, few research is conducted on the optimization of water system for a phosphoric acid plant. Besides, different from the processes for refinery and coal-based chemical plants, most process feed and effluent streams are aqueous phase or thick liquid for wet-process phosphoric acid production. The reuse/recycling of water stream with phosphorus content would lead to the recovery of phosphorus. The water-phosphorus nexus for wet-process phosphoric acid production has never been addressed. To achieve wastewater minimization and maximum recovery of phosphorus, this paper proposed the systematic procedure for water system optimization and water-phosphorus nexus analysis. A wet-process phosphoric acid production is taken as the case study. 2. Methodology The systematic procedure for water system optimization and water-phosphorus nexus analysis is illustrated in Figure 1. The first step is to carefully examine and fully understand the process flowsheet. Next, the water streams are extracted, and the flowrate balance is performed for the water system. Then the potential water sources and sinks can be identified, i.e. streams with low contaminant concentration (high quality), which are discharged to the wastewater treatment system or environment, can be identified as potential water sources. The process that currently uses freshwater and can be fed via water stream with higher contaminant concentration (lower quality) can be extracted as a potential water sink. The key quality (i.e. concentration of a certain contaminant) that govern the potential water reuse/recycling is determined as key contaminant. The flowrate and concentration for each water source and flowrate and limiting concentration (i.e. lower bounds and upper bounds) for each water sink can be exacted. Next, based on extracted data, the optimization model of water system integrated with flowrate balance can be used to optimize the water system. According to the optimized results, the basic process flowsheet can be improved. Next, the phosphorus substance flow analysis can be performed for the basis and improved processes. Finally, the water-phosphorus nexus can be addressed based on comparison of basis and improved processes.

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Figure 1. Systematic procedure for water-phosphorus nexus analysis

3. Process Description for Wet-Process Phosphoric Acid Production and Water System Optimization 3.1 Overall Process Description There are mainly five units in the process flow of the wet-process phosphoric acid production plant, homogenizing and milling unit (HMU), extraction reaction and filtration unit (ERFU), concentration unit (CU), tail gas washing unit (TGWU) and slurry and slag discharge unit (SSDU), as shown in Figure 2. After homogenizing and crushing treatment in HMU, the phosphate ore is sent into the milling process for further milling, grinding and thickening to obtain the qualified

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phosphate ore slurry which contents 38-40% water. The slurry is first fed to the storage tank, and then will be further sent to the extraction reaction unit (ERFU). The phosphate ore slurry reacts with the cycled phosphoric acid firstly to obtain the calcium dihydrogen phosphate slurry. Next, the slurry further mixes and reacts with the dilute sulfuric acid to produce the crystallization of CaSO4·2H2O and phosphoric acid. The reaction slurry is sent into the flash cooler for cooling. Most of the slurry is cycled to the extraction reaction unit (ERFU), a small part of the slurry flows into the digestion tank (for adjustment and crystallization), and then will be allocated to the filtration unit (ERFU) by a slurry pump. The slurry is separated to be the filtrate (phosphoric acid) and residue (phosphogypsum) by using the rotary table filter. After three-level countercurrent washing, the phosphogypsum (solid phase) is slurried again in SSDU by adding water. This phosphogypsum slurry with the concentration of 20% - 25% (mass fraction of P2O5, same hereinafter) is allocated to the slag field (SSDU). The filtrate which is the dilute phosphoric acid with 25% of P2O5 content is sent into the acid storage tank for clarification, and then the acid will be sent to the concentration unit (CU). The dilute phosphoric acid is concentrated by circulation vacuum evaporation to produce the final phosphoric acid. According to the production requirements, part of the concentrated phosphoric acid is further clarified and aged to produce the fertilizer grade commercial phosphoric acid, the sludge acid from this process is used to produce fertilizer. The fluoride-containing tail gas from the reaction unit is sent to the tail gas washing unit (TGWU) to remove harmful substances for purification. After washing and absorption by a venture scrubber, scrubbers and an empty tower, the tail gas which can meet the emission standard is discharged into the atmosphere, and the washing water is discharged from the circulating tank or slag discharge tank. As shown in Figure 2, freshwater, steam, desalted water and circulating cooling water are allocated to the water-using processes in the six units. The pool water generated from SSDU is delivered to the wastewater treatment station. The circulating cooling water will return to the cooling tower. The generated condensate water is treated in the desalted water station for the feed water of boiler.

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Figure 2. Schematic process diagram of water system for wet-process phosphoric acid production

3.2 Process Description of Each Unit and Water Flowrate Balance (1) Homogenizing and milling unit (HMU) The simplified diagram of water balance for the homogenizing and milling unit is shown in Figure S1. As shown, 40 t/h of fresh water is used for cooling, sealing of equipment and floor flushing and the used water will flow into the pit. The water as well as phosphate ore (23.6 t/h of water contained in the feed material) is allocated to mill, classifier and thickener in sequence to produce the phosphate slurry, which contains 128.8 t/h of water. The thick slurry from the classifier returns to the mill. Additional 37.1 t/h of fresh water will be supplemented into the mill. The dissolved flocculant water (i.e. 32 t/h) flows into the thickener, the overflow water in the thickener returns to the mill and classifier. The flowrate summation of intake fresh water ( water contained in the feed materials (

Gain f HMU, in )

Fresh f HMU, in

) of the homogenizing and milling unit and

equals to the flowrate summation of the water

contained in the outlet materials and water loss (

Loss f HMU, out )

of this unit, the water balance equation is

given in Equation (1). Fresh Gain Loss f HMU, in + f HMU, in = f HMU, out

(2) Extraction reaction and filtration unit (ERFU) 7


(1)


The diagram of water balance of extraction reaction and filtration units is shown in Figure S2. The phosphate slurry which contains128.8 t/h water is sent into the extraction tank I, and then will premix with the measured concentrated sulfuric acid which contains 2.7 t/h water, the back mixed acid which contains 210 t/h water from the filtration unit, and the sludge acid which contains 14.29 t/h water back from the dilute acid clarifying tank. Next, the mixed raw material is sent to extraction tanks II and III. The phosphate slurry, concentrated sulfuric acid and the back mixed acid react in the extraction tanks and digestion tanks. To ensure the reaction is completed, the concentrated sulfuric acid is added into digestion tank II. After the reaction is completed, the product will be sent to the filtration unit for further treatment. The heat generated by sulfuric acid dilution and reaction will raise the temperature of the reaction slurry. To maintain the suitable reaction temperature, the reaction slurry will be cooled in the lower-position flash evaporating condenser. The reaction slurry is recycled by the low flash-cold circulating pump which located in extraction tank VI, and the cooled slurry is returned from the low flash cooler by gravity to extraction tank I. The flash evaporating gas which contains 35 t/h water is sent to the pre-condenser and condenser of the tail gas washing unit for further treatment. The tail gas generated during the reaction which contains 15.2 t/h water will also enter the tail gas washing unit for treatment. The reaction slurry from digestion tank III enters the filter for filtration, the filter aid which contents 1 t/h water is added into the filtration process, and steam (0.2 MPa, 0.2 t/h) is used in the primary filtration area. The filter cake is washed by 3-stage counter-flow washing with fresh water. 30 t/h fresh water is used to wash the washing area III, and the washing solution from washing area III will be mixed with 180 t/h fresh water and then will be used to wash washing area II. The 210 t/h washing solution from washing area II is used to wash washing area I. Most filtered acid which contents 176.04 t/h water enters the dilute phosphoric acid aging tank. A small amount of filtered acid is mixed with 210 t/h washing solution which is from washing area I after counter-flow washing, and then this mixture will return to extraction tanks II and III as the back mixed acid raw material to react with phosphate ore slurry. After washing, the gypsum filter cake is discharged in the gypsum hopper, and after the gypsum is discharged, the filter cloth is sent into the filter cloth washing tank. The filter cloth will be flushed with 345.3 t/h hot water which is 8


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produced by cooling water after heating with the steam (0.2 MPa, 14.7 t/h) from the filter cloth washing tank. The washed-out gypsum which contents 48.44 t/h water is discharged into the gypsum slurring tank, and this flushing hot water is used for flushing the gypsum hopper with 620 t/h fresh water together, and then the flushing water and gypsum will be sent to the gypsum slurring tank (the water content of this material is 1080 t/h). The filter is operated under negative pressure. After the pumped-out gas is separated in the gas-liquid separator, it will be cooled and condensed with 600 t/h circulating cooling water from cooling tower I in the filtration condenser. 20 t/h condensation water returns to cooling tower I, while the non-condensable gas which contains 0.3 t/h water is pumped out with a liquid-ring vacuum pump, and then discharged after it is separated in the gas-liquid separator. 30 t/h circulating cooling water from cooling tower II is used for keeping the vacuum cooling system to work under negative pressure. The flowrate summation of intake fresh water ( units, water contained in the feed materials ( (

Circu f ERFU, in ),

and used water from troughs (

Gain f ERFU, in ),

Trou f ERFU, in )

Cond f ERFU, out ),

of the extraction reaction and filtration

steam (

Steam f ERFU, in ),

circulating cooling water

equals to the flowrate summation of the water

contained in the outlet materials and water loss ( condensation water (

Fresh f ERFU, in )

Loss f ERFU, out ),

circulating cooling water (

Circu f ERFU, out )

and

the water balance equation is given in Equation (2).

Fresh Gain Trou Steam Circu Loss Circu Cond fERFU, in + fERFU, in + fERFU, in + fERFU, in + fERFU, in = fERFU, out + fERFU, out + fERFU, out

(2)

(3) Concentration unit (CU) The simplified diagram for the water balance of the concentration unit is shown in Figure S3. The dilute phosphoric acid which contains 176.04 t/h water from the reaction and filtration unit is sent into the dilute phosphoric acid aging tank and dilute acid clarification tank for aging and clarification. After clarification, the dilute phosphoric acid is sent into the concentration circulating loop, and will be mixed with the heated circulating acid and enters the evaporator, where 116.67 t/h water is evaporated. After concentration, some concentrated phosphoric acid which contains 45.08 t/h water enters the concentrated phosphoric acid tank and will be sent to the phosphoric acid workshop. The remaining concentrated phosphoric acid is heated with 0.2 MPa, 110 t/h low pressure steam and then will keep circulating in the concentration circulating loop. 9



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101 t/h water condensed from the low-pressure steam in the steam saturator enters the condensate tank, and finally returns to the soft water tank. The steam loss in this process is 15 t/h, and the desalted water supply of the steam saturator is 6 t/h. The steam emitted from the evaporator contains fluoride and phosphorus pentoxide entrainment, so after separating with the entrainment separator, the steam enters fluorine absorber towers I and II for absorbing. Then the steam is sent into the steam condenser, 6300 t/h circulating cooling water is used for cooling the steam in the steam condenser, and finally 110.67 t/h condensation water is produced, which will return to cooling tower I with the circulating cooling water. The non-condensable gas which contains 6 t/h water from the condenser is pumped out and discharged to the atmosphere by the concentration vacuum jet system. Fluorine absorber towers I and II are operated by counter-flow absorption, and 15 t/h water from cooling tower II is used for absorbing fluorine in the steam. After absorbing, the produced fluosilicic acid which contains 15 t/h water will be sent to the fluosilicic acid recycling and utilization unit. 22.5 t/h pickling liquid is used to clean the evaporator, and will be back to the pickling liquid tank after cleaning. The flowrate summation of water contained in the feed materials ( steam (

Steam fCU, in ),


Circu f CU, in

Gain f CU, in

), desalted water (

), and water of pickling liquid (

Pickl fCU, in

flowrate summation of the water contained in the outlet materials and water loss ( circulating cooling water (

Circu f CU, out

liquid from concentration unit (

) and condensation water ( Pickl f CU, out ),

Cond f CU, out

Desalt fCU, in

),

) equals to the Loss f CU, out

),

), and effluent water of pickling

so the water balance equation is given in Equation (3).

Gain Desalt Steam Circu Pickl Loss Circu Cond Pickl fCU, in + fCU, in + fCU, in + fCU, in + fCU, in = fCU, out + fCU, out + fCU, out + fCU, out

(3)

(4) Tail gas washing unit (TGWU) The water balance diagram of the tail gas washing unit is shown in Figure S4.The tail gas which contains 15.2 t/h water and comes from the reaction unit first enters scrubbing tower I for washing, and then will be mixed with the tail gas which contains 2.92 t/h water from the filtration unit, to be sent into scrubbing tower II for washing again. After washing, the non-condensable tail gas which contains 8.7 t/h water is exhausted from the top of scrubbing tower II. 193 t/h fresh water is used for scrubbing tower I as the washing water, and 190.78 t/h cooling water is used by scrubbing tower II as the washing water. The flash gas from the flash condenser of the reaction unit (contains 10


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phosphoric acid and 35 t/h water) enters the pre-condenser, part of the steam of the flash gas condenses, the remaining flash gas enters the condenser, and will be further cooled by circulating cooling water (1350 t/h) from cooling tower I. The condensation water (60 t/h) enters the condensation seal tank. 200 t/h fresh water is heated with the steam (0.2 MPa, 5 t/h) and is used in the hot water seal tank. Then this hot water is discharged into the slag field with the washing solution produced by two scrubbing towers together, and the total flowrate is 573 t/h. The non-condensable gas which contains 0.2 t/h water is discharged from the condenser and enters the condensing demister, before pumped out by the liquid-ring vacuum pump. At last the gas is discharged into the atmosphere after separated with the gas-liquid separator. 13.5 t/h circulating cooling water from cooling tower II is used for keeping the vacuum cooling system to work under negative pressure. The flowrate summation of intake fresh water of the tail gas washing unit ( contained in the feed materials (

Gain fTGWU, in ),

steam (

Steam f TGWU, in ),

Fresh fTGWU, in ),

and circulating cooling water (

water

Circu fTGWU, in )

equals to the flowrate summation of the water contained in the outlet materials and water loss (

Loss fTGWU, out ),


Circu fTGWU, out )

and condensation water (

Cond fTGWU, out ),

and the water

balance equation is given in Equation (4). Fresh Gain Steam Circu Loss Circu Cond fTGWU, in + fTGWU, in + fTGWU, in + fTGWU, in = fTGWU, out + fTGWU, out + fTGWU, out

(4)

(5) Slurry and slag discharge unit (SSDU) The simplified diagram for the water balance of the slurry and slag discharge unit is shown in Figure S5. The phosphogypsum slurry which contains 1245 t/h water from the gypsum slurry tank enters the slag field, and will be discharged in front of the dam. The treated water (60 t/h) from the fluoride salts unit, the discharge water 573 t/h from the tail gas washing unit, the rainwater 164.52 t/h, the river water 57.15 t/h and the underground water 9.56 t/h are sent into the slag field. The water separated from the slurry returns to the pool through the backwater system. A small amount of pool water (123.33 t/h) is reused by the small phosphoric acid device, and the 1643 t/h pool water is sent to the waste water treatment unit. The pool water is reduced by evaporation (134.03 t/h), water leakage (29.58 t/h) and catching by phosphogypsum as the crystal water (179.29 t/h). The flowrate summation of water contained in the feed materials ( 11


Gain fSSDU, in

), rain


water/underground water/river water ( summation of the water loss (

Loss fSSDU, out )

Rain fSSDU, in

), and wastewater (

and pool water (

Pool fSSDU, out ),

Waste fSSDU, in

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) equals to the flowrate

and the water balance equation is

given in Equation (5). Gain Rain Waste Pool Loss fSSDU, in + fSSDU, in + fSSDU, in = fSSDU, out + fSSDU, out

(5)

In Equations (1) - (5), subscript pu represents each process unit, which is belong to the set PU (pu∈PU) and PU={HMU, ERFU, CU, TGWU, SSDU}. Superscript it denotes various types of inlet, which is belong to the set IT (Inlet Type) (it∈IT) and IT={Fresh, Gain, Desalt, Steam, Cond, Pool, Circu, Clean, Waste}. Superscript ot represents various types of outlet water, which is belong to the set OT (Outlet Type) (ot∈OT) and OT={Loss, Circu, Pool, Cond, Clean}. Equations (1) - (5) can be expressed in a generalized form shown in Equation (6).

∑

it f pu , in =

it∈IT

∑

ot f pu , out ∀pu ∈ PU

(6)

ot∈OT

(6) Flowrate balance for each type of water The types of water mainly include fresh water, desalted water, circulating cooling water, pool water, steam, condensate water, and water gain and loss. The water flowrate balance for freshwater, circulating cooling water and steam is performed. The fresh water from the fresh water station is sent to the phosphoric acid plant through pipeline and can be used as the equipment sealing and cooling water, supplementary water for classifier, water for flushing ground and preparing flocculant for the homogenizing and milling unit (HMU) (

Fresh f HMU, in ).The

(

Fresh f ERFU, in )

fresh water is also allocated to the extraction reaction and filtration unit (ERFU)

and will be used as washing water for washing area III and for preparing filter aid. The

fresh water is also used as the make-up water for cooling towers I ( and for producing picking liquid (PL) (

Fresh f PL, in ). The

Fresh f CTI , in

) and II (CTII) (

Fresh f CTII , in ),

flowrate balance is given in Equation (7).

Fresh Fresh Fresh Fresh Fresh Fresh fFWPL, out = fHMU, in + fERFU, in + fCTI, in + fCTII, in + fPL, in

(7)

Through the circulating cooling water pipeline, the circulating cooling water from cooling tower I (cooling tower I, CTI) (

Circu f CTI, out

reaction and filtration unit (ERFU) (

) and cooling tower II (CTII) ( Circu f ERFU, in

Circu f CTII , out )

is sent to the extraction

) of the phosphoric acid plant to be used for filtration 12


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condensation and other operations. The circulating cooling water is sent to the concentration unit (CU) (

Circu fCU, in

(TGWU) (

) to be used for concentrator condensation, and is also sent to the tail gas washing unit Circu f TGWU, in

) to be used for flash cooler condensation and other operations. The balance is

shown in Equation (8). Circu Circu Circu Circu Circu fCTI, out + fCTII, out = fERFU, in + fCU, in + fTGWU, in

The purchased steam (

Steam ) fSPL, out

(8)

is sent to the extraction reaction and filtration unit (ERFU) (

for filter cloth washing, to the tail gas washing unit (TWGU) ( seal tank, and to the concentration unit (CU) (

Steam f CU, in )

Steam ) fTWGU, in

Steam f ERFU, in

)

for heating the hot water

for indirectly heating the dilute phosphoric

acid and concentrating the phosphoric acid. The flowrate balance is given in Equation (9). Steam Steam Steam Steam fSPL, out = f ERFU, in + f TWGU, in + f CU, in

(9)

The flowrates of make-up water for cooling tower I and II are 450 t/h and 19 t/h. The total flowrate of cooling water for cooling tower I and II is determined as 8912.08 t/h via solving Equation (8). With the given data shown in Figures S1-S5, the total flowrate of freshwater can be calculated as 1803.98 t/h via solving Equation (7). The total flowrate of steam is determined as 129.9 t/h via solving Equation (9).

3.3 Analysis of Water Conservation Potential and Water System Optimization The flowrate balance analysis and calculation is performed in Section 3.2. The potential for water minimization is analyzed in this section. The mass fraction of P2O5 of water stream would govern the potential water reuse/recycling, and P2O5 is determined as the key contaminant. As described in Section 2, if water streams with high quality are discharged to the wastewater treatment system or environment, they can be identified as potential water sources. For instance, the mass fraction of P2O5 contained in the effluent from scrubbing tower I is 0.87%. It is acceptable for certain water-using processes, i.e. the hot water seal tank. The hot water seal tank can receive the water stream with P2O5% less than 1.1%, which is determined via the historical operation data. Currently, the hot water seal tank is fed by fresh water free of P2O5. Thus the hot 13



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water seal tank can be identified as a potential water sink and the effluent of scrubbing tower I can be determined as a potential water source. Similarly, all the data for the potential water sources and sinks are extracted and listed in Table 1. Next, a mathematical model is built up for the optimization of the water system. Table 1. Extracted data for potential water sources and sinks

Water sinks

Minimum flowrate(t/h)

Maximum flowrate (t/h)

Upper bound of concentration (P2O5%)

Washing area II

150

200

1.1

Gypsum hopper

600

650

1.5

Scrubbing tower I

190

210

1.2

Hot water seal tank

180

230

1.1

CCWS

Make-up water for cooling tower I

400

450

1.5

Unit

Water sources

Flowrate (t/h)

Concentration (P2O5%)

Effluent of hot water seal tank

180

0.87

Effluent of scrubbing tower I

200

0.87

Effluent of scrubbing tower II

193

0.94

pool water

1070

1.25

Unit

ERFU

TGWU

TGWU

SSDU

For each sink k, it can receive fresh water or process water sources. The flowrate balance can be given in Equation (10), 14


Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


∑f

s ,k

s

where

and

f u ,k

f s ,k

+ ∑ fu,k = fk ∀k ∈ K

(10)

u

denote the flowrates allocated from uth fresh water and sth process water

source to kth water sink.

fk

denotes the flowrate for kth water sink, which should be within the

range of minimum ( Fkmin ) and maximum ( Fkmax ) values. The constraint is shown in Equation (11). Fkmin ≤ f k ≤ Fkmax

(11)

The mass balance of P2O5 for each water sink can be expressed as Equation (12),

∑f

s ,k

⋅ X s + ∑ fu,k ⋅ X u ≤ Fk ⋅ X kUB ∀k ∈ K

s

where

and

Xu

source.

X kUB

Xs

(12)

u

denote the mass fraction of P2O5 for uth fresh water and sth process water

denotes the upper bound of mass fraction of P2O5 for kth water sink.

The allocated flowrate to all the water sinks from each process water source cannot exceed its available flowrate and it can be expressed as Equation (13),

∑f

s,k

+ fs,d ≤ FsUB ∀s ∈S

(13)

k

where

FsUB

denotes the upper bound of sth water source.

f s ,d

denotes the discharged flowrate

from sth water source to slurry and slag discharge unit (SSDU). The binary variable sinks. If

za ,b

za ,b

is introduced to express the connection status between sources and

equals one, it indicates the existence of the connection between source a and sink b

and the flowrate is within its lower and upper bounds. If

za ,b

equals zero, it indicates that there is

no connection and no flowrate between source a and sink b. The relationship between binary variable

za ,b

and continuous variable

f a ,b

can be formulated as Equation (14),

f a ,b − za ,b ⋅ FaUB ,b ≤ 0 f a ,b − za ,b ⋅ FaLB ,b ≥ 0 za ,b ∈ { zu ,k , zs ,k }

(14)

f a ,b ∈ { f u ,k , f s ,k , f s ,d }

where

FaUB ,b

and

FaLB ,b

are the upper and lower bounds of flowrate

f a ,b .

The total connection number ( ntotal ) between water sources and sinks can denote the network complexity and it can be expressed as Equation (15). 15



ntotal = ∑∑zu,k +∑∑zs,k u

k

s

k

Page 16 of 33

(15)

UB The total number of connections can be optimized via setting its upper bound ( N total ). The

upper bound can be reduced one by one until the minimum flowrate of freshwater begin to increase or in an acceptable scope. It can expressed as Equation (16), UB ntotal ≤ Ntotal

(16)

Note that, not all the outlet and in let water streams and for process units ( ∀pu ∈ PU ) are exacted as the water sources and sinks. To calculate the total flowrates of fresh water and cooling water, the water balance equation (i.e. Equation 6) should be included in the model. However, once the water network is optimized, the type of feed water for the inlet of process units would change, i.e. the feed water for washing area I would be changed from fresh water to the effluent of washing area II. Thus, Equation (6) should be revised to be Equation (17), ( ∑ f puit , in ) '+ it∈IT

where ( ∑

f puit , in ) '

∑

f puat , in =

at∈AT

∑

f puot , out ∀pu ∈ PU

ot∈OT

(17)

denotes the summation of the inlet flowrate for each type of water for process

it∈IT

at units. f pu ,in denotes the flowrate of altered type of water and AT represents the set of altered type

at of water. Worthy to mention, f pu ,in can be calculated via the summation of the flowrate of reused

process water sources for the water sinks included in the process units. It can be expressed as Equation (18), f puat , in =

∑∑f

s ,k

∀pu ∈ PU

k∈pu1 s∈S

pu1 ⊂ pu k ∈{washing II, hopper, tower I, seal, ccws} pu1∈{{wash II, hopper},{tower I, seal},{ccws}} PU={HMU, ERFU, CU, TGWU, SSDU} {wash II, hopper} ⊂ ERFU

(18)

{tower I, seal} ⊂ TGWU {ccws} ⊂ CCWS

where

pu1

denotes the set of water sinks for process units. Note that, only part of inlets of

process units are extracted as water sinks, thus set

pu1

is the subset of set pu, that is pu1 ⊂ pu .As

shown in Table 1, some water sinks (wash II, hopper) belong to filtration unit. The water sink, 16


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tower I, belongs to tail gas washing unit, and ccws belongs to circulation cooling water station. The minimum flowrate of fresh water for the wet-process phosphoric acid production is chosen as the objective function, which is given by Equation (19), Fresh OBJ = min f FWPL, out

(19)

The model P is considered for the water system optimization of the wet-process phosphoric acid production process. Fresh obj min f FWPL, out given in Equation (19)

s.t. Flowrate balance constraints for PUs (1)-(6); Flowrate balance constraints for each type of water (7) - (9); Flowrate and mass balance constraints of key component for extracted water sources and sinks (10) - (13) and (18); Relationship constraints for binary variables and continuous flowrate variables (14); Total number of connections (15) and (16); Flowrate balance constraints for altered type of water (17). It is worthy to mention that the binary variables za,b are included in model P and other equations are linear. It results in a mixed integer linear programming (MILP) problem. All the models are coded in GAMS 24.2.2 on a PC with Intel® Core™ i5-3330 3.2 GHz and 4.00 GB RAM, running Windows 10, 64-bit operating system. The MILP problem has 239 continuous variables, 25 binary variables and 91 constraints. It is solved in 0.01 CUPs using CPLEX solver. The absolute optimality tolerance for all solvers is set as 10-6. Firstly, the constraint for the total number of connections (Equation 16) is not included in model P. The optimal flowrate of freshwater is determined as 160.98 t/h. Afterwards, the constraint (Equation 16) is added in model P by setting

UB N total

.

UB Ntotal

could be decreased one by

one and it is reduced to be 5 and the flowrate of freshwater remains unchanged (160.98 t/h). The flowrate of pool water is determined as 1193.33 t/h, respectively. Note that, in the preliminary design, as shown in Figure S1, freshwater is used for equipment sealing and cooling and ground washing. The used water is reused in milling of phosphate ore. As shown in Figure 3(a), the effluent of washing area III is reused in washing area II, and washing area I reuses the effluent of 17



washing area II in sequence. However, washing area II, gypsum hopper, hot water seal tank and scrubbing tower I use 1193 t/h of freshwater and their effluents are discharged to slag discharge unit. Cooling tower I uses 450 t/h of freshwater as the make-up water. After optimization, as shown in Figure 3(b), pool water from slurry and slag discharge unit is recycled to replace the freshwater, i.e. 620 t/h of pool water allocated to gypsum hopper and 450 t/h of pool water directed to cooling tower I. In addition, the effluent of scrubbing tower II (193 t/h) is reused by scrubbing tower I. Its effluent (200 t/h) is reused in hot water seal tank. The effluent of hot water seal tank (180 t/h) is reused in washing area II in sequence. The flowate of the fresh water is reduced from 1803.98 t/h to 160.98 t/h and it indicates the optimization of water system reduces the flowrate of water sharply.

(a)

18


Page 18 of 33

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(b) Figure 3. Water sources and sinks: (a) preliminary design; (b) optimized design

4. Phosphorus Substance Flow Analysis and Water-Phosphorus Nexus In the preliminary design, washing area II in the filtration unit using countercurrent-washing, and freshwater is used for washing gypsum hopper plate, two tail gas washing towers, hot water seal tank and make-up water of circulating cooling water station. Figure 4(a) shows the phosphorus substance flow for the wet-process phosphoric acid production process. The P2O5 content in the feeding phosphate ore is 61.51 t/h. The most part of P2O5 flows into slurry (61.23 t/h P2O5) after homogenizing and milling treatment process, and 0.29 t/h P2O5 is lost. Then the slurry enters the extraction reaction unit and will be mixed with the concentrated sulfuric acid to carry out the extraction reaction. The reaction slurry will be sent to the filtration unit, and the flash evaporating gas containing phosphoric acid generated during the process is sent to the tail gas 19



Page 20 of 33

washing unit. The reaction slurry is filtered to produce the dilute phosphoric acid which contents 61.66 t/h of P2O5 and it is further sent to the concentration unit. The filtered phosphogypsum is sent to the slurry tank and generates gypsum slurry which contents 5.84 t/h of P2O5. The tail gas and the flash evaporating gas containing phosphoric acid which was generated from the filtration and reaction unit contents 0.88 t/h P2O5. Those tail gas streams enter the tail gas washing unit, and after washing treatment, the phosphorus contented in these tail gases will be absorbed by the washing water and be discharged to the slag field which contents 2.79 t/h of P2O5. The dilute phosphoric acid produced by the filtration unit enters the concentration unit to produce the concentrated phosphoric acid which contents 57.96 t/h of P2O5. In the concentration unit, the sludge acid produced by the dilute acid clarifying tank returns to the extraction reaction unit, and the P2O5 content of this acid is 3.70 t/h. The gypsum slurry is sent to the slag field, and after precipitation the pool water will be generated. Then this water will be discharged. There is another gypsum slurry which comes from the small phosphoric acid plant is sent to the slag field, and the P2O5 content of this slurry is 1.33 t/h. At the same time, part of the pool water is used in the small phosphoric acid plant, and the P2O5 content of this water is 0.55 t/h. In the preliminary design, the system input of P2O5 is 62.92 t/h, including the feeding phosphate ore (61.51 t/h) and part from small phosphoric acid plant (0.78 t/h). The content of P2O5 in the product, the concentrated phosphoric acid, is 57.96 t/h. The utilization efficiency of phosphorus element can be calculated as 94.22% via solving Equation (20), Ep =

1 mp,Prod CU, out 1 mp,Gain HMU, in

Prod

=

Prod1 1 XCU, out ⋅ FCU, out Gain

Gain1 1 XHMU, in ⋅ FHMU, in

×100%

where E p represents the utilization efficiency of phosphorus element (calculated as P2O5), denotes the content of P2O5 of the product phosphoric acid leaving the concentration unit,

(20) od1 m Pr p , CU, out

1 m Gain p , HMU, in

denotes the content of P2O5 of the phosphate ore entering the homogenizing and milling unit, Pr od1 X CU, out

denotes the concentration of P2O5 of the concentrated phosphoric acid leaving the

concentration unit, concentrated unit,

Pr od

FCU, out1 Gain1 X HMU, in

denotes the flowrate of concentrated phosphoric acid leaving the denotes the concentration of P2O5 of the phosphate ore entering the

homogenizing and milling unit, and

Gain

FHMU ,1 in

denotes the flowrate of phosphate ore entering the 20


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homogenizing and milling unit. Note that, the phosphorus element from the small phosphoric acid plant is not included in the utilization efficiency of phosphorus element.

(a)

(b) Figure 4. Phosphorus substance flow analysis and water-phosphorus nexus analysis (Unit of flowrate for water and P2O5 (in parenthesis): t/h): (a) preliminary design; (b) improved design;

The phosphorus substance flow for the improved design for wet-process phosphoric acid production is shown in Figure 4(b). In the improved design, the net input of phosphorus (P2O5) for the feeding phosphate ore is 61.51 t/h, the output of phosphorus (P2O5) content of products is 60.75 t/h. The utilization efficiency of phosphorus element (calculated as P2O5) can be calculated as 98.76% via solving Equation (20), which is 4.53% higher than that in the preliminary design. Compared with Figure 4(a), water stream from the tail gas washing unit (TGWU) and pool 21



Page 22 of 33

water stream from the slurry and slag discharge unit (SSDU) are reused in the extraction reaction and filtration unit (ERFU) in Figure 4(b). They would affect the utilization efficiency of phosphorus element. As shown in Figure 3(b), pool water is reused for the washing of gypsum hopper and the phosphorus content in the pool water will not flow into the phosphoric product. Note that, scrubbing tower II uses cooling water with 1% of P2O5. Its effluent (i.e. 193 t/h) is reused by scrubbing tower I. The effluent (200 t/h) of scrubbing tower I is reused in hot water seal tank. The effluent of the hot water seal tank (180 t/h) is reused in washing area II in ERFU in sequence. The

phosphorus content flows into the product with the dilute phosphoric acid. It leads to the increase of phosphorus content in the concentrated phosphoric acid. Compared with the preliminary design, the phosphorus (P2O5) content of concentrated phosphoric acid for the improved design is 60.75 t/h, which is increased by 2.79 t/h. Results comparison for water minimization and phosphorus recovery for preliminary and improved design is shown in Figure 5. In the preliminary design, the total consumption of the freshwater is determined as 1803.98 t/h, and the flowrate of discharged pool water is 1643.00 t/h. After the optimization of the water system and improvement of process flowsheet, the total consumption of the fresh water is reduced sharply to 160.98 t/h and it is reduced by 91.08%. Due to the water reuse/recycling, the phosphorus element (calculated as P2O5) contained in the water streams is also recovered and the utilization efficiency of phosphorus element (calculated as P2O5) is increased from 94.22% (preliminary design) to 98.76% (improved design). All the pool water is reused. The zero-water discharge has been achieved although there is a small amount of water loss. To evaluate the economic performance of the improved design, the additional annualized profit can be determined as 248.8×106 CNY/a via the annualized revenue minus annualized investment cost as given in Equation (21). The annualized revenue includes the additional benefit of recovered phosphorus and reduced flowrate of freshwater and discharged wastewater. The investment cost mainly includes the additional investment and construction cost of the new added pipeline. AP = H ⋅ (mrec p ⋅ SPp + RFu ⋅ UCu + RFd ⋅ UCd ) − Af ⋅ IC pipe

22


(21)

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where

denotes the additional annualized profit.

AP

hours).

SPp

H

denotes the operating hours (i.e. 7200

represents the selling price for the phosphorus (i.e. 6000 CNY/t, calculated as P2O5).

mrec p

denotes the mass flowrate of recovered phosphorus (i.e. 2.79 t/h, calculated as P2O5).

and

RFd

RFu

denote the reduced flowrate of freshwater and discharged wastewater, which are the

same (i.e. 1643 t/h).

UCu

and

UCd

represent the unit costs for freshwater and discharged

wastewater, which are 4 CNY/t and 7 CNY/t.

IC pipe

denotes the additional investment and

construction cost for the new added pipeline, which is estimated to be 15×106 CNY via the economic engineer.

Af

denotes the annualized factor for the investment cost and it can be

expressed as Equation (22). Af =

where

fi

denotes interest rate (i.e. 4%),

ny

fi ⋅ (1 + fi ) ny (1 + fi ) ny − 1

(22)

denotes depreciation period (i.e. 10 years).

Flowrate of fresh water (t/h) 1803.98

160.98 preliminary design

improved design (a)

23



(b) Figure 5. Results comparison for water minimization and phosphorus recovery for preliminary and improved design scenarios: (a) Flowrate of freshwater; (b) Utilization efficiency of phosphorus element (calculated as P2O5)

5. Conclusion A systematic methodology for water system optimization and water- phosphorus nexus analysis is firstly proposed for a wet-process phosphoric acid production process. Once the preliminary process flowsheet is fully understood, the water flowrate balance can be performed, and the potential water sources and sinks as well as the key components that govern the water reuse and recycling are extracted. The mathematical model for the optimization of water system with water flowrate balance is introduced. According to the optimized water network, the process flowsheet can also be improved. The utilization efficiencies of phosphorus element (calculated as P2O5) for the preliminary design and improved design are determined. Worthy to mention, the wet-process phosphoric acid production process is mostly aqueous phase. Due to the reuse and recycling water stream with phosphorus content, the flowrate of freshwater is reduced from 1803.98 t/h (preliminary design) to 160.98 t/h (improved design). The utilization efficiency of phosphorus 24


Page 24 of 33

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


element is increased from 94.22% (preliminary design) to 98.76% (improved design). The improved design can get the additional annualized profit of 248.8×106 CNY/a, and it is very attractive for the phosphorus production plant. The water-phosphorus nexus is firstly addressed for the wet-process phosphoric acid production process. The proposed systematic procedure can be utilized for the wastewater minimization and phosphoric recovery for other phosphorus fertilizer and chemical plants.

Notation Sets AT

—

set of altered type water

IT

—

set of inlet type water

K

—

set of water sinks

OT

—

set of outlet type water

PU

—

set of water-using unit

pu1 —

sub set of water-using unit PU

S

—

set of water sources

U

—

set of fresh water sources

F

—

required water flowrate, t/h

H

—

operation time, hour

IC

—

additional investment cost, CNY

RF

—

reduced flowrate, t/h

SP

—

selling price for the phosphorus, CNY/t

UC

—

unit cost, CNY/t

X

—

mass fraction of P2O5, ppm

Parameters

Variables

25



Af

—

annualized factor of investment cost

AP

—

annualized profit, CNY/a

E

—

utilization efficiency of phosphorus element

f

—

water flowrate, t/h

fi

—

interest rate

m

—

mass flowrate of phosphorus, t/h

n

—

connection number

ny — z

depreciation period, a

—

binary variable for connection

—

type of altered water

Superscripts at

Circu —

circulating cooling water

Cond

—

condensation

Desalt

—

desalted water

Fresh —

fresh water

Gain —

generated water or water contained in the feed materials

Gain1

—

phosphate ore in process unit 1

it

—

type of inlet water

LB

—

lower bound of water flowrate

Loss

—

lost water and water contained in the product materials

ot —

type of outlet water

Pickl —

pickling liquid water

Pool —

pool water

Prod1

—

Rain — rec — Steam

—

Trou —

final product-concentrated phosphoric acid rain/underground/river water recovered phosphorus steam water from troughs

26


Page 26 of 33

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


UB

—

upper bound of water flowrate

Waste

—

wastewater

a

—

index of source

b

—

index of sink

CCWS

—

circulating cooling water station

CT I

—

cooling tower I

CT II

—

cooling tower II

CU

—

concentration unit

d

—

discharged to slurry and slag discharge unit

ERFU

—

extraction reaction and filtration unit

EU

—

extraction reaction unit

FU

—

filtration unit

Subscripts

FWPL —

fresh water pipeline

HMU

—

homogenizing and milling unit

in

—

inlet

k

—

index of water sink

out — p

—

outlet phosphorus element

pipe —

added pipeline

PL —

pickling liquid

pu —

process unit

Reservior

—

pooling water reservoir

s

—

index of water source

SDU

—

slag discharge unit

SU

—

slurry unit

SSDU

—

slurry and slag discharge unit

SPAP

—

small phosphoric acid plant

27



SPL — TGWU —

steam pipeline tail gas washing unit

total

—

total number

u

—

index of water utility or fresh water

Supporting Information Figures S1-S5 are included in the Supporting material.

Author Information Corresponding Authors * E-mails: [email protected] (Xiao Feng) and [email protected] (Chun Deng) Notes The authors declare no competing financial interest.

Acknowledgements Financial support from the National Natural Science Foundation of China (21736008) is gratefully acknowledged. The research is also supported by Science Foundation of China University of Petroleum, Beijing (No. 2462015BJB02).

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Table of Contents (TOC) Graphic The water-phosphorus nexus of wet-process phosphoric acid production leads to water conservation and phosphorus recovery for sustainability.

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