Phosphoric Acid Process Comparison by Donal S. Tunks, Process Engineer, Jacobs Engineering S.A. (JESA) firstname.lastname@example.orgExecutive SummaryFrom a rigorous analysis of the capital and operating costs of 9 different processes toproduce phosphoric acid from phosphate rock, the different processes can be ranked bytheir economic efficiencies. The rankings established in this analysis are as follows: 1. Dihydrate Process 2. Hemi-Dihydrate Process 3. Hemi-Dihydrate Recrystallization Process 4. Hemihydrate Process 5. Di-Hemihydrate Process 6. Nitrophosphate Process 7. Furnace Process 8. Thermal Process 9. Hydrochloric Acid RouteThe Dihydrate Process surpassed recrystallization processes due to the high additionalcost of recrystallization processes. This extra investment is not compensated by theincreased P2O5 recovery and by the increased electrical production from lower steamconsumption. The Hemihydrate Process comes at a slightly lower investment cost thanthe Dihydrate Process yet there is a significant drop in P2O5 recovery, so tworecrystallization processes surpassed it in economic efficiency. The Di-HemihydrateProcess has the highest total investment cost of all processes with acidulation withsulfuric acid which makes the Hemihydrate process more economical even with adifference in P2O5 recovery of 6%. Implementing the Nitrophosphate Process requires ahuge investment which has a slightly lower payback than processes with sulfuric acidacidulation. The Furnace Process does have a lower operating cost than the DihydrateProcess yet not enough to justify an extra 123 million USD investment cost of a 500,000tpy P2O5 Phosphoric Acid Plant. Due to the high cost of electricity, the Thermal Process(Electric Arc Furnace Process) will not return a profit when used to make PhosphoricAcid. The Hydrochloric Acid route showed to be more unprofitable than the ThermalProcess if Hydrochloric Acid was purchased so it received the lowest ranking.Some of the assumptions made in this analysis includes that the phosphate rock used wasquality K09 from the Kingdom of Morocco, all sulfuric acid plants will havecogeneration with HRS, and that an ammonia plant is required for the implementation ofthe Nitrophosphate Process.IntroductionOver the years, many different Phosphoric Acid Processes have been developed, some ofwhich have fallen out of use and others have not yet been fully developed. This paper
will present the underlying economics behind several different routes of phosphoric acidproduction. The paper is based on generic processes for each of the routes rather thanprocesses offered by specific licensors.The Phosphoric Acid Production Processes that will be analyzed are as follows: • Dihydrate Process • Hemihydrate Process • Hemi-Dihydrate Process • Di-Hemihydrate Process • Hemi-Dihydrate Recrystallization Process • Nitrophosphate Process • Hydrochloric Acid Route • Thermal Process • Furnace ProcessIn the evaluation of these processes, the total cost to produce phosphoric acid will becompared against the total installed cost of a facility. The processes will then be rankedby determining the incremental rate of return between different facilities. To determinethe incremental rate of return, the increase in profit of a facility is divided by the increasein cost of a facility. If the incremental rate of return is greater than the rate of return ofthe facility it is being compared against, then this facility is considered to be moreeconomically efficient.To determine the cost of the different facilities, various news articles and technical paperswere consulted and these costs were scaled up to a 500,000 tpy P2O5 (1500 tpd P2O5)facility based on the 2010 USD. The total installed cost of the different facilitiespresented in this paper does not have any relation to what would offered as a competitivebid offered by Jacobs Engineering Group or by Jacobs Engineering S.A. (JESA).In the analysis of the different wet processes with acidulation with sulfuric acid, sulfurconsumption was set at 1t/t P2O5 for the Dihydrate Process and the sulfur consumptionwas increased or decreased based on the P2O5 recovery. This assumption was based onthe fact that citrate soluble losses (Cocrystallized Phosphoric Acid Losses) and watersoluble losses (Filtration Losses) would vary with P2O5 recovery and the citrateinsoluble losses (Unreacted Phosphate Rock Losses) will stay constant. The P2O5 that islost due to C.S. & W.S. losses first requires acidulation of phosphate rock into phosphoricacid. Therefore a decrease in P2O5 recovery increases the consumption of sulfur.The effect of gypsum quality and acid concentration on filtration rates was excluded fromthe analysis. In general, the filtration rate for hemihydrate tends to be higher than fordihydrate due to the irregularity of the hemihydrate crystals. Also, higher P2O5concentrations increase the filtration rate because the viscosity of phosphoric acidincreases exponentially with concentration.
Basis of ComparisonThe following Raw Material and Products Costs were used to estimate the operatingcosts:Raw Material / Product CostCalcium Ammonium Nitrate 250 USD/tonneHydrochloric Acid (33%) 160 USD/tonnePhosphate Rock 130 USD/tonneSulfur 150 USD/tonnePhosphoric Acid 800 USD/tonneNatural Gas 18.4 USD/GJPetroleum Coke 50 USD/tonneCalcium Carbonate 10 USD/tonneDihydrate for Building Material 35 USD/tonneHemihydrate for Plaster 50 USD/tonneSand 6 USD/tonneElectricity (Purchase Price) 50 USD/MWhElectricity (Selling Price) 25 USD/MWhDihydrateThe Dihydrate Process involves the acidulation of Phosphate Rock with Sulfuric Acidwhich results in the production of Phosphoric Acid and the precipitation of CalciumSulfate Dihydrate. Following acidulation, the Dihydrate is filtered from the reactor slurryand the phosphoric acid is sent to storage. In the evaluation of this process it will beassumed that phosphoric acid at a concentration of 28% P2O5 will be produced with anoverall recovery of 95%.The Dihydrate Plant considered in his comparison consists of a Sulfuric Acid Plant withcogeneration & HRS, Phosphate Rock Grinding, Reaction/Filtration, Filter AcidClarification & Storage, Phosphoric Acid Concentration, and Merchant Grade AcidClarification & Storage.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the DihydrateProcess was estimated to be 240 million USD.The annual profit for the Dihydrate Process was determined by tabulating the revenuefrom phosphoric acid and electrical sale and subtracting the cost of phosphate rock,sulfur, general operating costs, and gypsum disposal. General operating costs includeoperating labor, maintenance labor, reagents, plant overhead, maintenance, operatingsupplies, insurance, taxes, and depreciation. General operating costs was assumed to be40USD/t P2O5. With the exception of the Di-Hemihydrate process, all wet processeswith acidulation with sulfuric acid will include a gypsum stack. The cost of a gypsumstack was spread out over the operating lifetime of the plant. With all that said, the
annual profit of a phosphoric acid plant by the Dihydrate Process was determined to be88 million USD.HemihydrateThe Hemihydrate Process involves the precipitation of Calcium Sulfate Hemihydrate andthis process can produce phosphoric acid at a significantly higher concentration than theDihydrate Process. To promote the formation of Hemihydrate, the reactor must maintaina higher temperature and a higher %P2O5 than the Dihydrate process. The Hemihydrateprocess experiences significantly higher lattice losses and the overall P2O5 recovery istypically around 92%.The cost of a Hemihydrates Plant will differ from a Dihydrate Plant due to the following: • Less Phosphate Rock Grinding is required Since a Hemihydrate Reactor operates at a higher temperature and a higher %P2O5, a larger particle size can digested. This is further accomplished through the use of a low sulfate section followed by a high sulfate section in the reactor. • Number and size of Reactor Flash Coolers plus auxiliary equipment is different Two Reactor Flash Coolers are required in the Hemihydrate Process. Although the Reactor Flash Coolers are smaller in size, the cost of both greatly outweighs the single Flash Cooler required for the Dihydrate Process. • Reduced Size of Filter Acid Clarification & Storage Since phosphoric acid is at a higher concentration (40% P2O5) than the Dihydrate Process, the volume of filter acid that needs to be clarified and stored is significantly less. • Reduced Size of Phosphoric Acid Evaporation Evaporating phosphoric acid from 40% P2O5 to 54% P2O5 can be done with 46% less evaporator capacity than with the Dihydrate Process.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by theHemihydrate Process was estimated to be 224 million USD.The calculation of the annual profit for the Hemihydrate Process uses the same methoddescribed for the Dihydrate Process. The annual profit of a phosphoric acid plant by theHemihydrate Process was determined to be 80 million USD.
Hemi-DihydrateThis process is a two stage filtration process where Hemihydrate is crystallized andfiltered in the first stage and recrystallized to Dihydrate and filtered in the second stage.The Acid produced in the Hemihydrate stage is sent to storage, and the liquor from therecrystallization section is used on the Hemihydrate filter. The Hemi-Dihydrate processproduces a high strength phosphoric acid similar Hemihydrate process and overcomes thehigh lattice losses experienced in the Hemihydrate process by recrystallizing theHemihydrate into Dihydrate. The P2O5 recovery for this process is 98%.The differences between a Hemihydrate Plant and a Dihydrate Plant applies to a Hemi-Dihydrate Plant as well. The following differences are also included in evaluating a costof a Hemi-Dihydrate Plant to a Dihydrate Plant: • Recrystallization Tank is required The Recrystallization Tank is what transforms the Hemihydrate crystals into Dihydrate. The retention time of the Recrytallization Tank was assumed to be one hour at 40% solids. • Second Filter Building is required The addition of a second filter building is extremely expensive and this fact is what brings into question the economic efficiency of two stage processes. • Larger Fume Scrubber The increased filtration area, as well as the recrystallization tank require aeration. This increases the amount of air that needs to be scrubbed which increases the size of the fume scrubber. • Larger Overall Installed Capacity Due to discontinuities between filter washes of the first and second stages. A 5% extra overall capacity is required to produce the design capacity.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the Hemi-Dihydrate Process was estimated to be 271 million USD.The calculation of the annual profit for the Hemi-Dihydrate Process uses the samemethod described for the Dihydrate Process. The annual profit of a phosphoric acid plantby the Hemi-Dihydrate Process was determined to be 99 million USD.
Di-HemihydrateThe Di-Hemihydrate Process can be characterized as a Hemi-Dihydrate Process inreverse. The acidulation section consists of crystallization of Dihydrate, Dihydratefiltration, Hemihydrate Recrystallization, then Hemihydrate filtration. The claimedadvantage of this process is that the recrystallized Hemihydrate can be used as a buildingmaterial without any drying because all the residual moisture is absorbed when theHemihydrate recrystallizes again into Dihydrate.Like the Hemi-Dihydrate Process, the Di-Hemihydrate Process requires aRecrystallization Tank, a Second Filter Building, Larger Fume Scrubber, and LargerOverall Installed Capacity. For a description of these units, please refer to the previoussection regarding the Hemi-Dihydrate Process.The Di-Hemihydrate, Hemi-Dihydrate, and Hemi-Dihydrate Recrystallization Processesall produce relatively pure gypsum which can be sold as a construction material in mostcountries. For the Hemi-Dihydrate and Hemi-Dihydrate Recrystallization Processes,gypsum sale will not be considered because the profit loss to dry and sell the gypsum willbe greater than the cost of stacking gypsum. For the Di-Hemihydrate Process,Hemihydrate sale will be considered.The recrystallized Hemihydrate from the second stage will have a residual moisture of35%, if K09 phosphate is used. Of this 35% residual moisture, only 12% can beabsorbed leaving a 23% residual moisture. This 23% residual moisture makes drying theresulting Dihydrate for sale unprofitable. Even though more water needs to beevaporated to produce Hemihydrate, Hemihydrate can be sold at a higher price. Due tothe high fuel consumption of drying gypsum, a very small profit is returned forHemihydrate sale. A gypsum post-processing unit for Hemihydrate sale has beenestimated to be 35 million USD.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the Di-Hemihydrate Process was estimated to be 287 million USD without Gypsum post-processing and 322 million USD with Gypsum post-processing.The calculation of the annual profit for the Di-Hemihydrate Process uses the samemethod described for the Dihydrate Process except that gypsum stacking was replaced byHemihydrate sale. The cost of fuel and electrical consumption from the Gypsum post-processing unit was also included. The annual profit of a phosphoric acid plant by theDi-Hemihydrate Process was determined to be 108 million USD.Hemi-Dihyrate RecystallizationHemi-Dihyrate Recystallization is a single filtration phosphoric acid process whereHemihydrate is initially formed in a high temperature reaction with sulfuric acid. TheReactor Slurry is subsequently cooled in a set of crystallizers to recrystallize theHemihydrate into Dihydrate. Due to the recrystallization from Hemihydrate into
Dihydrate, the majority of the lattice losses from the initial Hemihydrate step arerecovered. The process produces phosphoric acid with a concentration of 32% P2O5 andthe theoretical P2O5 recovery is 98%.This process requires an enormous reaction section which significantly increases the costof the facility. The reaction section for this process is relatively small and the remainderof the reactor consists of the recrystallzation section. A large recrystallization volume isrequired for this process due to the fact that the recrystallization of Hemihydrate inDihydrate happens slowly at higher %P2O5 concentrations. Other additional costs aredue to additional equipment to cool the slurry in the Recrystallizers and a larger scrubberto handle the increased aeration of the reactor. Credit is also given for the smaller FilterAcid Clarification & Storage volume and the smaller Phosphoric Acid Evaporators.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the Hemi-Dihydrate Recrystallization Process was estimated to be 270 million USD.The calculation of the annual profit for the Hemi-Dihydrate Recrystallization Processuses the same method described for the Dihydrate Process. The annual profit of aphosphoric acid plant by the Hemi-Dihydrate Recrystallization Process was determinedto be 97 million USD.Nitrophosphate ProcessThe definition of a Nitrophosphate Process is a process that involves acidulation ofPhosphate Rock in Nitric Acid. The Nitrophosphate Process that will be considered is asfollows:Phosphate Rock is attacked with Nitric Acid the resulting mixture stays in solution.Calcium Nitrate Tetrahydrate (CNTH) is then crystallized at a temperature ranging from0-20°C, which is then filtered and the phosphoric acid is sent to storage. The CNTH isdissolved then reacted with CO2 to remove a portion of the calcium and the resultingCalcium Carbonate is separated. The solution is then ammoniated and granulated toproduce calcium ammonium nitrate (CAN). All other Nitrophosphate processes that havebeen physically or conceptually devised are beyond the scope of this paper.It was assumed that if a nitrophosphate process was to be implemented for the productionof phosphoric acid, the construction of an ammonia plant, nitric acid plant, nitrophoshateplant, and CAN granulation plant would be required. Due to the high volumes ofammonia required to produce nitric acid, it would not be practical to purchase ammoniatherefore an ammonia plant was included with the cost of a nitrophosphate complex.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by theNitrophosphate Process was estimated to be 1.1 billion USD.The total annual cost of Natural Gas, phosphate rock, electricity, general operating costsand catalyst for the nitric acid plant was compared against the sale of phosphoric acid and
CAN. The General Operating Costs were assumed to be 60 USD/t P2O5 for the entirecomplex which includes an ammonia plant, nitric acid plant, nitrophoshate plant, andCAN granulation plant. From this, the annual profit was determined to be 369 millionUSD.Hydrochloric Acid RoutePhosphoric Acid can also be produced by reacting phosphate rock with hydrochloric acid.Since the Calcium Chloride cannot be precipitated at a reasonable temperature range, thephosphoric acid is removed from solution via solvent extraction. This process canproduce super phosphoric acid and with less waste than a process that uses sulfuric acid.A limitation to this process is that it can only be implemented near the sea for disposal ofCalcium Chloride into the ocean. Therefore, the environmental impact of the HCl routeis much greater than a process with gypsum stacking.The total cost of phosphate rock, hydrochloric acid, electricity, plus 40 USD per tonne ofP2O5 of general operating cost was compared the sale price of 800 USD/tonne P2O5. Itwas determined that the production of phosphoric acid using hydrochloric acid could notreturn a profit in today’s market.The cost of a Plant to produce 500,000 t P2O5/year of phosphoric acid from acidulationwith hydrochloric acid was determined to be 300 million USD.Thermal ProcessThe Thermal Process (Electric Arc Furnace Process) was the primary mention ofproducing phosphoric acid before being dominated by the wet process with sulfuric acid.In the Thermal Process, phosphate rock, silica, and coke are fed to the furnace and anelectric current is applied. The calcium in the phosphate rock fuses with the silica whilethe carbon in the coke combines with the oxygen in the phosphate which allows P4 toevaporate, and the P4 is condensed once leaving the electric arc furnace. In theproduction of phosphoric acid, P4 is then burned forming P4O10 which is then scrubbedwith water to make phosphoric acid.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the ThermalProcess was estimated to be 379 million USD.The thermal process consumes about 13,000 kWh/t P2O5 and general operating costswere assumed to be 40 USD/t P2O5. The P2O5 recovery for the Thermal process wasassumed to be 92%. Applying these factors, it was determined that the Thermal Processcannot return a profit for the production of phosphoric acid.Furnace ProcessThe furnace process is similar to the Thermal Process, but the heat required to evaporateP4 comes from the burning of petroleum coke instead of from electricity. Upon the
evaporation of P4, the P4 reacts with the oxygen in the furnace to form P4O10. The gasladen with P4O10 is scrubbed with water to form phosphoric acid. The P2O5 recovery ofthe furnace process is 87%.The total cost of a phosphoric acid plant to produce 500,000 tpy P2O5 by the FurnaceProcess was estimated to be 363 million USD.In the evaluation of the annual profit of the Furnace Process, the total cost of phosphaterock, coke, silica, electricity, and general operating costs were compared against the saleof phosphoric acid. The annual profit was determined to be 103 million USD.ComparisonThe total cost of each facility along with the expected profit in today’s market and therate of return can be seen in the table below.Process Cost Annual Profit Rate of (Million USD) (Million USD) ReturnDihydrate 240 88 36.5%Hemihydrate 224 80 35.7%Hemi-Dihydrate 271 99 36.3%Di-Hemihydrate 322 108 33.6%Hemi-Dihydrate Recrystallization 270 97 36.0%Nitrophosphate 1100 369 33.5%Hydrochloric Acid Route 300 -396 -132%Thermal 379 -172 -45.4%Furnace 363 103 28.5%Even though the rate of return on investment for a phosphoric acid production facilityseems high, these numbers only consider the profit gained for the market conditions as ofearly 2011 and the time taken to construct a facility was not taken into account. Theconstruction time for the sulfuric acid based processes will be about the same since thesefacilities are very similar. The Nitrophosphate Process will have a higher constructiontime since it is a much larger facility.Some selected incremental rates of return are as follows:Base Process Upgraded to Incremental Upgrade Rate of Return Economically Efficient?Hemihydrate Dihydrate 49.0% YesDihydrate Hemi-Dihydrate 35.1% NoDihydrate Di-Hemihydrate 25.2% NoDihydrate Hemi-Dihydrate 31.9% No RecrystallizationDihydrate Furnace 12.9% NoDihydrate Nitrophosphate 32.7% No
By calculating the incremental rate of return between all the different facilities, theranking of the different plants by economic efficiencies developed. The result of thiscalculation gave the following result: 1. Dihydrate Process 2. Hemi-Dihydrate Process 3. Hemi-Dihydrate Recrystallization Process 4. Hemihydrate Process 5. Di-Hemihydrate Process 6. Nitrophosphate Process 7. Furnace Process 8. Thermal Process 9. Hydrochloric Acid RouteEven at the high cost of phosphate rock and sulfur, the Dihydrate Process still proves tobe the most economically efficient process.