TECHNOLOGY
Lower-Cost Processes Developed for Phosphoric Acid Production Ready for commercialization, both routes offer producers ways to counter the effects of high energy costs and decreasing rock quality Joseph Haggin, C&EN Chicago
Two new processes for the production of phosphoric acid have been developed and are ready for commercialization. One is the BESA-2 process, developed by Bohna Engineering & Research, San Francisco; the other is Occidental Research Corp/s KPA process. But whether they will be put into service in the near future will depend on the evolving economics of a depressed industry. At present, phosphoric acid manufacture is dominated by two processes. Fertilizer-grade phosphoric acid usually is made by the wet
process—digesting phosphate rock with sulfuric acid. And higher-grade phosphoric acid and phosphates ordinarily are made from elemental phosphorus produced in electric furnaces. High electricity costs and decreasing rock quality adversely affect the economics of the phosphate and fertilizer industries and cause related adverse effects in the upstream sulfuric acid business. Electricity is estimated to account for more than one third of the cost of making highpurity furnace acid. In recent years, escalation of electricity costs has helped depress an already endangered market. The decreasing quality of phosphate rock significantly increases the cost of wet-process acid, which is similarly suffering from the depletion of high-grade ore resources and the corresponding increase in natural impurities. P h o s p h o r i c acid is t h u s in a chicken-and-egg situation that has yet to be resolved. But whatever
the present state of the business, there are at least two new processes available for commercialization. And both offer the prospects for significantly reduced manufacturing costs. The BESA-2 process is b e i n g readied for commercialization by the exclusive licensee, Bechtel Petroleum Inc. Bechtel performed its own evaluation of the process in 1984 and subsequently acquired an exclusive license to design and build plants. Planning is now under way for construction of a demonstration plant to prove the process on a commercial scale, according to Bechtel's Allen Rubin. In the BESA-2 process, phosphate rock and sulfuric acid react in two steps. The first is the conventional means of producing single superphosphate, gypsum, and monocalcium phosphate. In the second step, there is digestion of the single superphosphate in methanol with sulfuric acid.
BESA-2 process employs two reaction steps Vent
i i
Fluorine scrubber
Ground phosphate* rock H20«
Sulfur-
• To gypsum pond 20% P205
Single superphosphate plant 93% H 2 S0 4
Sulfuric acid plant
Digestion and extraction Gypsum
Extract solvent recovery
Extract filtration
Superacid concentration
Fjlter cakeT
— Residue solvent recovery
T
64% P 2 0 5
72% P205 Solvent storage
Methanol
I Gypsum slurry
Discard
Methanol
October 21, 1985 C&EN 23
Technology
Economics are attractive for BESA-2 process Process
Capacity, tons of P2O5
Superacid grade
Plant cost, $ millions
Market price, $ per ton P2O5
Furnace acid BESA-2 acid Wet-process acid BESA-2 acid
300 300 1100 1100
Technical Technical Fertilizer Fertilizer
$ 79 59 116 106
$738 453 320 300
Note: Comparative economics are for U.S. Golf Coast location, third-quarter 1984 U.S. dollars. Source: Bechtel Petroleum
tel. The BESA-2 process is projected to be significantly more attractive than either competitor. The second new method, the KPA process, has been developed through the pilot stage by Occidental Research Corp. It reduces phosphate ore to phosphorus in a high-temperature rotary kiln. It utilizes the heats of combustion of phosphorus and carbon monoxide in the kiln to provide most of the energy required for reduction and replaces the electric power used in the furnace process with fossil fuel as the energy source. The process has been piloted with the aid of an industrial consortium under the general direction of Occidental. According to the developers, the kiln is a good heat transfer device but a rather poor device for mass transfer. This means that the kiln will operate with welldefined oxidation and reduction zones. In the KPA process, the kiln is fed countercurrent to the hot kiln gases. The feed consists of spherical
The slurry of the extract and the gypsum are separated by settling and decantation. Clean phosphoric acid is recovered by evaporation of the methanol. Acid of ultrahigh purity may be obtained by ion-exchange of the extract prior to evaporation. Extraction of impurities is thereby avoided. Instead the impurities are rejected during manufacture and are discharged with residue solids. The BESA-2 process produces acid at a 64% P2O5 content without further concentration. Rubin claims that, because of the conventional first step in the process, rock quality is not critical. The process, he says, also may be retrofitted to existing wet-process production plants. There is considerable flexibility after the first step. One of the advantages claimed for the process is a combination of lower product-acid viscosity and less tendency to form sludge, both the result of low impurities content. Comparative costs for BESA-2, furnace, and wet-process phosphoric acids have been developed by Bech-
KPA process uses rotary kiln Vent
t Gas scrubbing Water
Coke ^ ^ Raw feed Silica —i+> prep Phosphate — + > rock
1
1
L
24
October 21, 1985 C&EN
Fuel
1
\ Balling preheat
t 1
JL_.
r* 1 1 | 1
t
Rotary kiln
Acid absorption
1 Heat recovery
1 Air
Acid cleaning
1
l
Spent solids
Product acid (70% P205)
pellets (Vrinch diameter) made from phosphate ore, coke, and silica. The pellets are tumbled in the usual way and travel slowly through the kiln. Within the solid layer of the pellets, reduction occurs, generating elemental phosphorus vapor and carbon monoxide. When these vapors transfer to the gas phase, above the pellet layer, oxidation to phosphorus pentoxide and carbon dioxide occurs. The heat generated in the oxidation is transferred back into the pellet layer, mostly by radiation. The kiln operates at solids temperatures of up to 1500 °C. The hot gases leaving the kiln pass through a cyclone to remove dust and then to an absorber, where the P2O5 is absorbed in a stream of phosphoric acid. The cooled gases are further scrubbed of any fluorine compounds and acids, and the remainder are vented to the atmosphere. Additional filtration can be used to remove ultrafine solids, if necessary. Acid strengths of more than 70% P2O5 can be produced in this way. Spent pellets exit through a grate cooler where residual heat is recovered in an air stream. The heated air is used as secondary air for the kiln and for dryers in feed preparation. Cooled pellets are disposed of by dumping. Tests have demonstrated that spent pellets would not be classified as hazardous wastes. The major requirements for a successful process are reaction rates fast enough to accommodate available residence times in the kiln, avoiding excessive melting of the solids in the beds, and avoiding excessive oxidation of the carbon reductant. Controlling the Si0 2 /CaO ratio in the pellets appears to control the phosphate reduction and melting in the pellets. The high silica content effectively makes any melts formed too viscous to be troublesome. Control of the air injection effectively controls carbon oxidation. A computer simulation of the process indicates that the scaleup to a large commercial unit would result in better conversion than that in the pilot plant. Yields of more than 90% are predicted for kilns over 300 feet long, with carbon losses of about 10%. These losses might be dimin-
KPA process reactions take place in kiln Elemental phosphorus Is extracted from fluorapatite In phosphate ore by reduction with carbon at high temperature: CaioO(P04)6F2 + 15C + 9SI0 2 — %P4 + 15CO + 9CaO-SI02 + CaF2 (AH = 12,000 Btu per lb P4) In producing phosphoric acid, the phosphorus is burned with carbon monoxide in two separate reactions: PA + 50 2 — 2P 2 0 6 (AH = -11,000 Btu per lb P4) CO + 1 / 2 0 2 - * C0 2 (AH = -9700 Btu per lb P4) The last two reactions liberate more than enough heat to drive the reduction.
ished with appropriate process controls. Economic projections for the KPA process aren't available, but it is expected to be more cost effective than the conventional wet process. •
Salvaging plastic from bottles gains ground Moves are under way in Europe to recover poly (ethylene terephthalate) (PET) from used bottles made from the polymer. A U.K. company has linked up with a West German firm to offer a package system claimed to have distinct technical and economic advantages over others now available. And, in West Germany, Coca Cola Co. is operating a trial PET bottle return program. The new polymer recovery system, called Plas/PET, stems from many years of collaboration between Amberger Kaolinwerke (AKW) of Hirschau (some 60 miles east of Nurnberg, West Germany), a specialist manufacturer of waste recycling plants, and Plas/Tech, a supplier of reprocessing technology, based in Birmingham, England. "The result," claims Rod Fox, Plas/ Tech's director, "is a highly efficient and cost-effective integrated
system that produces recycled PET to a quality suitable for a wide range of applications." Key to the Plas/PET system is a series of separation steps using hydrocyclones operating at acceleration rates of up to 200 G. The first of these removes polyethylene and p o l y p r o p y l e n e (most commonly used for making the bottles' base support cups), light mineral cont a m i n a n t s , a n d p a p e r from t h e shredded, granulated, and washed PET containers. Next comes air drying, followed by removal of residual paper, aluminum, and any colored PET polymer. This involves proprietary technology that AKW declines to reveal. Dehumidifying reduces entrained moisture to about 0.0001% prior to extrusion, pelletizing, and crystallizing of the recovered PET. These latter steps took up to three years to perfect, notes Guido Ropertz, AKW's manager of secondary raw materials recovery systems. Ongoing studies are aimed at removing
AVAILABLE FROM SNPE AROMATIC CHLOROFORMATES
ETHYLHEXYL NITRATE
INTERMEDIATES
CETANE IMPROVER FOR DIESEL OILS
o-ct &
C4H9-CH-CH2-0N02 i
CI
C2H5
CARBAMATES
DIMETHYL CARBONATE
PESTICIDES
INTERMEDIATE
gr
0 - C - NHCH3
CH3-0^ C= 0
ii
0 SNPE, YOUR PARTNER IN PHOSGENE CHEMISTRY AND NITRATION. SNPE, PRESENT IN 56 COUNTRIES. CHEMICAL DEPT12 QUAI HENRI IV 75181 - PARIS 04 FRANCE. PHONE: 33.1.42771570. TLX: 220380 F.
CH3-0
GSSNPE October 21, 1985 C&EN
25
