From phosphate rock to phosphoric acid advantages over conventional wet methods
by this flowsheet-five
PHOSPHATE
+/
ROCK DRYER
GRINDER
5-STAGE COUNTERCURRENT LEACHING SYSTEM
n20-
A LT ER NAT IV E
TRAVELING PAN FILTER
I
C. C. LEGAL, Jr., J. N. PRYOR, T. 0. TONGUE, and P. L. VELTMAN Davison Chemical Co., Division of W.
R.
Grace & Co., Baltimore 3, Md.
Phosphoric Acid by the Clinker Process Strong, wet, relatively pure phosphoric acid is obtained directly from phosphate rock without concentration
C o . in the United Srates. \vith CONSIDERABLE Dorr Kunsrdunger-Patent Ver~vertungs
effort has been directed toward development of a \vet phosphoric acid process, in Ivhich strong, relatively pure phosphoric acid can be directly obtained without concentration but no such method has gained widespread commercial acceptance. All the wet acid produced in the United States and most of that produced in the bvorld is manufactured by a relatively weak phosphoric acid process (5); strong acid is obtained by concentration. The manufacture of phosphoric acid by \vet methods has been studied by many investigators. beginning about 1872 (5). One of the most active, the
334
X.G., Sivitzerland! has developed a so-called strong acid process which directly gives a product of about 33y0 phosphorus pentoxide content. This process has en.joyed practically exclusive commercial acceptance. Tordengren in Siveden produced a strong acid by an anhydrite process. Coleman (3:.J) developed a wet process for producing phosphoric acid of high concentration and high purity in u p to 51Yc phosphorus pentoxide concentration. Shoeld of the Davison Chemical Co. produced 40 to 42% phosphorus pentoxide acid by treating
INDUSTRIAL AND ENGINEERING CHEMISTRY
granulated s i i ~ ~ e ~ ~ ~ ~ ~ 11 i oi tsl ipsull'iiric. Ii;it~~ acid and then lcaching o i i t the risii1i;inr phosphoric acid (7). .\ n r ~ vrnc.iliot1 for obtainilq srrong ivct phosphoric acid \%.ithou t co tic en rra ti on is dr sc ri hccl I I r r r . Chemistry of Phosphate Rock Conversion Ll'hcn phosphatc rock is treatcd \vir11 sulfuric acid, the fluorapaiite can bc converted to compounds of varjing percentages of phosphorus pcntoxide. depending upon the amount o f sulfuric acid used. The equation sho\cing the
Dried and ground phosphate rock and strong sulfuric acid are mixed together in proportion to form a puttylike solid and convert all the calcium oxide to calcium sulfate anhydrite with a slight excess of sulfuric acid. I n so doing, substantially all of the phosphorus pentoxide is converted to phosphoric acid. The reaction produtt is heated at 200' to 240' C. for 20 to 60 minutes to form a hard clinker. The phosphoric acid is separated from the insoluble calcium sulfate by countercurrent extraction using filtration or a leach tank system,
conversion of phosphate rock to phosphoric acid is:
+ lOHzSO4 = GHsPOi + + 2HF (reacts with SiO? in
rock to give SiF4)
The chemistry of converting the phosphate to phosphoric acid is relatively simple as shown. However, process difficulties arise when the separation of the phosphoric acid from the calcium sulfate is attempted. Any process which deals with this separation is termed a phosphoric acid wet process or simply a wet process. Commercial processes currently used yield a relatively weak, impure product suited primarily for the fertilizer industry. Despite this, more wet process acid is produced than electric furnace acid. During 1955, 1,478,000 tons of phosphoric acid (calculated to 50% phosphoric acid) were produced from elemental phosphorus, a 6% increase over 1954; 1,962,000 tons of phosphoric acid were produced by wet processing, a 26% increase over 1954 (2). A demand exists for a cheap, relatively impure phosphoric acid. A process which directly yields a concentrated phosphoric acid in a more pure form would meet an even wider demand.
Materials and Procedure All experimentation was conducted using Florida pebble phosphate rock. Different samples were used, of varying composition and screen analysis as shown in Table I. The process was developed using 98% technical grade sulfuric acid exclusively. Laboratory Procedure. The sulfuric acid was heated to approximately 100 O C. and intimately mixed with the ground phosphate rock, using a kitchen-type mixer,' for about 1.5 to 2 minutes. At this stage, the mixture formed stiff puttylike particles, about to 3/4 inch in size. Copious quantities of fluorine compounds were evolved during mixing. Temperatures attained were 210' to 235' C. The mix was placed in trays and heated for 20 to 70 minutes at a temperature of 200' to 240' C. At this point the mass was hard and somewhat porous. During the heating process fluorine compounds continued t o be evolved. The dried product or clinker was immediately processed in a countercurrent leaching system made up of either leach columns or filters. In this process the clinker was dumped without cooling into the strong acid wash, from which the product was withdrawn, treated with successively weaker portions of phosphoric acid, and finally washed with hot water. Five stage extraction was required in the leach tank system to gain
Exploratory Experiments Extensive laboratory experiments were carried out in attempts to find conditions whereby phosphate rock could be treated with concentrated sulfuric acid to yield a readily filterable calcium sulfate, and, a t the same time, give a phosphoric acid product of high strength. Direct acidulation of phosphate rock gave a slimy, nonfilterable calcium sulfate, from which the phosphoric acid could not be extracted in a reasonable time. Not until heating experiments were conducted was it possible to extract the phosphoric acid. A process (Figure 1) was evolved as follows:
Table 1.
Moisture, yo PsOa (dry basis), %
BPL % coz, % B, %
CaO, % RrOa, % Lb. H z S Orequired/lb. ~ rock to convert to H s P O ~
a 49 to SOTophosphorus pentoxide phosphoric acid product and eight-stage extraction in the filtration system to gain a 47 to 48y0 phosphorus pentoxide product, The calcium sulfate by-product was discarded to waste. In one test 198 grams of 98?, sulfuric acid heated to 90" C. in a 6.5-inch porcelain casserole was mixed with 240 grams of rock (sample 2, Table I) using an electric mixer of the eggbeater type. Attained temperature of the mix was 220' C. The mix (see Table I1 for screen analysis) was transferred to a tray and heated for 1 hour in an oven maintained at 250' C. The material temperature was measured a t 230' C. for 30 minutes. Chemical analysis showed 97.8% water-soluble phosphorus pentoxide and 98.2YG citrate-soluble phosphorus pentoxide. Then, without cooling, the mixture was placed in 42.5% phosphorus pentoxide quench liquor for acid extraction. A weight ratio of 0.777 quench liquor to 1 of clinker yielded 0.47 weight parts of 47% phosphorus pentoxide phosphoric acid product and 1.02 weight parts of filter cake after eight washing stages. Details on filtration are given in Table 111. Analysis (wet basis) of the cake gave: moisture, 24.96%; total phosphorus pentoxide 1.OO% ; citrate-insoluble phosphorus pentoxide, 0.567, ; available phosphorus pentoxide, 0.44Yc ; waterinsoluble phosphorus pentoxide, 0.20%. A material balance showed 94.8% recovery of the total phosphorus pentoxide and 99.0% recovery of the available phosphorus pentoxide. Pilot Plant Operation
In the pilot plant, rock and hot 98% sulfuric acid were mixed together continuously in a pug mill mixer. Rock feed was set as high as 300 pounds per hour. The mixer product was discharged to either a Roto-Louvre dryer or cocurrent direct-fired dryer fitted with lifting flights. The dried product was caught in insulated cans and discharged to a five-stage leach tank sys-
Rock Analysis and Sulfuric Acid Requirements 1
2
Sample No. 3
4
0.94 33.32 72.80 2.35 3.61 46.33 2.66 0.808
1.33 33.14 72-50 2.46 3.60 46.70 2.44 0.816
0.52 31.57 68.98 3.53 3.61 46.12 2.41 0.807
0.40 30.85 67.41 4.15 3.72 46.30 2.01 0.809
0.25 30.60 66.99 4.17 3.56 45.51 2.92 0.795
1.4 2.2 20.8 34.4 41.2
6.4 5.8 36.0 17.2 34.6
8.6 6.4 13.6 27.0 44.4
1.2 4.0 5.2 28.8 60.8
4.6 9.6 30.S 8.0 47.0
Screen anal3:ses, % On 60 On 80 On 100 On 200
- 200
VOL. 49, NO. 3
MARCH 1957
335
Table II.
Screen Analysis of Dryer Feed Cumulati~-e .o
On 2 On 6 On 12 O n 28 Through 2 8 Total
ci
CI
Tyler-hIeih
~
,c
...
2.5 45.0 29.4 19.4 3.7 100.0
47.5 76.9 96.3
tem, where the phosphoric acid product was extracted from the calcium sulfate. The filtration sjstem !vas not proved on a pilot plant scale. Failure to prove out the process using a rotarv pan-type filter was attributed to mechanical rather than process difficulties. A DorrOliver traveling pan filter ( 7 ) \vould, it is believed, be suitable. Operating Factors
Effect of Rock Particle Size. The particle size of the phosphate rock affects the filtering or leaching properties of the clinker in the extraction s)-stem. Acidulation with coarse rock is unsatisfactory because the larger rock particles remain unreacted and a sloppy mix results, Extremely fine rock is objectionable because the clinker slakes down to a mud and cannot be leached or filtered. The rock qrind should be controlled ivithin the limits of 40 to 55% through 200 mesh for best results. Moisture Content of Rock. The moisture content of the rock affccts the physical condition of the mix. It should not be greater than lyGand preferably not greater than 0.5. Phosphate Rock-Sulfuric Acid Ratio. The ratio of phosphate rock to sulfuric acid should be controlled, so that the residual or free sulfuric acid in the phosphoric acid product is vithin 1 to 2yc. T h e approximate amount of sulfuric acid required can be calciilated from the total calcium content of the phosphate rock, based on the stoichiometric requirements. Holyever, the ex-
act requirements are determined by analyzing the phosphoric acid product. ; i deficiency of sulfuric acid ultimately leads to filtering problems; hoxever, an excess simply dilutes the phosphoric acid product. Effects of Mixing. The phosphate rock and sulfuric acid are mixed until ail the phosphate rock is \vetted by the sulfuric acid. Mixing must be stopped short of reaching a dense compressed mass. The mix should be slightly damp but not sticky. One and one half to 4 minutes of mixing is usually sufficient to gain the proper condition. The particle size distribution of a mix is $\,en in Table 11. Effect of Sulfuric Acid Temperature. Stickiness and particle size of rhe mix should he kept at a minimum. They can he decreased to a great extent by increasing the acid temperature. Temperatures of 88' to 105' C. are satisfactory. Effect of Drying, Drying is necessary to condition the rock-acid mix for the succeeding leaching step. It so modifies the properties of the mix that an impossible process becomes practical. Tf filtration lvithout previous drying is atrempted. the rock-acid mix slakes down to a n impervious mud. .After proper drJ-ing, eight filtration stages may be carried out in approximatel>- 2 minutes. Heating time is less critical than heating temperature. The time can be varied from 20 to 60 minutes at a particular temperature to give a good filtering clinker. The material temperature of the clinker can be varied bettveen 200' and 240' C. An)- variation from these temperatures gives poor leaching. The dr!.ing can be carried out in either a stationary oven o r a rorary dryer. I n the laboratory sEationary and rotary dr)irig \rere used interchangeabl!.. \vhile in the pilot plant. rotary drying \vas used exclusively. Eirher a Roto-Louvre or a dryer fitted n i t h lifting flights gave satisfactory operation. The rotary dryers ivere direct oil fired. using cocurrent Ao\v of heating gases. Inlet temperatures were controlled to give the material temperatures required. The exact reason why a specific set of drying conditions gives a nonslaking
Filtering Time and Strengths of Wash Liquors during Filtration of Clinker Slurry in a Bed 2 Inches Deep Filtet ing Time,
Filtrate S o . 1 Product 2
Quench liquor
3 4 5 6 7 8
336
Seconds
Baume'
PnO5, %
5 25
56-57 51 40 27 17
47.0 42.5 33.5 22.5 14.0 8.5 3.5 1.5
16
12 10 10
8
10 10
4 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
Materials of Construction Corrosion trsts were rtiadr 0 1 1 a t i m i brr of difyerenr metals and alloys in connection with choosing proprr materials of construction for use in contact with the phosphoric acid. Lead and Type 316 stainless steel gave thi. l x s t i.c:sults for use in leach tanks a n d filters. 'Type 3 1 6 stainlcss steel save {he best results of any materials that arc rryularly made into filtrr rloth. .UO)YX 20 or Carlientcr 20 stainless strcl and Teflon \\'ere satisfactor),. but becausr of rxliciisr and limitrd forms in Ii-hicii thc facturcd are limited in thc, strel gave satisfactory starvice for t h r mixer shell. suitablr matc.i,ial o f ~011struction for thr mixer blades rciriains a problem. Table I\' gives results of corrosion tests Fluorine Evolution The LISC of high teniper~aturrs. strong sulfuric acid, and hiqh acid-rock ratio rcsulrs in a considcralA>,grcarrr fluorine r\,olution from tiic rock t h a n is ciistomarily found in normal or tri1)1
