Controllable reaction medium single reactor for phosphoric acid production

Apparatus for the continuous production of phosphoric acid from phosphate-containing rock and sulfuric acid, consisting of a reaction tank with a cylindrical side wall and a planar horizontal bottom and cover, wherein the improvement comprises a rigid vertical internal S-shaped baffle with one end smoothly merging into the said cylindrical side wall of the reaction tank and the other end freely projecting into the tank so as to permit of free circulation and flow of the reaction mixture from one side of the baffle to the other but dividing the reaction tank into two distinct zones.

This invention relates to a single compartment reactor for the manufacture 
of phosphoric acid by the wet process from phosphate-containing rock and 
sulfuric acid with resultant formation of calcium sulfate as a by-product. 
The decomposition of phosphate rock attacked by sulfuric acid involves a 
rather large reaction volume taking place at a low speed. 
The known process procedure for the continuous manufacture by the wet 
process of phosphoric acid involve the use of multiple tank reactors 
mounted in cascade or a single large reactor which may be divided into 
compartments. 
The cascade-type reactors differ widely from the point in their design and 
function. 
A smaller first vessel called pre-mixer is used for the preparation of a 
mixture formed of phosphate and recirculated slurry. A second tank is 
provided for the homogenization of the mixture pouring from the first 
similar vessel. 
The next succeeding tanks have higher capacities and are used for the 
achievement of reaction between the sulfuric acid and the phosphoric acid 
of a lower concentration resulting from the filtration with phosphate 
slurries at the preliminary mixing stage. 
The number and the capacity of such reactors are so selected as to achieve 
the complete reaction of the phosphate feed. At the end of the multiple 
tank reactors, the slurry is partly recirculated to the first pre-mixer 
and partly introduced into a final buffer tank for filtration. 
The cascade reactors permit control of the intermediate reaction stages but 
they have the inherent disadvantages of complexity and occupying a large 
space. 
The approach made to simplify the apparatus by the concept of a single tank 
reactor has resulted in a useful volume increase of 20% compared to the 
cascade reactors. However, the single reactor does not permit the correct 
control of the process flow because the intermediate stages are altogether 
eliminated. 
Other designs have been developed to improve the single reactor by 
providing it with compartments involving the intermediate control, but 
these single compartment reactors present themselves numerous 
disadvantages. 
Another form of single reactor conceived was in the form of a polygonal 
vat, the number of walls varying from six to ten, of which two opposite 
walls are longer. Centrally disposed along the longitudinal axis of the 
tank is a vertical wall of which one end extends through a baffle to a 
side wall of the reactor tank. The reactants entering in on one side of 
the central wall are evacuated through the opposite side. The orifice made 
in a second baffle permits a slurry recirculation (French Pat. No. 
1,526,980). 
This type of reactor still presents a number of difficulties. 
The polygonal configuration of the reaction tank makes the presence of 
corners and edges unavoidable, in which the solids from the suspension of 
reactants filter out thus limiting the reactor's capacity. The polygonal 
design in comparison with the cylindrical tank is less resistant to 
hydraulic shock. 
The seam joining the centrally-disposed vertical wall and the oblique 
baffle along the side wall and the connection of that baffle to the said 
wall are not likely to show greater resistance, but to lead to still 
pockets permitting solids to filter out. 
The orifice provided in the baffle for the purpose of slurry recirculation 
makes the reactants in the supply zone intermix with the final reaction 
product in the event that the agitators fail to operate. 
BRIEF SUMMARY OF THE INVENTION 
This invention relates to a reactor for production of phosphoric acid from 
phosphate-containing rock and sulphuric acid, which satisfactorily 
combines the advantages of cascade reactors concerning the process flow 
developing in stages and the control possibility during the reaction 
intermediate stages with those of a single tank regarding the space 
limitation. 
Another object of the invention is to eliminate the drawbacks demonstrated 
by the compartment reactors and the polygonal tank with central wall and 
baffle.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, the single tank reactor of the present 
invention shown comprises a generally cylindrical vessel having a bottom 2 
and a planar horizontal cover 3. Inside the reactor vessel there is 
located an S-shaped vertical "baffle 4, of which one end merges into the 
side wall 5 of" the reactor, the other end being free. At the junction 
with the side wall of the reactor vessel, the baffle face has an arcuate 
configuration which prevents the formation of still pockets for the 
reaction fluid and the deposit of solid substances from the slurried 
reactants. The baffle extends the full height of the tank, with its lower 
part resting solidly on the reactor bottom 2 and its upper end supporting 
the cover 3. 
It is obvious that the S-shaped baffle 4 could be reversed as a mirror 
image of the configuration shown in FIG. 1. 
The cover 3 is provided with a number of openings reguired for the 
introduction of reactants into the reaction tank at the points marked A, B 
and C. A plurality of agitators 9 are so arranged in the reactor as to 
hold the substances in suspension and assure a homogenized slurry flow. A 
pump 10 is fitted for withdrawing the reaction product. 
The cylindrical vessel wall is also provided with openings for slurry 
withdrawal and recirculation 6, a discharge outlet 7 at the reaction tank 
and an inspection manhole 8. 
The side wall 5, the reactor bottom 2 and the sinusoidal baffle 4 are made 
either of carbon steel plate or concrete with a rubber lining giving inner 
protection, anti-acid brick and graphite brick. The cover 3 is made from 
metal and is protected on its inner face with a rubber lining or high 
alloy steel. 
The S-shaped baffle divides the reaction tank into two zones. 
The first zone to the right of the baffle, as shown in FIG. 1 is the 
so-called reaction zone with a smaller section and capacity. 
The second zone adjacent the left face of the baffle is called the reaction 
completion zone, and has a higher capacity 
The approximate ratio of the cross-sectional area along the line D--D is 
1:2. 
The reactants consisting of phosphate containing rock, sulfuric acid, 
recirculated phosphoric acid, recirculated phosphate slurry with an 
anti-foaming agent are continuously introduced into the reaction zone 
through the inlets A and B, in the pockets formed between the right side 
of the baffle, the arcuate junction and the inner wall of the reaction 
vessel. 
The agitators located in this zone provide for rapidly homogenizing the 
reactant mixture. Due to the sinusoidal shape of the baffle, the reaction 
mixture is forced to travel up to and around the tip of the baffle and 
along its front side up to a point when the reaction product is withdrawn 
by the filter feed pump 10. 
The supply of reactants should be adjusted in such a manner as to permit a 
slight excess of SO.sub.4 ions in the reaction zone, approximating 
1.0-1.4%, which is thought to obtain the highest yield, and avoids the 
rapid precipitation of gypsum and coating of the mineral particles which 
would prevent the reaction coming to completion. 
At the end of the first reaction zone at the point mark C, the excess of 
SO.sub.4 ions appears to increase to 2.0-2.5% by the addition of sulfuric 
acid which causes the reaction to continue up to completion and the gypsum 
crystals to grow in the second reaction zone. 
About the middle of the reaction completion zone, by means of a pump 
installed at the point 6 outside the reaction vessel, part of the 
phosphate slurry is withdrawn, cooled down and reintroduced into the 
circulating reactants through the reactant inlet. 
The rest of slurry is directed to the filter pump. The ratio between the 
recycled slurry and that sent to filtration is of the order of 10:1. As 
outlined before, due to the sinusoidal configuration of the baffle 4, the 
area of the reaction zone is smaller than the reaction completion zone, 
which results in an increased rate of circulation and homogenization in 
the first zone, and to a longer residence time of the reaction mixture in 
the second zone. In the result of all this, there is obtained a suspension 
with large gypsum crystals which are uniform and easily filterable, and 
higher efficiency of the reaction exceeding 97%. 
The configuration of the baffle radius, its position on the bottom of the 
reaction vessel, the arrangement of the inlets for the reactants and 
outlets for the recycled slurry can be varied depending on the grade of 
the commercial phosphate used for processing. 
A variant of the invention shown in FIGS. 3 and 4 uses a pre-mixer wherein 
the reactant mixture is prepared before introducing it into the reaction 
vessel. 
The pre-mixer consists of a vertical cylinder 11, with a bottom 12 and a 
planar, horizontal cover 13, providing with a single agitator 14. The 
bottom and the side wall of the pre-mixer are made of carbon steel plate 
lined with rubber foil, anti-acid brick and graphite brick 15. Both the 
cover and the agitator are made from high alloy steel which is resistant 
to attack from phosphoric acid. 
Located on the cover 13 are openings 16 and 17 designed for introducing the 
reactants and a reaction gas exhaust outlet 18. The outlet 19 formed in 
the cylindrical wall 11 is intended for the discharge of the reaction 
mixture. 
A vertical baffle 20 prevents the reactants from flowing directly from the 
supply inlets 16 and 17 to the discharge outlet 19, and in this way 
homogenization of the mixture is assured. 
The capacity of the pre-mixer may widely vary but it is recommended that it 
bears a relation of 1 to 20 to the capacity of the reactor. The use of the 
pre-mixer renders the process of the flow control more flexible and 
permits higher efficiencies even when the phosphate grades used appear 
less reactive and have characteristics such as to make the processing 
difficult, i.e. have a higher content of organic substances. 
Depending upon the characteristics of the processed rock, the process may 
be varied. Thus, if the rock used has a high reactivity and a relatively 
high content of CO.sub.2 and organic matter, the feed introduced into the 
pre-mixer will consist of recirculated slurry and a mixture of 
recirculated phosphoric acid and sulfuric acid, while the phosphate rock 
is directly fed into the reactor. Alternatively, if the kind of rock used 
is of lower reactivity and tendency to foam, the recyled slurry along with 
phosphate rock is introduced into the pre-mixer and a mixture formed of 
recirculated phosphoric acid and sulfuric acid is added to the reactor 
proper. There are, however, cases when the rock used is of low reactivity 
and poor CO.sub.2 and organic contents and when the best obtainable 
results can be obtained if all the reaction media are introduced into the 
pre-mixer. In all cases, a close control of the concentration in SO.sub.4 
ions is necessary and the proper corrections must be made in the reactor 
as the process goes on. 
Two examples using the apparatus of the invention are given below: 
EXAMPLE 1 
A single reactor with an S-shaped baffle has been used to process 
finely-ground phosphate rock with the following composition: 
______________________________________ 
Percent (by weight) 
______________________________________ 
P.sub.2 O.sub.5 33.1 
CaO 51.3 
R.sub.2 O.sub.3 0.7 
CO.sub.2 4.4 
SiO.sub.2 3.4 
F 3.6 
Bound water and 
organic matter 1.7 
______________________________________ 
FIGS. 1 and 2 are schematic views of the single reactor according to the 
invention with the ratio between the cross-sectional areas of the first 
and second zones being 1:2. 
Through the feed hole made in the reactor cover 3 at the point marked A, 
recirculated slurry and finely-ground rock are introduced, while through 
suitably-disposed openings B and C suitable amounts of sulfuric acid and 
recirculated phosphoric acid enter the reactor. The amount of acid is 
adjusted to comply with the excess of SO.sub.4 ions in the slurry and 
enhance the ratio of decomposition. Due to the direction of the rotation 
of agitators 9 and configuration of baffle 4, the mixture of reactants is 
directed toward the second reaction zone. The agitators located in each 
zone permit the mixture to be homogenized. 
The reaction gases are vented off through an appropriate hood and are 
directed to a gas washer-scrubber. 
Before reaching the suction orifice 6, part of the slurry is sucked up by 
the recirculating pump 10 and returned to the circulating mass through 
feed hole A passing first through a vacuum cooler in order to maintain the 
optimum temperature required for the reaction. 
The slurry is conveyed to the submerged pump 10 for further treatment in 
the filter section. The ratio between the amount of recycled slurry and 
the slurry sent to the filters is about 10:1. 
The suspension obtained in this manner contained large and uniform gypsum 
crystals which were easily filterable and washable. The filtration was 
performed on a tilting-bucket filter. The resultant product contained 30% 
P.sub.2 O.sub.5 by weight of phosphoric acid and phospho-gypsum containing 
a total of 0.8-1.0% P.sub.2 O.sub.5 of which 0.2-0.3% was soluble P.sub.2 
O.sub.5 
EXAMPLE 2 
To the single reactor described in Example 1 having attached the pre-mixer 
that can be seen in FIGS. 3 and 4, finely-ground phosphate rock was added 
with the following average composition: 
______________________________________ 
Percent (by weight) 
______________________________________ 
P.sub.2 O.sub.5 
33 
CaO 52 
R.sub.2 O.sub.3 
0.5 
CO.sub.2 4.0 
SiO.sub.2 2.6 
F 3.7 
organic 
matter 0.3 
______________________________________ 
Recirculated slurry and recirculated phosphoric acid mixed with sulfuric 
acid were introduced into the pre-mixer. The agitator 14 effected an 
intimate and rapid homogenization of these reaction components that are 
successively passed by the overflow 19 to the single reactor provided with 
the sinusoidal baffle. 
The finely-ground rock was introduced into the reactor directly along with 
the proportion of sulfuric acid necessary for the correction of SO.sub.4 
ions. The rest of the process flow follows the scheme described in Example 
1. 
The slurry obtained contained macro-crystals of gypsum and exhibited the 
best filtration and washing characteristics. The filtration operation was 
developed on a tilting-bucket filter. 
The reaction products were: 30% P.sub.2 O.sub.5 phosphoric acid and 
phospho-gypsum containing less than 0.8% P.sub.2 O.sub.5 (total) of which 
0.3% were soluble P.sub.2 O.sub.5.