CHEMICAL INSTALLATION FOR THE PRODUCTION OF PHOSPHORIC ACID EQUIPPED WITH A PHOSPHOGYPSE ​​EVACUATION SYSTEM

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to a chemical plant for the production of phosphoric acid, which is equipped with a system for removing the phosphogypsum obtained primarily during the synthesis of phosphoric acid. At the end of the synthesis, the phosphogypsum precipitate is separated from the phosphoric acid by filtration and then removed from the chemical plant via the removal system. ETAT DE LA TECHNIQUE

[0002] A phosphoric acid production plant includes a reactor in which the phosphoric acid is synthesized. The reaction product from this synthesis contains phosphoric acid, which is recovered, but also a phosphogypsum precipitate, which is extracted from the reaction product by filtration and collected in a suitable receiving device, usually a hopper.

[0003] Phosphogypsum filtration is a well-known and commonly used process in the chemical industry. Generally, a horizontal rotary filter with pivoting screens is used. Such filters are described, in particular, in US patents 2004 / 0089599 and 938378. These filters comprise filter cells arranged to form a cylinder and are rotatable around the cylinder's axis. Once filtration is complete, the cylinder is tilted, and the filtrate, in this case the phosphogypsum precipitate, is transferred to a receiving device located directly above the filter, such as a hopper. RU patent 2008 254 C1 describes a chemical plant for the production of phosphoric acid according to the preamble of claim 1.

[0004] Once the phosphogypsum is collected in the hopper, it is usually discharged using a carrier fluid, most often water. However, this method has the drawback of significantly wetting the phosphogypsum, making it very heavy and therefore more difficult to move. Furthermore, most phosphogypsum recovery processes require it to be dry. The water jet discharge method significantly increases the subsequent drying time, resulting in substantial time and financial losses.

[0005] To solve this problem, a motorized screw conveyor, positioned at the hopper outlet, can be used instead of a water jet. The phosphogypsum collects in the screw's flutes and is then discharged by its rotation. Phosphogypsum is thus discharged "dry," meaning without the use of a carrier fluid, and in particular without water. This screw conveyor system also has the advantage of allowing control of the phosphogypsum discharge rate by adjusting the screw's rotation speed and its intrinsic characteristics, such as the dimensions of its flutes.

[0006] However, the auger's splines regularly become clogged and blocked, significantly reducing the discharge rate. This causes frequent system shutdowns, resulting in considerable downtime and additional production costs.

[0007] Furthermore, the method of removing phosphogypsum by the screw conveyor does not allow control of the fraction, or relative quantity, of phosphogypsum removed compared to the initial quantity of filtered phosphogypsum arriving at the hopper inlet, since all of the phosphogypsum at the hopper outlet is removed by the screw conveyor. EXPOSE DE L'INVENTION

[0008] The present invention therefore aims to remedy the drawbacks of the prior art by proposing a chemical plant for the production of phosphoric acid comprising a phosphogypsum evacuation system allowing control of the fraction, also designated as the relative quantity, of phosphogypsum evacuated, in relation to the initial quantity of phosphogypsum obtained after filtration at the inlet of the receiving device.

[0009] The chemical plant's phosphogypsum removal system is also designed to remove phosphogypsum "dry," meaning without the use of a carrier fluid, such as water, which is the most commonly used carrier fluid. The removed phosphogypsum is then readily usable, and its physical and chemical properties remain unchanged.

[0010] To this end, the invention relates to a chemical installation for the production of phosphoric acid, comprising: a reactor for the production of phosphoric acid, a device for receiving the phosphogypsum obtained after filtration of the phosphoric acid produced in the reactor, a system for evacuating phosphogypsum from the receiving device, wherein the drainage system comprises a chute whose inlet is located inside the receiving device and is arranged to communicate with said receiving device, and a movable flap about an axis of rotation, said flap being capable of opening at a predetermined angle corresponding to a predetermined passage section of a fraction of phosphogypsum through the inlet of the chute, and to close in order to prevent the passage of phosphogypsum into the chute.

[0011] Phosphogypsum is removed from the chemical plant via a dry process. The fraction of phosphogypsum removed is controlled, notably by adjusting the opening of the damper, and any potential loss of phosphogypsum during removal is minimized by positioning the chute inlet within the receiving device.

[0012] Depending on other optional characteristics of the chemical plant for the production of phosphoric acid, taken individually or in technically feasible combinations: the opening angle of the flap is between 0° when said flap is closed and 90°, preferably 75°, when said flap is open; the flap is operated by at least one pneumatic or electric cylinder; the evacuation system further includes a conduit communicating with the outlet of the chute, allowing the evacuation of phosphogypsum from the chute; the flap opens towards the interior of the phosphogypsum receiving device.This is particularly advantageous when the cross-section of the chute is greater than that of the pipe, and the flap is designed to operate in the chute and therefore has a size that allows it to move in the chute but not in the pipe; the axis of rotation of the flap is arranged in the chute at the outlet of the chute communicating with the pipe; the chemical installation further includes a second phosphogypsum evacuation system, the latter allowing the evacuation of a second fraction of the quantity of phosphogypsum initially present at the inlet of the receiving device, so that the sum of the two fractions corresponds to said quantity of phosphogypsum initially present at the inlet of the receiving device; .

[0013] The invention also relates to a method of evacuating phosphogypsum from a phosphogypsum receiving device, in a chemical installation as defined above, comprising a step of opening the access flap to the chute at a predetermined angle, and a step of circulating a fraction of phosphogypsum from the receiving device into the chute.

[0014] According to another optional feature, the process further includes a step of transferring the phosphogypsum fraction from the chute to a pipe, allowing the phosphogypsum to be removed from the chute. BRÈVE DESCRIPTION DES FIGURES

[0015] Other advantages and features of the invention will become apparent from the following description, given by way of illustrative and non-limiting example, with reference to the attached Figures which represent: There Figure 1 , a general side view of the phosphogypsum evacuation system within a chemical plant for the production of phosphoric acid, according to the invention; The Figure 2 , a cross-sectional view along axis AA of the drainage system Figure 1 There Figure 3A , a schematic diagram of the phosphogypsum evacuation system according to the invention, in which the flap is closed; The Figure 3B , a basic diagram similar to that of the Figure 3A , in which the shutter is half open; The Figure 3C , a basic diagram similar to that of the Figures 3A et 3B in which the shutter is fully open. DESCRIPTION DETAILLEE DE L'INVENTION

[0016] The following description relates mainly to a phosphogypsum evacuation system from a receiving device within a phosphoric acid production facility, and to a process for evacuating phosphogypsum from such a receiving device.

[0017] The phosphogypsum evacuation system and process are described in parallel in the remainder of this text in order to simplify the presentation.

[0018] The phosphogypsum drainage system is specifically, but not exclusively, designed to be used in conjunction with another drainage system, referred to hereafter as a third-party system, of which only the fluid nozzles are shown in the diagram. Figures 1 And 2 .

[0019] At the entrance to the chemical installation, the incoming phosphogypsum F is filtered by a suitable filtration device, allowing the phosphogypsum to be separated from the phosphoric acid during the synthesis of phosphoric acid.

[0020] The phosphogypsum removal system is located downstream of the filtration device. It is particularly, but not exclusively, suited for removing phosphogypsum that has been previously filtered by a horizontal rotary filter with pivoting screens.

[0021] Once the phosphogypsum has been filtered, it is poured into a receiving device, which is usually a hopper as shown with reference 2 on the Figures 1 And 2 This refers to the quantity of phosphogypsum initially present at the inlet of the receiving device, or more simply the initial quantity of phosphogypsum, denoted Mi.

[0022] A fraction of the initial quantity of phosphogypsum, or relative quantity, is then removed by the removal system (partial removal), while the remaining fraction is preferably removed by a third system, usually well known to the person skilled in the art.

[0023] A phosphogypsum fraction is defined as the ratio between a defined quantity of phosphogypsum and the initial quantity Mi of phosphogypsum.

[0024] The evacuation system therefore allows control of the quantity MS (in volume or mass for example) of phosphogypsum evacuated by dry means by said system, and the associated fraction FS corresponding to the quantity MS of phosphogypsum evacuated divided by the initial quantity Mi of phosphogypsum (FS = MS / Mi).

[0025] On the other hand, it allows, indirectly, the control of the quantity MH (in volume or mass for example) of phosphogypsum evacuated by wet means by the third-party system, and the associated fraction FH corresponding to the quantity MH of phosphogypsum evacuated divided by the initial quantity Mi of phosphogypsum (FH = MH / Mi) whose evacuation is most often carried out via a carrier fluid.

[0026] With reference to Figures 1 And 2, the phosphogypsum evacuation system 1 includes a chute 3 whose inlet 4 communicates with the hopper 2, in order to allow the passage of a fraction FS of phosphogypsum from the hopper 2 to the chute 3 when necessary.

[0027] Advantageously, chute 3 is partially located inside the hopper, so as to prevent any loss of phosphogypsum outside the discharge circuit. In particular, the chute's inlet 4 is advantageously located inside hopper 2 for this same purpose.

[0028] The dimensions of the chute 3 and its positioning relative to, and where applicable inside, the hopper 2 are adapted both to maximize the evacuation capacity of the phosphogypsum by the evacuation system 1 out of the hopper, and to minimize the bulk of the portion of the chute located in the hopper.

[0029] In particular, when the phosphogypsum has been filtered by a horizontal rotating filter with pivoting cloths, the inlet 4 of the chute 3 allows for the collection of some of the phosphogypsum discharged by the filter cloths, and the dimensions of the chute 3 as well as its positioning relative to, and where appropriate within, the hopper 2 are adapted to maximize the flow of phosphogypsum falling from a pivoting cloth and passing orthogonally through the passage section 13 of the chute 3.

[0030] The phosphogypsum evacuation system 1 further includes a movable flap 6 rotating about an axis 7, the latter being substantially horizontal, that is to say substantially parallel to the ground on which the chemical installation including the evacuation system rests.

[0031] The flap 6 is advantageously positioned near the entrance 4 of the chute 3. Depending on the embodiment of the Figure 2 , axis 7 of flap 6 is arranged at the entrance 4 of the chute, and allows the passage of phosphogypsum from hopper 2 into chute 3.

[0032] According to another embodiment illustrated on the Figures 3A, 3B, et 3C , axis 7 of flap 6 is arranged in the chute at the outlet 5 of the chute, and jointly allows the passage of phosphogypsum from hopper 2 into chute 3 and the passage of phosphogypsum from the chute to the pipe 11 located in the extension of the chute.

[0033] The flap 6 permits or prohibits the passage of phosphogypsum into the chute 3 respectively by opening or closing by rotation around its axis 7. In addition, preferably, the flap 6 opens towards the inside of the hopper 2 and is located, when open, inside the hopper.

[0034] Preferably, flap 6 is trapezoidal in shape.

[0035] With reference to Figures 3A, 3B, et 3C , the opening angle of the flap 6, noted α, allows adjustment of the passage section 13 of the phosphogypsum in the chute and then in the conduit 11, and thus control of the fraction of phosphogypsum evacuated by the evacuation system 1.

[0036] Thus, the larger the opening angle α of flap 6, the larger the cross-sectional area 13 of phosphogypsum through the inlet 4 of the chute, and the greater the fraction of phosphogypsum removed by the drainage system 1. Conversely, the smaller the opening angle α of flap 6, the smaller the cross-sectional area 13 of phosphogypsum through the inlet 4 of the chute, and the lower the fraction of phosphogypsum removed by the drainage system 1.

[0037] By extension, other associated quantities such as, for example, the phosphogypsum evacuation flow rate (with continuous operation of the filtration device and the evacuation system) by the evacuation system, are also known and controlled by varying the opening angle of the flap.

[0038] The opening angle α of the shutter 6 is preferably between 0° when said shutter is closed, and 90°, more preferably 75°, when the shutter is open.

[0039] Preferably, the flap 6 is actuated by one or more pneumatic cylinders 8, the operation of which is shown on the Figures 3A, 3B et 3C .

[0040] One end of the cylinder is fixed to the pipe 11 which communicates with the outlet 5 of the chute 3, said pipe allowing the dry phosphogypsum to be conveyed to a belt conveyor 12.

[0041] The free end of the cylinder 8 is articulated to an arm 9, itself articulated, and preferably fixed, to the flap 6. When the flap is closed, on the Figure 3A Cylinder 8 is compressed and arm 9 is lowered. The phosphogypsum cannot then pass through chute 3. The extension of cylinder 8 causes arm 9 to rise, which in turn causes flap 6 to open by rotating around its axis 7. The phosphogypsum can then pass through chute 3 and enter conduit 11.

[0042] Thus, the shutter is partially open at an angle α1, corresponding to a passage section 13, on the Figure 3B , and fully open at an angle α 2 , corresponding to a larger passage section 13, on the Figure 3C .

[0043] Alternatively, shutter 6 is operated by one or more electric motor(s). Of course, shutter 6 can also be operated by any other suitable means designed for this purpose.

[0044] The phosphogypsum passes through chute 3 and then from chute to pipe 11, to be conveyed to belt conveyor 12. Flexible flaps 13 are advantageously provided to ensure sealing between the outlet of pipe 11 and the belt conveyor 12. The latter then handles the phosphogypsum to a defined area.

[0045] It should be noted that the chute 3, the flap 6, and the conduit 11 are designed to ensure optimal phosphogypsum conveying speed without a carrier fluid. Furthermore, the materials used in these three components are resistant to corrosion and abrasion caused by phosphogypsum.

[0046] The remaining phosphogypsum fraction FH, not recovered by the phosphogypsum removal system 1, is collected at the bottom of the hopper 2 and then removed from said hopper by the third system. As previously mentioned, this is generally a water jet generated by nozzles 10 designed for this purpose. The flow rate of the water jet is controlled by valves according to the phosphogypsum flow rate in order to minimize water and energy consumption.

[0047] At the outlet of the chemical plant, a dry fraction (FS) of phosphogypsum was recovered via the phosphogypsum discharge system 1, and a wet fraction (FH) of phosphogypsum via the third system. These two fractions are controllable and adjustable via the opening angle α of the damper 6; that is, the relative quantities of phosphogypsum discharged by each discharge system can be adjusted in relation to the initial quantity Mi of filtered phosphogypsum.

[0048] We then know the ratio of dry fraction to wet fraction R = FS / FH .

[0049] Controlling the R ratio allows for a better response to industrial needs arising subsequently to the described process.

[0050] In conclusion, the described phosphogypsum evacuation system allows for the evacuation of a phosphogypsum flow rate independently of the production rate of the filtration device, and without altering the physical and chemical properties of the phosphogypsum, including the moisture content and phosphoric acid content, and without disturbing the operating parameters of the filtration device. REFERENCES

[0051] US 2004 / 0089599 US 938378

Claims

1. Chemical plant for the production of phosphoric acid, comprising : - a reactor for the production of phosphoric acid, - a receiving device (2) for receiving the phosphogypsum obtained after filtration of the phosphoric acid produced in the reactor, - a phosphogypsum evacuation system (1) from the receiving device (2), the chemical installation being characterized in that the evacuation system (1) comprises a chute (3), the inlet (4) of which is located inside the receiving device (2) and is arranged so as to communicate with the said receiving device (2), and a flap (6) which is movable about a rotation axis (7) said flap (6) being able to open at a predetermined angle (α) corresponding to a predetermined section of passage (13) of a fraction of phosphogypsum through the inlet (4) of the chute (3), and to close in order to prevent the passage of phosphogypsum into the chute.

2. The chemical plant of claim 1, wherein the angle (α) of opening of the flap (6) is between 0° when said flap is closed and 90°, preferably 75°, when said flap is open.

3. The chemical plant according to any of the preceding claims, wherein the flap (6) is actuated by at least one pneumatic or electric cylinder (8).

4. The chemical plant according to any of the preceding claims, wherein the phosphogypsum evacuation system (1) further comprises a pipe (11) communicating with the outlet (5) of the chute (3), allowing the discharge of phosphogypsum from the chute (3) to said pipe (11).

5. The chemical plant of claim 4, wherein the flap (6) opens into the interior of the receiving device (2) for receiving the phosphogypsum.

6. The chemical plant according to one of claims 4 or 5, wherein the rotation axis (7) of the flap (6) is arranged in the chute (3) at the outlet (5) of the chute communicating with the pipe (11).

7. The chemical plant according to any one of the preceding claims, further comprising a second phosphogypsum evacuation system, the latter allowing the discharge of a second fraction of the amount of phosphogypsum initially present at the inlet of the receiving device (2), so that the sum of the two fractions corresponds to said amount of phosphogypsum initially present at the inlet of the receiving device (2).

8. A method of discharging phosphogypsum from a phosphogypsum receiving device (2) in a chemical plant according to one of claims 1 to 7, comprising a step of opening the access flap (6) to the chute (3) at a predetermined angle (α), and a step of circulating a phosphogypsum fraction from the receiving device (2) into the chute (3).

9. The method according to claim 8, further comprising a step of transferring the phosphogypsum fraction from the chute (3) to a pipe (11), allowing the phosphogypsum to be discharged from the chute (3).