Process and apparatus for the continuous preparation of alpha calcium sulfate hemihydrate

Abstract

A process for the continuous production of alpha calcium sulfate hemihydrate is described, the process including mixing particulate gypsum, a crystal habit modifier, and water to form a gypsum slurry. The process further includes calcining the gypsum slurry to provide an alpha calcium sulfate hemihydrate slurry, filtering the alpha calcium sulfate hemihydrate slurry to separate at least one fluid stream from the slurry, measuring a first control parameter of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry, and combining the at least one fluid stream with additional gypsum, additional water, and additional crystal habit modifier to form an additional gypsum slurry, the process further including varying an amount of the additional crystal habit modifier in response to the measured value of the first control parameter. An apparatus for performing the described process is also provided.

Description

[Technical field]

[0001] The present invention relates to a process for the continuous preparation of alpha calcium sulfate hemihydrate. The present invention further relates to an apparatus for providing this process. [Background technology]

[0002] Gypsum is calcium sulfate dihydrate (CaSO 4 2(H 2 Gypsum occurs naturally as a raw material in the form of calcium sulfate hemihydrate (CaSO). Gypsum-containing products such as plasterboard contain calcined or anhydrous gypsum, i.e. calcium sulfate hemihydrate (CaSO 4 0.5(H 2 O)) with water to form a settable slurry which is then cast into a desired shape. The calcium sulfate hemihydrate reacts with the water and rehydrates to dihydrate crystals which are then allowed to harden or dry to a solid state.

[0003] Calcium sulfate hemihydrate can be classified into two basic forms, alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate. Beta calcium sulfate hemihydrate is typically formed by heating gypsum in air at ambient or elevated humidity and ambient (i.e., atmospheric) pressure. Alpha calcium sulfate hemihydrate, on the other hand, is produced from gypsum in steam or water under conditions of high temperature and pressure. Both alpha and beta calcium sulfate hemihydrate can be used to create gypsum products, with gypsum products made with alpha hemihydrate tending to be harder and have higher strength and density.

[0004] In commercial industrial settings where gypsum products are produced continuously, it is often desirable or necessary to produce alpha calcium sulfate hemihydrate continuously, which can be more difficult than a batch production process due to the need to control the production process parameters continuously and in real time.

[0005] Continuous wet calcination processes for the production of alpha calcium sulfate hemihydrate typically involve combining gypsum particles with water, and then the gypsum slurry is calcined at elevated temperature and pressure conditions to produce an alpha calcium sulfate hemihydrate slurry. The alpha calcium sulfate hemihydrate particles are then separated from the majority of the water in the slurry in a filtration or separation process to produce an alpha calcium sulfate hemihydrate cake and a separate water stream.

[0006] Alpha calcium sulfate hemihydrate produced in solution via the "wet alpha" process is a single crystal with distinct crystal faces. The aspect ratio (length vs. width) of the alpha calcium sulfate hemihydrate crystals is influenced by elements, compounds, and substances present in the gypsum slurry during the calcination process. These elements, substances, and compounds, known as crystal habit modifiers, can change the aspect ratio of the alpha calcium sulfate hemihydrate particles by preferentially adsorbing onto certain faces of the crystals during formation and substituting themselves into the crystal lattice or by reacting with calcium and / or sulfate ions in the solution to reduce the concentration of these ions. Crystal habit modifiers can be present due to impurities in the gypsum used in the calcination process (e.g., magnesium from dolomite) or can be purposely incorporated into the gypsum slurry (e.g., succinic acid).

[0007] In commercial processes, for the production of alpha calcium sulfate hemihydrate, a crystal habit modifier is used to ensure that the particles produced have the desired aspect ratio. Generally, this desired aspect ratio is a width to length ratio of about 1, preferably in the range of 0.8:1 to 1.2:1. In commercial processes today, the most commonly used crystal habit modifier is succinic acid. The aspect ratio of the alpha calcium sulfate hemihydrate particles produced during continuous calcination is sensitive to the concentration of succinic acid used, with the optimum succinic acid level often being understood to be 0.86 g per liter of slurry, or 1.2 g per kg of gypsum, where the water-gypsum ratio is 1:1. Generally, about one-third of this amount is believed to be consumed during the calcination process. Thus, a significant proportion of the succinic acid introduced into the calcination process remains in the alpha calcium sulfate hemihydrate slurry upon completion of the calcination process.

[0008] Since succinic acid and other crystal habit modifiers are often relatively expensive consumables in the alpha calcination process, there is an incentive to reduce waste of these materials. Although it is very difficult to recover crystal habit modifiers that are bound to particles or otherwise and retained within the alpha calcium sulfate hemihydrate cake, crystal habit modifiers may also be found in the water stream separated from the alpha calcium sulfate hemihydrate during the filtration process. Thus, these crystal habit modifiers may be recovered and reused by combining this water stream with fresh water and gypsum particles at the start of the calcination process.

[0009] However, such methods of recovering the crystal habit modifier have drawbacks. As previously described, the aspect ratio of the alpha calcium sulfate hemihydrate particles produced in the calcination process can be sensitive to the concentration of succinic acid or other crystal habit modifiers. Thus, small variations in the concentration of the crystal habit modifier in the gypsum slurry can have a large effect on the quality of the alpha calcium sulfate hemihydrate particles produced. Small differences in the amount of crystal habit modifier recovered in the water stream are often seen in the continuous process due to small changes in the quality of the gypsum input. Thus, there is a need to improve the consistency of the alpha calcium sulfate produced by the prior art systems. Summary of the Invention

[0010] According to a first aspect of the present invention, a process for continuous production of alpha calcium sulfate hemihydrate is provided, the process including mixing particulate gypsum, a crystal habit modifier, and water to form a gypsum slurry; calcining the gypsum slurry to provide an alpha calcium sulfate hemihydrate slurry; filtering the alpha calcium sulfate hemihydrate slurry to separate at least one fluid stream from the slurry; measuring a first control parameter of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry; and combining the at least one fluid stream with additional gypsum, additional water, and additional crystal habit modifier to form an additional gypsum slurry, the process further including varying an amount of the additional crystal habit modifier in response to the measured value of the first control parameter.

[0011] In this manner, a process is provided in which a parameter of at least one fluid stream or alpha calcium sulfate hemihydrate slurry is measured to allow for monitoring the quality and consistency of the process output over time. The process includes varying the amount of additional habit modifier in response to the measurement of the first control parameter. In this manner, the quality and characteristics of future alpha calcium sulfate hemihydrate slurries produced by the process can be varied depending on the characteristics of the current alpha calcium sulfate hemihydrate slurry produced by the process. In this manner, the characteristics and / or quality of the alpha calcium sulfate hemihydrate can be adjusted and optimized during the continuous production process.

[0012] Preferably, the process includes measuring a first control parameter of both the at least one fluid stream and the alpha calcium sulfate hemihydrate slurry. In this manner, the process can be more closely monitored, potentially allowing for a higher level of control and / or more precise control.

[0013] Preferably, the process comprises comparing the measured value of the first control parameter with a set value. Preferably, the set value is predetermined. More preferably, the amount of the further crystallization habit modifier is increased if the measured value is lower than the set value. Alternatively, the amount of the further crystallization habit modifier is decreased if the measured value is lower than the set value.

[0014] Preferably, the step of measuring the first control parameter comprises continuously measuring said first control parameter. If the first control parameter is continuously measured, this may provide the advantage of allowing continuous control of the process. Alternatively, the step of measuring the first control parameter comprises periodically measuring said first control parameter. Preferably, the period between measurements is 1 hour or less, more preferably 30 minutes or less, even more preferably 10 minutes or less, and most preferably 1 minute or less.

[0015] Preferably, the first control parameter is the pH of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry. If the crystallization habit modifier comprises an acid, measuring the pH may provide a quantification of the amount of crystallization habit modifier remaining in the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry after the calcination process.

[0016] Alternatively, the first control parameter is a volumetric flow rate of at least one fluid stream or the alpha calcium sulfate hemihydrate slurry. The volumetric flow rate may provide a quantification of the amount of water trapped between the alpha calcium sulfate hemihydrate particles in the slurry. The amount of water bound to the particles is related to the shape of the particles. Since the shape of the alpha calcium sulfate hemihydrate particles depends on the concentration of the crystal habit modifier in the slurry, the volumetric flow rate can be used to infer the amount of crystal habit modifier present in the slurry during the calcination process.

[0017] Preferably, the process comprises the further step of measuring a second control parameter of at least one fluid stream or of the alpha calcium sulfate hemihydrate slurry. More preferably, the step of measuring the second control parameter comprises periodically measuring said second control parameter. Preferably, the period between measurements of the second control parameter is longer than the period between measurements of the first control parameter, for example the period between measurements of the second control parameter is 1 hour or more, more preferably 6 hours or more, even more preferably 12 hours or more, most preferably 1 day or more.

[0018] Preferably, the step of measuring the second control parameter comprises the addition of an enzyme. More preferably, the addition of the enzyme comprises adding succinyl-CoA synthetase. Preferably, the addition of the enzyme comprises adding pyruvate kinase. Preferably, the addition of the enzyme comprises adding L-lactate dehydrogenase. Most preferably, the addition of the enzyme comprises adding succinyl-CoA synthetase, pyruvate kinase, and L-lactate dehydrogenase, in that order.

[0019] Preferably, the process comprises analysing the product of the addition of the enzyme by ultraviolet light. More preferably, the ultraviolet light has a wavelength of 340 nm. Preferably, the process comprises detecting the presence and / or amount of nicotinamide adenine dinucleotide (NAD+) in the product of the addition of the enzyme.

[0020] Preferably, the step of measuring the second control parameter includes an assay by which succinic acid is converted to nicotinamide adenine dinucleotide (NAD+). Such an assay may allow for quantification of succinic acid in the at least one fluid stream and / or the alpha calcium sulfate hemihydrate slurry, since such an assay converts succinic acid to NAD+ in a 1:1 ratio. Preferably, the second control parameter is the concentration of succinic acid in the at least one fluid stream or the concentration of succinic acid in the alpha calcium sulfate hemihydrate slurry.

[0021] Preferably, the step of measuring the second control parameter comprises ion chromatography. More preferably, the step of measuring the second control parameter comprises ion chromatography using an inductively coupled plasma detector. More preferably, the step of measuring the second control parameter is performed using an in-line ion chromatography device.

[0022] Preferably, the step of measuring the second control parameter comprises near infrared spectroscopy. Preferably, the method further comprises measuring a near infrared transmission spectrum. Alternatively, the method further comprises measuring a near infrared reflectance spectrum. Preferably, the measuring step comprises a Fourier transform.

[0023] Preferably, the step of measuring the second control parameter comprises combining a copper and / or copper ion source with at least one fluid stream or alpha calcium sulfate hemihydrate slurry. Preferably, the method further comprises measuring the presence and / or amount of one or more copper complexes in the at least one fluid stream or alpha calcium sulfate hemihydrate slurry. More preferably, the copper complexes measured comprise copper(II) and succinate ions. More preferably, the complexes consist of a dimer in which each copper atom is linked through two carboxylate oxygen atoms from a tetradentate succinate ligand and four nitrogen atoms from two chelating 2,20-bipyridyl groups, with the succinate ligand bridging the two Cu atoms.

[0024] Preferably, the process includes calibrating the measurement of the first control parameter with the measurement of the second control parameter, in this way drift or error in the measurement of the first control parameter over time can be detected and calibrated against.

[0025] Preferably, the process includes measuring the first control parameter more frequently than measuring the second control parameter.

[0026] Preferably, the step of measuring the second control parameter includes taking a sample of at least one of the fluid streams or the alpha calcium sulfate hemihydrate slurry and measuring the second control parameter offline. In this way, a wider variety of analytical techniques can be implemented to measure the second control parameter. Furthermore, it may be possible to more accurately determine the second control parameter using offline measurements. Here, offline should be interpreted to mean that the sample is removed from the apparatus such that the measurement of the second control parameter occurs when the sample is isolated from the fluid streams and slurries in the apparatus.

[0027] Preferably, the measurement of the second control parameter is slower than the measurement of the first control parameter. Thus, the measurement of the first control parameter may be regular or constant, while the measurement of the second control parameter may be more intermittent.

[0028] Preferably, the measurement of the second control parameter is more accurate than the measurement of the first control parameter, where more accurate should be interpreted as meaning that the measurement has a smaller range of experimental error.

[0029] Preferably, the crystallization habit modifier comprises succinic acid.

[0030] Preferably, the alpha calcium sulfate hemihydrate slurry contains particles having a length to width ratio of 0.8:1 to 1.2:1 inclusive. More preferably, the alpha calcium sulfate hemihydrate slurry contains particles having a length to width ratio of about 1:1. Such a length to width ratio may be preferred to ensure good slurry fluidity throughout the process.

[0031] According to a second aspect of the present invention, there is provided an apparatus for providing the process described above, the apparatus comprising: a mixing zone for mixing a habit modifier, water, and gypsum particles; a calciner containing or located downstream of the mixing zone and in fluid communication with the mixing zone; a filter located downstream of the calciner and in fluid communication with the calciner; a fluid path for carrying at least one fluid stream from the filter and for introducing at least one fluid stream into the mixing zone; and at least one measurement device configured to measure a first control parameter of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry, the apparatus further comprising control means for modifying an amount of habit modifier added to the mixing zone in response to the measured value of the first control parameter.

[0032] Thus, an apparatus is provided that is capable of providing the advantageous processes described above.

[0033] Preferably, the mixing zone is in a mixing vessel.

[0034] Preferably, the apparatus comprises a second measurement device configured to measure a second control parameter of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] An embodiment of the present invention will now be described, by way of example only, with reference to Figures 1-3. [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 shows a diagram of a process for the continuous production of alpha calcium sulfate hemihydrate. [Diagram 2] FIG. 2 shows a diagram of a second embodiment of the process of FIG. [Diagram 3] FIG. 3 shows a diagram of a third embodiment of the process of FIG. [Figure 4] FIG. 4 shows a graph containing data obtained from batch calcination experiments plotting filtrate pH versus aspect ratio of dried gypsum particles obtained from the corresponding slurries.

[0037] The process 100 shown in Figure 1 includes mixing particulate gypsum 101, a crystal habit modifier 102 (here, succinic acid), and water to form a gypsum slurry. The gypsum slurry is calcined in a wet calcination step 104 to provide an alpha calcium sulfate hemihydrate slurry 103. To achieve low aspect ratio crystals and to maintain a low viscosity during calcination 104, the gypsum slurry includes 0.86 g / l of succinic acid 102. After calcination 104, the alpha calcium sulfate hemihydrate slurry 103 includes particles with a length-to-width ratio of about 1:1, ensuring good slurry fluidity throughout the process 100.

[0038] A filtration step 105 follows, in which the alpha calcium sulfate hemihydrate slurry 103 is filtered to separate a primary fluid stream 109 from the slurry 103. In this manner, excess fluid is removed from the slurry 103, thereby allowing the formation of a plaster cake 106. In this embodiment, the filtration step 105 comprises a Buchner filtration.

[0039] To improve the efficiency of the process 100 and to reduce costs, heat demands, and resource demands, the water containing the crystallization habit modifier 102 is recycled and re-injected into the gypsum slurry. Thus, after the filtration step 105, the primary fluid stream 109 is separated in a separation step 107 into a secondary fluid stream 110 containing fluid for re-injection into the process 100 and excess water 108 for removal. The excess water 108 is removed for reuse or disposal. For example, the excess water 108 that is not re-injected into the gypsum slurry can be reused within the process 100, such as in a board line to warm the slurry. The secondary fluid stream 110 is combined with additional gypsum 101, additional water, and additional crystallization habit modifier 102 to form additional gypsum slurry.

[0040] The primary fluid stream 109 flows substantially continuously during the process 100, and after filtration 105 the primary fluid stream 109 is pumped into a mixing tank.

[0041] However, to ensure that the concentration of the crystal habit modifier 102 in the further gypsum slurry remains constant and at an optimal level, the concentration of the further crystal habit modifier 102 mixed with the further particulate gypsum 101 and the further water must be monitored and adjusted. Thus, the process 100 includes a regulation loop 111 for regulating the concentration of the crystal habit modifier 102 in the gypsum slurry taking into account the concentration of succinic acid in the secondary fluid stream 110 in the further crystal habit modifier 102. In this embodiment, the concentration of succinic acid in the secondary fluid stream 110 is about 0.7 g / l and the concentration of succinic acid in the crystal habit modifier 102 is about 20 g / l. The regulation loop 111 includes a proportional-integral-derivative (PID) controller.

[0042] The process 100 includes measuring a first control parameter CP1 of the secondary fluid stream 110. Succinic acid is consumed both during the calcination 104 and during the filtration 105, as well as by dilution mechanisms. For example, the concentration of succinic acid 102 tends to decrease after the calcination 104, because bound water is liberated and succinate ions remain to some extent adsorbed on the surface of the alpha plaster crystals. Thus, the pH of the filtrate is greatly influenced by the concentration of the crystallization habit modifier 102, i.e., succinic acid. Thus, measuring the pH value of the secondary fluid stream 110 as the first control parameter CP1 provides an indication of the concentration of the crystallization habit modifier 102 in the secondary fluid stream 110.

[0043] The step of measuring the first control parameter CP1, i.e., the pH of the secondary fluid stream 110, includes measuring the first control parameter CP1 every minute. In this manner, the first control parameter CP1 is measured regularly. The first control parameter CP1 is compared to a predetermined set value. In response to the comparison against the set value, the regulation loop 111 adjusts the concentration of the crystallization habit modifier 102 in the gypsum slurry by modifying the amount of the further crystallization habit modifier 102 added to the gypsum slurry. The amount of the further crystallization habit modifier 102 is increased if the measured value is lower than the set value, and the amount of the further crystallization habit modifier 102 is decreased if the measured value is higher than the set value.

[0044] The process 100 includes the further step of periodically measuring a second control parameter CP2 of the secondary fluid stream 110 offline and calibrating the measurement of the first control parameter CP1 with the measurement of the second control parameter CP2. In this way, drift or errors in the measurement of the first control parameter over time can be detected and eliminated by calibration. In this embodiment, the step of measuring the second control parameter CP2 includes the addition of an enzyme. The period between measurements of the second control parameter CP2 is one day. The addition of the enzyme can therefore be used as a periodic control to detect process deviations.

[0045] The enzyme addition is carried out as follows: In the presence of succinyl-CoA synthetase (SCS), succinate anions present in the fluid stream (by dissociation of succinate) are converted to succinyl-CoA by adenosine triphosphate (ATP), resulting in the formation of adenosine diphosphate (ADP) and inorganic phosphate, according to the following reaction pathway: Succinate ion + ATP (SCS) → Succinyl CoA + ADP + Pi

[0046] In the presence of pyruvate kinase (PK), ADP then reacts with phosphoenolpyruvate (PEP) to form pyruvate and ATP: ADP+PEP (PK) →ATP + pyruvate

[0047] Finally, pyruvate converts L-lactate and NAD + It is reduced by nicotinamide adenine dinucleotide (NADH) in the presence of L-lactate dehydrogenase (L-LDH) to: Pyruvate + NADH (L-LDH) →L-lactate + NAD +

[0048] NAD formed during the last reaction step + is stoichiometrically related (1:1) to the succinate ion content. As a result, the initial succinate ion content can be derived from the NADH consumption measured by UV absorbance at 340 nm. Finally, the succinate concentration (in g / l) can be expressed as: C AS =0.5136ΔA NADH In the formula, ΔA NADH is the variation in absorbance (uA) of NADH during the enzymatic reaction. The enzyme addition schedule described above can be used as part of any embodiment of the present invention.

[0049] The measurement of the second control parameter CP2 is slower than the measurement of the first control parameter CP1, and the measurement of the first control parameter CP1 occurs more frequently than the measurement of the second control parameter CP2. In this way, the measurement of the first control parameter CP1 is periodic, while the measurement of the second control parameter CP2 is more intermittent, allowing for off-line and more accurate measurements.

[0050] In the following description, like reference numerals are used for like parts of the second embodiment of the invention outlined in Figure 2. A process 200 for the continuous production of alpha calcium sulfate hemihydrate is shown, where the crystallization habit modifier 202 comprises succinic acid. The process 200 of Figure 2 is similar to the process 100 of Figure 1 with the following differences:

[0051] During the filtering step 205, the alpha calcium sulfate hemihydrate slurry 203 is filtered to separate a primary fluid stream 209 from the slurry 203. In this embodiment, prior to the separating step 207, the process 200 includes measuring a first control parameter CP1′ of the primary fluid stream 209 using a flow meter 215.

[0052] The first control parameter CP1′ is the volumetric flow rate of the primary fluid stream. The volumetric flow rate provides a quantification of the amount of water bound to the alpha calcium sulfate hemihydrate particles in the gypsum slurry. Because the shape of the alpha calcium sulfate hemihydrate particles depends on the concentration of the crystal habit modifier 202, the volumetric flow rate can be used to infer the amount of crystal habit modifier 202 present in the slurry during the calcination process 204.

[0053] A separation step 207 follows, where the primary fluid stream 209 is separated into a secondary fluid stream 210 and excess water for removal. Measurement of the second control parameter CP2′ occurs after the separation step 207, where the second control parameter CP2′ of the secondary fluid stream 210 is periodically measured. The measurement of the first control parameter CP1′ is calibrated with the measurement of the second control parameter CP2′, and a regulation loop 211 regulates the concentration of the further crystallization modifier 202 in the further gypsum slurry. The step of measuring the second control parameter CP2′ includes the addition of an enzyme as described with respect to FIG. 1 and occurs offline.

[0054] The process 200 further includes a PID controller 212 for controlling the separation step 207 .

[0055] 3, a third embodiment of a process 300 of the present invention is shown, similar to process 100 of FIG. 1. Here, a crystallization habit modifier 302, also succinic acid, is pumped into a gypsum slurry 313 via pump 314. Process 300 includes measuring a first control parameter CP1″ of the alpha calcium sulfate hemihydrate slurry 303 rather than a control parameter of at least one fluid stream. The first control parameter CP1″ is the volumetric flow rate of the alpha calcium sulfate hemihydrate slurry 303 and is measured using a first flow meter 315.

[0056] The process 300 also includes a filtration step 305, in which the alpha calcium sulfate hemihydrate slurry 303 is filtered using belt filtration to separate a primary fluid stream 309 from the slurry 103. The efficiency of the filtration step 305 can be determined by the shape of the plaster crystals and therefore the succinic acid concentration. A reliable filtration efficiency criterion is the relative amount of water removed by belt filtration in the filtration step 305, which is an indicator of the water demand of the plaster. If the calcination process is well controlled, i.e., there is a suitable water-to-gypsum ratio and 100% conversion to alpha calcium sulfate hemihydrate, and the filtration is stable, the amount of water extracted during the filtration step 305 can be used to adjust the concentration of succinic acid 302, as shown in FIG. 3. The adjustment of the concentration of succinic acid 302 takes into account the residence time of the calcination process.

[0057] The step of measuring the second control parameter CP2" involves the addition of enzyme as described with respect to Figure 1 and occurs offline. Process 300 further includes calibrating the measurement of the first control parameter CP1" with the measurement of the second control parameter CP2", as described with respect to Figures 1 and 2.

[0058] In some embodiments, a second flow meter is positioned to measure the second control parameter CP2″. In such embodiments, the volumetric flow rate of the primary fluid stream 309 is the second control parameter CP2″. In this manner, both the first control parameter CP1″ and the second control parameter CP2″ are measured online. By calibrating the measurement of the first control parameter CP1″ with the measurement of the second control parameter CP2″, any errors in the measurement from the alpha calcium sulfate hemihydrate slurry 303 relative to the measurement from the primary fluid stream 309 can be detected.

[0059] experiment As part of the development of the present invention, the inventors conducted a series of tests. Alpha calcium sulfate hemihydrate slurries were produced by batch wet calcination in a reactor using succinic acid as a habit modifier. In three experiments, four different gypsum sources were used, and in each case the gypsum was calcined at a temperature of 135±5° C., a pressure of 300000±50000 Pa, and a water to gypsum ratio of 1 for 40 minutes.

[0060] In each case, after calcination, the resulting alpha calcium sulfate hemihydrate slurry was filtered to recover 69% to 77% of the water introduced into the reactor as filtrate. After removal of the filtrate using a Buchner funnel equipped with filter paper and a vacuum pump, the alpha calcium sulfate hemihydrate cake was immersed in isopropanol and filtered twice in a Buchner apparatus. Finally, the cake was placed in a drying oven at 40°C to eliminate residual isopropanol. Analysis of the alpha calcium sulfate hemihydrate particles was then performed using a scanning electron microscope (SEM) to characterize the size and dimensions of the alpha calcium sulfate hemihydrate particles.

[0061] The compositions and particle size distributions of the four different gypsum sources used in the experiments are detailed in Tables 1 and 2 below. [Table 1] [Table 2]

[0062] Here, the D10, D50, and D90 measurements for each gypsum represent the values ​​at which 10%, 50%, and 90%, respectively, of the diameters measured within the gypsum are smaller than the specified value.

[0063] In the first experiment, each of the gypsums A-D was calcined and filtered as described above. The concentration of succinic acid (measured using an enzyme kit), the percentage of the total water added to the slurry that was recovered in the filtrate, and the pH of the filtrate were then measured in each case. Four tests were performed with gypsums A, B, and D, and three tests were performed with gypsum C. In the first test with each gypsum, succinic acid was added to the slurry at a concentration of 0.86 g / l. The filtrate from the first test was then added to the slurry during the second test, and this filtering and reintroduction process was repeated throughout the tests, resulting in a continuous decrease in the succinic acid concentration in each test.

[0064] The different concentrations of succinic acid in the filtrate were measured after calcination using a reference method, and these measurements are shown in Table 3. [Table 3]

[0065] As can be determined from Table 3, the measured pH was dependent on the concentration of succinic acid in the filtrate as expected. Therefore, it can be determined that the concentration of succinic acid in the filtrate can be determined by measuring the pH of the filtrate.

[0066] As can also be determined from Table 3, the concentration of succinic acid in the filtrate also correlates with the percentage of total water recovered in the filtrate. As the percentage of water recovered in the filtrate decreases, the concentration of succinic acid in the filtrate also decreases. As the percentage of total water recovered in the filtrate decreases, the total volume of the filtrate also decreases. Therefore, the relative volume of the filtrate to the initial slurry volume, or the volumetric flow rate of the filtrate, can be used to determine the concentration of succinic acid in the filtrate.

[0067] In a further step of the experiment outlined above, the alpha calcium sulfate hemihydrate within the produced slurry was retained and analyzed using a scanning electron microscope, where the aspect ratio (length to width) of the alpha calcium sulfate hemihydrate particles was measured, and this data is presented in Table 4 below. [Table 4]

[0068] The above data is further illustrated in Figure 4. As can be seen from Table 4 and Figure 4, a clear correlation exists between the pH of the filtrate and the aspect ratio of the alpha calcium sulfate hemihydrate particles. Since the pH of the filtrate is directly related to the pH of the slurry during the calcination process, it can therefore be determined that the aspect ratio of the alpha calcium sulfate hemihydrate particles correlates with the pH of the slurry during the wet calcination process. As shown in Figure 3, the pH depends on the concentration of succinic acid habit modifier present during the wet calcination process.

[0069] In view of the above, the concentration of the crystal habit modifier during the wet calcination process has a significant effect on the aspect ratio of the alpha calcium sulfate hemihydrate particles produced. Therefore, it is essential to monitor the concentration of the crystal habit modifier in the filtrate containing a variable amount of crystal habit modifier when the filtrate is reintroduced into the wet calcination process as described herein. Furthermore, the amount of additional crystal habit modifier introduced into the further slurry must be controlled to ensure that alpha calcium sulfate hemihydrate particles of the desired aspect ratio are obtained.

[0070] Ion Chromatography In previous experiments, direct measurement of succinic acid concentration was performed using an enzymatic kit. In an alternative approach, ion chromatography can be used to directly measure the succinic acid concentration in the filtrate.

[0071] To demonstrate the validity of this approach, the inventors obtained a filtrate containing the ions and expected amounts of succinic acid as listed in Table 5 by mixing a known amount of succinic acid with water and gypsum, filtering the gypsum from the mixture, and collecting the filtrate. [Table 5]

[0072] The concentration of succinic acid in each of these filtrates was then measured by ion chromatography using an inductively coupled plasma detector (IC-ICP). Here, an eluent pump was used to create a stream of deionized water that carried the sample from the injector to the separation column. The separation column contains a small tube filled with a polymeric ion exchange resin that reacts differently depending on the ion, so that each component in the sample exits the column after a certain duration (elution time). The measurements were performed using a CI DIONEX ICS2100 A-03-71 chromatography system. On this instrument, the elution time of succinic acid is 32 minutes.

[0073] As the ions emerge from the column, their quantity is measured by a detector. In the ICP detector, the sample is atomized by a mist generator. The resulting aerosol passes through an argon torch and a high-power induction bobbin to create plasma generating conditions. The ionized atoms are separated by mass-to-charge ratio in a chamber with a continuous potential, providing a signal proportional to the ion concentration. The results of these experiments are shown in Table 6. [Table 6]

[0074] As can be seen above, ion chromatography was an accurate method to measure the concentration of succinic acid in each of the filtrates, with each of the measured values ​​falling within 10% of the expected value. Therefore, ion chromatography is an alternative to an enzymatic kit for the direct measurement of the concentration of succinic acid in the filtrates.

Claims

1. A process for the continuous production of calcium sulfate hemihydrate, comprising: mixing particulate gypsum, a crystallization agent and water to form a gypsum slurry; calcining the gypsum slurry to provide a calcium sulfate hemihydrate slurry; filtering the calcium sulfate hemihydrate slurry to separate at least one fluid stream from the slurry; measuring a first control parameter of the at least one fluid stream or the calcium sulfate hemihydrate slurry; combining the at least one fluid stream with additional gypsum, additional water and additional crystallization agent to form additional gypsum slurry; and further comprising varying the amount of additional crystallization agent in response to the measured value of the first control parameter. A process.

2. The process according to claim 1, wherein the step of measuring the first control parameter comprises continuously measuring the first control parameter.

3. The process according to claim 1, wherein the first control parameter is the pH of the at least one fluid stream or the calcium sulfate hemihydrate slurry.

4. The process according to claim 1, wherein the first control parameter is the volumetric flow rate of the at least one fluid stream or the calcium sulfate hemihydrate slurry.

5. The process according to claim 1, further comprising measuring a second control parameter of the at least one fluid stream or the calcium sulfate hemihydrate slurry.

6. The process according to claim 5, wherein the step of measuring the second control parameter comprises adding an enzyme.

7. The process according to claim 5, further comprising calibrating the measured value of the first control parameter with the measured value of the second control parameter.

8. The process according to claim 5, further comprising measuring the first control parameter more frequently than the second control parameter.

9. The step of measuring the second control parameter includes taking a sample of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry and measuring the second control parameter offline, the process according to claim 5.

10. The measurement of the second control parameter is slower than the measurement of the first control parameter, the process according to claim 5.

11. The measurement of the second control parameter is more accurate than the measurement of the first control parameter, the process according to claim 5.

12. The crystal seeding agent includes succinic acid, the process according to claim 1.

13. The alpha calcium sulfate hemihydrate slurry includes particles having a length-to-width ratio of 0.8:1 to 1.2:1 (including both ends), the process according to claim 1.

14. An apparatus for performing the process according to any one of the preceding claims, a mixing zone for mixing a crystal seeding agent, water and gypsum particles, a calciner including or located downstream of the mixing zone and in fluid communication with the mixing zone, a filter located downstream of the calciner and in fluid communication with the calciner, a fluid path for carrying at least one fluid stream from the filter and introducing the at least one fluid stream into the mixing zone, at least one measuring device configured to measure a first control parameter of the at least one fluid stream or the alpha calcium sulfate hemihydrate slurry and further comprising control means for changing the amount of crystal seeding agent added to the mixing zone in response to the measured value of the first control parameter.

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