The final step is where the ions are advected throughout the solution. These steps are shown in Fig. Liu and Nancollas noticed that the rate of dissolution is proportional to the difference between the instantaneous concentration C, at time t, and the gypsum saturation solubility in water, C S.
This statement can be formulated mathematically as follows:. The value of the saturation concentration of the calcium ions in water can be derived by knowing that 2. Since the molar mass of gypsum is Building upon the findings of previous growth and dissolution studies of other mixed sludges, it was concluded that the dissolution rates of these minerals are governed by solvation affinity for the near-surface divalent metals.
The results of this study suggest that the dissolution kinetics of sulphates is governed by a similar, but inverse, rate-limiting step. That is, dissolution rates are proportional to the hydration or solvation affinity of metals contained in these structures. In gypsum, the divalent atoms of Ca are coordinated to join with the SO 4 tet-rahedra. Rates of gypsum dissolution are also quite rapid and rates are limited by transport control in most conditions Liu and Nancollas, ; Barton and Wilde, However, it was also noted that CaSO 4.
The MgCO 3. From the reaction equilibrium and the results of this study, it was apparent that decreasing the temperature and lowering CO 2 pressures will result in an improved conversion of Mg OH 2 to Mg HCO 3 2.
Keeping the pressure low will help in lowering the solubility of the CaCO 3 , hence increasing the separation rate. Based on our current application, the recommended optimum operational conditions of temperature and pressure on the separation of Mg OH 2 and CaSO 4.
Applying the results of this study will contribute to the improvement of the CSIR-ABC process, designed to meet the criteria for maximum value of treated water and by-products, coupled with lowered running and sludge disposal costs.
It is also foreseen that the cost can be kept low in other applications by producing CO 2 on site by burning coal and scrubbing the off-gases in water rather than purchasing pure CO 2. Temperature and pressure had a significant impact on the dissolution of the mixed sludges when contacted with CO 2.
Saudi Aramco J. Faraday Soc. Natl Bur. Acta 72 Acta 48 Nuclear Chem. Hydrometallurgy 41 MUCCI A The solubility of calcite and aragonite in seawater at various salinities, temperatures and 1 atmosphere total pressure. It is well known that magnesium has a good cutting performance since its cutting resistance is low. Therefore, magnesium is utilized for engineering applications [ 1 , 2 , 3 ]. On the other hand, magnesium batteries using metallic magnesium at an electrode have been investigated as batteries replacing lithium batteries [ 4 , 5 ].
Industrial magnesium is usually produced from magnesium ore which is mainly distributed in China, Russia, Turkey and some other countries. However, seawater contains abundant mineral resources. It is well known that concentration of magnesium ion is the second most in all positive ions contained in seawater.
Moreover, lithium and strontium ions are also included in seawater. According to W. Zhang, the purity of salt increases by adopting a monovalent ion exchange membrane in the ED system [10]. On the other hand, the effluent treatment for the concentrated seawater drained from reverse osmosis process RO has been required in terms of environmental pollution [11].
Under the circumstance, zero discharge desalination technology ZDD was proposed for recovery system of drinking water and metal resources, which are combined with RO desalination process and ED process etc. This technic is expected as the resource recovery method with low cost due to reusing the concentrated seawater drained from RO process. In some ZDD processes, the method for recovering the magnesium resource consists of the concentration process of seawater by ED and the formation process for the magnesium compound by using chemical [ 12 , 13 , 17 ].
Thus, chemical is needed for creating magnesium compound from seawater in the conventional method. Furthermore, the complicated processes including the pretreatment process are needed to recover high purity magnesium hydroxide.
Generally, the batch process with large space and high cost is adopted for magnesium resource recovery. As far as our knowledge, continuous magnesium resources recovery system has never been proposed.
In this study, the continuous resources recovery system utilizing the water electrolysis reaction is proposed for recovering magnesium resources from seawater. A set of experiments for creating magnesium hydroxide from the deep-ocean water were carried out at a cathode channel separated by an ion exchange membrane.
Substantially, the effect of deaeration from seawater on the purity of magnesium hydroxide was investigated by adopting the deaeration methods of both the acid supplying and the boiling. In this study, deep-ocean water obtained from the depth m at Yaizu in Japan was used as experimental solution for magnesium recovery from seawater. Note that sodium concentration is excepted from this figure since its concentration is much higher than the other ions concentration.
Therefore, it is found that following sodium, magnesium, calcium, potassium and silicon are mainly included in the deep-ocean water. Positive ions concentrations in deep-ocean water obtained from the depth m at Yaizu.
An experimental apparatus is illustrated in Fig. Note that each solution passed through the test section only once.
Na 2 SO 4 solution was adopted not to generate Chlorine gas Cl 2 in this study. The inside electrodialysis stack is illustrated in Fig. The detailed specification for the CMB membrane provided by manufacturer is tabled in Table 1. When applying the electric current between the electrode plates in both anode and cathode, electrolysis reaction takes place on the electrode plates.
Therefore, a cation exchange membrane was adopted to prevent the OH - ions generated at cathode from moving to anode. Moreover, this cation exchange membrane plays an important role not to transport Cl - ions from cathode to anode.
On the other hand, positive ions penetrate through the cation exchange membrane. The molarity affects the ratio of ions passing through the ion change membrane. Note that this device is different from conventional ED system. Generally, the conventional ED stack consists a lot pairs of dilute and concentrate compartments by alternately arranged cation exchange membranes and anion exchange membranes.
Moreover, a pair of electrode components is placed at both ends of electro-dialysis stack. However, the present stack consists of one cation membrane. The size of effective membrane area, which is a region contributing to pass the current, was set to same size of electrodes i. A filter press electro dialysis stack was placed perpendicular to the ground. Each solution was fed to the reverse direction of the gravity so as to prevent the root clogging by the formation in the channel.
In this experiment, the root clogging was never observed in any parts of the test section. When applying the electric current between the electrode plates in both anode and cathode, the following electrolysis reactions may take place on the electrode plates:.
Moreover, positive ions are transported through a cation exchange membrane from anode to cathode solution under the influence of the electrical potential difference. Current density was set not to reach the limiting current density so that water dissociation does not take place on a cation exchange membrane [ 18 , 19 ]. Therefore, the pH value increases at cathode solution, while it decreases at anode solution. In the present method, OH - ions generated by water electrolysis are utilized for forming magnesium hydroxide.
The solubility of magnesium hydroxide Mg OH 2 is 1. When calcined dolomite CaO. MgO is used, the reaction proceeds as follows:. It will be observed that in the above reaction, both the calcined dolomite and the aqueous liquid furnish magnesium for the magnesium hydroxide. Thus, the invention may be practiced efficiently and economically with calcined dolomite as one of the reactants. In step 2 of the process, a settling aid or flocculating agent is added to the magnesium hydroxide suspension.
The settling aid or flocculating agent causes an initial, at least partial, agglomeration of the suspended magnesium hydroxide particles and a slurry is thereby formed. Settling aids or flocculating agents are described in U. The settling aids disclosed therein include the group of polyelectrolytes, and particularly the group of organic copolymers of acrylamide having molecular weights estimated at between two and three million.
It should be understood that any settling or flocculating agent which produces magnesium hydroxide agglomerates to a substantial degree can be employed in the process.
However, the group of anionic polyelectrolytes and particularly the group of anionic, high molecular weight polyacrylamide resins work best. Particular polyacrylamide resins of this type useful in carrying out the invention include a number of commercially available products.
Betz and , manufactured by Betz Manufacturing Corp.. Preferably, the settling aid is added to the suspension continuously in the feed line leading to the separation vessel. It is generally desirable to add the settling aid in the form of a dilute aqueous solution at a point in the feed line which insures its proper dispersion. The rate at which the settling aid is continuously added to the suspension should be metered so as to produce the most effective final dosage.
While the most effective dosage depends on a variety of factors, such as the nature of the settling aid, it has been found that dosages ranging from 1. In step 3 of the process, the resulting slurry is introduced as influent feed into a separation vessel beneath the upper boundary of a settling zone contained therein.
The settling zone is filled with a previously formed liquid slurry having a higher concentration of agglomerated solid magnesium hydroxide than is present in the influent feed. Preferably, the previously formed liquid slurry is gently agitated by the influent feed as it is introduced into the midst of the settling zone. No additional agitation is required. In step 4 of the process, the path of the influent feed is deflected radially and outwardly in order to increase contact between the magnesium hydroxide particles in the influent feed and the magnesium hydroxide agglomerates already present in the settling zone.
The effect of this intermixing is to cause additional agglomeration and coalescence of the solids into relatively large agglomerates which settle rapidly to the bottom of the separation vessel. A baffle or other arresting means positioned closely adjacent to the influent feed line may be used to deflect the path of the influent feed. In the next step of the process, a dense liquid solids slurry fraction is continuously removed from the separation vessel.
This fraction is removed at a point near the bottom of the settling zone and at a rate which maintains the upper boundary of the settling zone substantially stationary. In the final step of the process, the magnesium hydroxide agglomerates are separated from the dense liquid solids slurry fraction.
The separation may be accomplished by any conventional method, for example, by evaporation. A preferred method is filtration since this method is swifter and more efficient than evaporation.
The following examples are presented in further illustration of the process of this invention. Influent feed containing magnesium hydroxide particles and flocculating agent for agglomeration of the magnesium hydroxide particles is introduced into the settling zone of a clarifier along the lines described in U. The flocculating agent is added to the influent feed a suitable distance, e. The influent feed may also usefully include a static mixer.
A mechanical rake connected to a drive or motor is used to move the agglomerated solids at the bottom of the settling zone for discharge therefrom.
This movement of the agglomerated magnesium hydroxide solids is effected with minimum or substantially no agitation of the settling zone per se. A baffle positioned closely adjacent to the outlet from the standpipe is employed to arrest the vertical motion of the influent feed emerging from the standpipe and to deflect its path radially and outwardly within the settling zone. The dense underflow is removed through a bottom outlet while a clarified overflow is removed simultaneously from near the top of the separation vessel.
In a first series of tests conducted in a device of this type with a 3. A rake was operated at 12 rpm to move the sludge bed for discharge. Betz was used as the settling aid or flocculating agent. The results of these tests are set out in Table I:. Further effects of variations in feed rate and settling aid dosage were tested and the results of these tests are set forth in Table II:.
It than reacts with nitrogen from air to form magnesium nitride Mg 3 N 2. When attempts are made to extinguish magnesium fires with water, magnesium aggressively reacts with hydrogen gas. To prevent any damage, a magnesium fire must be covered in sand. An example of a magnesium compound is magnesium phosphide Mg 3 P 2 , an odorous, grey solid. When this compound comes in contact with water or moist air, it is decomposed and phosphine PH 3 is formed.
This is a toxic compound, and it is also very flammable in air. In seawater it can also be found as MgSO 4. A large number of minerals contains magnesium, for example dolomite calcium magnesium carbonate; CaMg CO 3 2 and magnesite magnesium carbonate; MgCO 3. Magnesium is washed from rocks and subsequently ends up in water. Magnesium has many different purposes and consequently may end up in water in many different ways.
Chemical industries add magnesium to plastics and other materials as a fire protection measure or as a filler. It also ends up in the environment from fertilizer application and from cattle feed. Magnesium sulphate is applied in beer breweries, and magnesium hydroxide is applied as a flocculant in wastewater treatment plants.
Magnesium is also a mild laxative. Magnesium alloys are applied in car and plane bodies. During World War II magnesium was applied in fire bombs, to cause major fires in cities. The development of these bombs introduced a method to extract magnesium from seawater.
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