IL29630A - Process for the production of magnesium oxide - Google Patents

Description

ISRAEL C- If OHO PJ i3MPS FOUH N0½ 3 PATENTS AND DESIGNS ORDINANCE SPECIFICATION "PROCESS FOR THE PRODUCTION OF MAGNESIUM OXIDE" oi*»3Aon nsmnn •ns»»,> T»*?nn v/o, ALLIED CHEMICAL CORPORATION, a Corporation organized and existing under the laws of the State of New York, United States of America, of 61, Broadway, New York 6, New York, United States of America, do hereby declare the nature of this invention and in what manner the cane is to be performed, to be particularly described and ascertained in and by the following statement: THIS INVENTION relates to a method of producing magnesium oxide from magnesium chloride utilizing a fluidized bed wherein the magnesium chloride is hydrolysed to form the magnesium oxide and hydrogen chloride. It relates particul rly to the continuous production of magnesium oxide particles of relatively low calcium oxide m calciyrj chloride content £rou rines rich in magnesium chloride but which are cont minated with calcium chloride. The hydrogen chloride gas may be dissolved in water to form hydrochloric acid.

It is well known that magnesium chloride can be thermally decomposed in the presence of water to form magnesium oxide and hydrochloric acid. Although the theoretical basis of the reaction is well known,adoptationof the reaction to an economical commercial process has not been entirely satisf ctory. Magnesium chloride hydrate melts at about its decomposition temperature, particularly if contaminated, thus forming an agglomerate mass which is difficult to handle, resulting in poor reaction rates and inefficiencies.

It has been proposed in U. S. Patent No.3, 251,650 that the problem of agglomeration may be avoided by using a fluidized bed of heated refractory particles wherein magnesium chloride hydrate contacts the particles and is converted to a porous magnesium oxide ash v/hich is carried away from the reaction zone by exit gases. It has been found, however, that in such a process if the magnesium chloride is contaminated with significant proportions of calcium chloride, agglomeration of the fluidized bed is a troublesome problem and a magnesium oxide product results which contains significant amounts of calcium oxide as an impurity, which is difficult to separate from the desired magnesium oxide product, also a small amount of hydrogen chloride is given off. Since brines rich in magnesiur. chloride, but highly cont minated with calcium chloride, are available at relatively low cost, e.g., as salt lakes, it is advantageous to produce high grade :nr,gnesiur.i oxide and an appreciable amount of hydrogen chloride from such brines.

It has now been found that aagnesiun oxide of relatively high purity can be obtained from the aforesaid brines, in a fluidized bed process, in which the bed is composed of magnesia particles which are recovered as the product.

The process according to the invention comprises heating a brine containing magnesium chloride and 10 to 0%, by weight of the magnesium chloride, of calcium chloride, at a temperature of 60oPc. to lloaC, preferably 700° to 950°C., in a fluidized bed of particles having a magnesium oxide content of at least 35 wt. % and a calcium chloride content of not more than 20 wt. %, and recovering from the bed a product of larger particle siae consisting predomi antly of magnesium oxide.

In this process the magnesium chloride in the brine is converted subst ntially to magnesium oxide and hydrogen chloride, whilst the calcium chloride remains substantially unchanged; the magnesium oxide and calcium chloride form a coating on the particles so that they increase in siae and eventually drop out of the fluidized bed and are recovered. These coated particles preferably have a magnesium oxide content of at least 75 wt. % and not more than 20 wt, % of calcium chloride, and may be further purified if desired* In a preferred embodiment , silica sand particles are also added to the bed in amounts such that the molar ratio of silica to the calcium chloride in the brine is in the range 0,85 ί1 to 3.0:1. preferably 1 :1 to 2.0:1. The addition of this small amount of sand allows easy conversion of the calcium chloride, which may become sticky and cause agglomeration of the particles, to calcium silicate (CaSiO^) which is hard and refractory. 6θ vrt, % but preferably - and especially when silica sand is added - 25 to 35 wt. %· The calcium chloride con ent is 10 to 30 %, preferably 1 to 2 % by weight of the magnesium chloride in the brine. (The proportions of constituents in the brine are given in terns of the anhydrous materiel, ignoring the formation of hydrates such as MgCl ·6Κ 0. ) The brine may be an aqueous solution or slurry having a solids content of preferably 10 to 40 wt %; the molar ratio of water to magnesium (H 0 to Mg) in the brine is preferably betv.'een 3 * 1 and 7 : 1 .

The fluidized bed is composed of particles containing at least 35.wt % magnesium oxide, preferably at least 75%, ο·9· 80%, or preferably 55 to 6 % when silica sand is added. The calcium chloride content of the particles is not more than 20%, preferably not more than 15%, and not more than 8% ivhen silica sand is added. High calcium chloride contents will cause the particles to stick together. The size of the particles is preferably 6 to 50 U.S. standard mesh size, or 65Ο - 1000 , preferably 650 - 850 , microns in median diameter.

Smaller sised particles may however be used adv ntageously if it is desired to recycle some of the product particles to the bed to provide nuclei for building new particles.

The silica sand, if used, should have articlcu of median diameter 5Ο to 800 microns to avoid contaminating the magnesia product with sand* No more sand should be added than is needed, preferably no more than a 1 niol excess over the amount recuired to stoichiometrically react with the calcium ions in the brine.

The bed temperature X3 600° to 1100°C., preferably 700° to 950°C. or 750° to 950°C» when sand is added. We have found that no MgSiO is formed from the sand if the reaction tomoercture is maintained 3 sufficiently high, and the H Q to MgCl ratio is sufficiently high to the I'igCl hydrolysis reaction is fast enough to prevent KgSiO 2 j formation at temperatures above 600°C« with a K O/MgCl mol ratio in the brine between 2 : 1 , and 10 : 1 . Reaction temperatures above o 1100 C. cause appreciable hydrolysis of CaCl to CaO before the CaSiO^ forms and result in an unsatisfactory magnesia product.

The pressure in the fluidized bod should be near atmospheric, e.g. 3 to 7 p.s.i.g. (or 0.21 to 0.49 kg/cm ), and the rate of flow 1.5 of gas through the bod may be 5 to 8 feet per second to 2.4 metres per second).

The gases leaving the bed contain hy:"rogen chloride, which may bo recovered and used to form hydrochloric acid.

A portion of the particulate magnesia product may bo recycled to the fluidized bed to keep the calcium chloride content thereof low, but this is not necessary if sand is add^d which reacts with the calcium chloride. The magnesia may be used directly as fertilizer, or may bo purified e.g. by washing with water, a product containing 84 - 99% magnesium oxide and not more than 1% of calcium oxide.

The invention will now bo described with rofercnco to the accompanying drawing which is a schematic illustration of apparatus for carrying out the process of the invention.

I-iagnesium chloride brine contaminated with calcium chloride is fed from a source 9 | through a spray 10, into a reactor 11 containing a fluidized bed 12 of magnesium oxide-calcium chloride particles resting on a conventional bed grate 13. Preferably, the brine is fed into the reactor 11 at a location within the fluidized bed in order to achieve a good contact with the bed particles therein. Fuel, and air 17 from sources 14, and 15 ore mixed and burnt in a combustion chamber and the combustion products are fed into the reactor at 18 to heat the bod. The rate of brine feed is controlled by suitable means, not shown. 29630/2 to avoid exceeding the heating capacity of the bed, since excessive feeding of brine will *ause the bed to cool thereby resulting in a retardation of the decomposition of the MgCl , The bed particles also are heated by a burning of the fuel, at a burner 19, within the bed itself. Significantly, the direct burning of fuel v/ithin the bed itself creates very high temperatures-approaching flame temperatures, directly on the surface of the magnesia particles thereby accelerating the decomposition reactions. The bed temperature is maintained from about 6θΟ° to 950°C. , preferably 700° to 950°C. , in a conventional manner wherein the gas mixture is caused to move upwardly through the reactor at a rate sufficient to suspend the solid particles therein to form a homogeneous turbulent bed.

Effluent gases and elutriated fine solids pass from the reactor 11 via line 20 into a cyclone separator 21 wherein they are separated. The elutriated fine solids, consisting mainly of magnesium oxide, are of 50 to 325 mesh size and are returned directly to the fluidized bed through line 22 to serve as fresh nuclei in the reactor. Alternatively, the solids nay also be returned to the reactor in a manner such that they are slurried into the input brine. The effluent gases pass from the cyclone separator vi line 23 into a heat exchanger 24 where they are cooled to about 350-400°C. by air passing through line 25. The heated air and combustion products leave the heat exchanger 24 through line 26 and are preferably recycled through the combustion chamber 17 into the reactor 12 » The gas leaving the heat exchanger through line 27 contains HC1, H O and combustion products. The gas passes through a scrubber 28 in which it is contacted with water which enters through line 29 » which absorbs the HC1 to form dilute hydrochloric acid (l to 13 wt. % of HCl) which passes as product from the scrubber through line 30. The combustion products substantially exit from the scrubber as overhead gases through line 31 and are discarded therefrom.

The heavier magnesiun-calcium chloride particles 32 , i.e. those larger than about 10 niesh median size, fall from the fluidized bed and are continuously discharged froa the lower portion of the reactor, while still hot, through lino 33 to a cooler 3 , where they are cooled to about 20 to 100°C, and then they are washed with water in chamber 35 one or more tines to decrease the content of water-soluble salts, e.g. calcium chloride, therein. A3 a result, a particulate magnesia product is recovered in n ypn l /Htj said product having a magnesium oxide content of at least 90 weight %» A portion of the magnesia product is preferably recycled through line 37 into the reactor 11 to maintain the calcium chloride content in the bed below about 15 wt. %.

In an alternative embodiment, silica sand particlac in wot I or dry form are also introduced into the bed 12 from a source 38 via a line 39.

The magnesia particles produced at 33 when sand has been added may be used directly as fertilizer-grade MgC, or they may be purified as shown by passing into a tank 40 wherein they are mixed , o with water from 41 at about 90 C. to provide a slurry. The hydrated crude magnesium mixture passes into another tank 42 wherein it is blended again with water, froa 4£ at ambient tonporuturu. The slurry mixture containing preferably 5-20 weight % solids is then pumped into a settling tank 3 and retained therein for a desired time until the heavy SiO and CaSiOj particles are settled. Alternatively a liquid cyclone separator (not shown) can be utilized to separate the solids in lieu of the settling tank 3. The raagnesiuo oxide is -largely slaked to Kg(OH) > essentially all the magnesia being suspended as a light and very fine slurry. The magnesium hydroxide slurry is passed into a filtering unit 44 wherein the magnesium hydroxide is filtered and the solids washed with an equal weight of water, from lino 4 , to remove the chloride.

The filtered magnesium hydro:d.dc is then passed to a heater 46 where it is dried end calcined to the final product, na;.:ely relatively pure magnesium hydro;:ide« The following examples are illustrative of the invention* SXAMPLE 1 Aqueous brine solution containing 29.1% MgCl , 6,0¾ CaCl , 1,1% NaCl and 0.7% C1, by weight, was sprayed at a rate of 102 pounds per hour for 10,8 hours into a fluid bed reactor lined with fire brick having a cross-sectional area of 1,77 square foet and a static bed depth of 4,6 foet.

The bed initially consisted of 920 pounds of magnesia particles. Distribution of particle size at the start of the run, using U,S, standard screens, was approximatoly 6 to 40 nesh material.

The magnesia particles at the start of the run had the following composition: Component eight % MgO 82.5 CaO 3.9 HgCl2 2.2 CaCl 5.8 Si02 5.5 The fluidized bed was maintained at an average temperature o of about 730 C, by burning natural gas at 10,5 cubic feet per minute (standard conditions) within the bed itself, Fluidiz ion was achieved by burning this amount of gas with lo9 cubic foet per minute of air (standard conditions). This is equivalent to a fluidization velocity of 7 feet per second.

The hot gases left the reactor as overhead and were conducted through a cyclone separator to remove elutriated fine solids, the gases wore then passed through a water-scrubber and discharged.

After 10.8 hours, a total of 1120 pounds of solids wore recovered, 960 pounds fron the reactor end l60 pounds from the cyclone separator, thus representing a recovery of solids of about The particles recovered from the rorctor h d the following composition: Component Weight ¾ MgO 75.0 CoO 0.7 MgCl2 1.4 CaCl2 16.5 Si02 6.5 Particle size distribution of sclids recovered from the reactor was 1.9 weight % on a 6 mc3h sieve end 99.2 weight % on 40 nesh whereas the size distribution of solids fron the cyclone separator was «5 weight % on 30 nosh and 99.5 weight °/i on 200 nesh.

The gas discharged from the cyclone separator contained 1.8¾ by volume of HC1, indicating virtually complete decomposition of HgCl · EXAMPLE 2 A process similar to that describee in Example 1 was carried out and the product from the reactor was recovered and then slurried and filtered o three times with water at 90-100 C. resulting in a marked decrease in calcium ion and chloride ion content thereof as indicated in the Table presented below. Values are in weight TABLE 1 Component Before Washing After has i g, MgO 75.39 89.41 3i02 4.5 5.3 Calcium ion 6.0 0.8 Chloride ion 8.14 0.35 Sodium ion 0.67 s than 0.1 Potassium ion 1.94 s than 0.1 EXAMPLE 3 In E:iar.-;ple 1 , the product was contaminated with SiO from the refractory lined reactor walls . The reactor was relined with magnesi a brick to eliminate thi s contamination and the following run was made : Aqueous brine solution containing an average of 25.6 weight % MgCl , 6.5 wei ght % CaCl , 0.5 wei ght % NaCl , and 0.5 weight % KC1 was sprayed at a rate of 114 pounds per hour for 9 hours into a fluid bed reactor having a cross sectional area of 1 .77 square feet and a st atic depth of feet .

The bed initi ally consisted of 1000 pounds of 95.6 wei ght % MgO. Distribution of particle size at st art of run. using U . S. st andard screens , was approximately 10 to 50 mesh material - about 750 microns medi an size.

The fluidized bed was maint ained at an average temperature o of 92 C. by burning natural gas at 10 scfm within the bed itself , and 4.7 scfm in a Dutch oven ai r preheater. Fluidization was achieved by burning this amount of gas with 180 scfm air which is equivalent to a fluidization velocity of 9 feet per second.

During operation of the brine feed, the bed particles tended to grow. The rate of growth was controlled between 950-1150 microns medi an size (16-I8 U.S .S. mesh) by recycle of about 1.9 pounds of -20 of 46% cyclone fines and 54% nagncsi fines* The hot gases left the reactor as overhead and were conducted through a cyclone separator to remove elutriated fine solids, the gases, containing 1#7 volume % HC1 , then passed through a water scrubber and then discharged to atmosphere.

During a 94-hour period of operation, 1950 pounds of brine solids and 1774 pounds of recycle solids were fed to the reactor. At the end of the operotioa, 3831 pounds, of product solids wore' collected for an 8l% solids recovery. The equilibrium bed solids analysis was 83.2 weight % MgO, 13.4 weight % CaCl , 1.1 weight % CaO and 1.5 weight % NaCl plus Cl. The crude product was slurried and filtered throe tines with water at 90-100°C. The washed product contained 99 weight % MgO. 5XAHPLE 4 The fluidized bed reactor used in this example was constructed of a 36-inch diameter carbon steel pipe 20 feet tall, lined with one course of insulating brick and an inner course of magnesia refractory brick. The inside disr.ieter of the reactor was 18 inches. The reactor was provided with a fluidizing riser or plate containing 21 air distri utors. This plate was located 3 feet up fror.i the bottom of the reactor and contained a solids discharge line for renoval of crude magnesia.

An aqueous brine solution containing 28.3% MgCl , 5·7% CaCl , 1.0% NaCl and 0.6% KCl, by weight, was introduced at a rate of 110 pounds per hour for 36 hours into a fluid bed reactor lined with fire brick having a cross-sectional area of 1.77 square feet and a static bed depth of 4.6 feet.

The bed initially consisted of 920 pounds of r.iagnesio particles from a previous run. The distribution of particle size at the start of the run, using U.S. standard screens, wa a roximately 10 to 40 mesh material . During the runs, silie? s~nd (about 28 mesh) was added to the bed at about 5 pounds per hour.

The magnesia particles at the start of the run had about the following composition: Component Weight 55 gO 56 CaSiO„ 22 NaCl 4 XC1 3 CaCl2 5 SiO 10 2 The fluidizod bed was maintained at an average temperature about 850°C. by burning natural gas at 12.3 cubic feet per minute (standard conditions).

The hot gases left from the reactor as overhead and were conducted through a cyclone separator to remove fine solids, the gase were then passed through a water-scrubber and discharged.

After all flows to the system had been stabilized and the system had been operated for 24 hours, operating data for the system were taken throughout a 12-hour test period as described below.

During the test period there was fed 110 pounds per hour of a magnesium chloride brine v/ith the following analysis: Component Weight ½ gCl2 23.8 Ca.Cl2 5.7 NaCl 1.0 KCl 0.7 63.8 H2° The sand feed rate was 5 «0 pounds par hour. The send particle median diameter was 6oO microns. The bed temperature was maintained 850°C» by burning 12.3ccf per minute of natural gas in the bod. Air feed through the reactor grate was l60 scf par minute. The combustion gases were removed iron the reactor top through a cyclone nnd had the following analysis: Component Weight % HCl 3.06 C0„ 10.1 2 0 4.40 2 N 67.01 H O 15.39 100.00 Bed solids were removed at a rate of 24.17 pounds per hour in order to maintain a constant solids level in the reactor. The analyses of these solids were: Component ί/eight % MgO 55.53 CcCl 5.19 Cr.SiO 21.71 aCl 4.55 Kci 3.19 3iC 9.83 100.00 The solid particle median diameter was 750 microns. No MgSiO^ v/cs found in the bed.

These solids from the bed are suitable for use as fertilise] grade magnesium oxide. Altenatively, the solids may be purified s described in Exam le 4 to produce a relatively pure magnesium oxide suitable for use as a fertiliser or for other uses such as in preparation of refractory materials.

E AMPLE 5 The crude solids from the fluid bod reactor of E m le 4 were slurried in 35 ·76 pounds per hour of water at 90°C. and the fixture was agitated in pugmill for about 1 »5 hours. The resulting hydrated crude magnesia v/cs then discharged to a nixing vessel and blended with 210.65 pounds per hour of additional water at ambient temperature. The slurry was then pumped to a settling tank which provided about 15 minutes retention tine for settling of the heavy SiO,. c and CaSiO^ particles. The feed rate of slurry to this settler was 270.6 pounds per hour.

The overflow magnesium hydroxide slurry from the settler was 211 .12 pounds per hour v/ith the following composition: Component Weight % Mg(0H)2 5.52 CaSiO 0.17 SiO 0.08 CaCl 0.48 MaCl Ο. 3 C1 0.30 95.02 H2° 100.00 This represents about a 60½ recovery of magnesium hydroxide. is Mg(Ori) stream was filtered and the solids were washed v/ith water 2 remove the chlorides. Analysis of the dried and calcined magnesia Component V/eight % MgO 93.0 CaSiO. .3 SiO„ 2.0 Others 0.7 100.0 Higher magnesia recoveries and purity can be obtained by using core settling stages, more efficient settling equipment and by the use of noro water in the slurry feed to classification. The crude magnesia feed containing higher percentages of MgO produces purer MgO product under com arable operating conditions. For example, heavy impurities settled froni crude magnesia containing about 30 weight % MgO gave a product containing 84 weight MgO; whereas, r. crude product containing about 78 weight ½ MgO gave a final magnesi containing over 99.3 weight % KgO.

Claims (14)

WZ CLAIM*

1. A process for producing magnesium oxide, comprising heating a brine containing magnesium chloride end 10 to 30%, by weight of the magnesium chloride, of calcium chloride at a temperature of 600°C. to 1100°C, preferably 700° to 950°C., in a fluidized bed of particles having a magnesium oxide content of at least 35 wt» % and a calcium chloride content of not more than 20 wt. %t and recovering from the bed a product of larger particle size consisting predoninantly of magnesium oxide*

2. · A process according to claim 1, wherein particles of silica sand are also edded to the bed in amounts such that the molar ratio of silica to the calcium chloride in the brine is in the range 0,85:1 to 3·0:1, preferably 1:1 to 2.0:1»

3. » A process according to claim 2, v orein ti:- tejfipcrr.ture of the bed is 750° to 950°C.

4. » A process according to claim 1, 2 or 3, wherein the brine used has a magnesium chloride content of 10 to 60 \rt» % or, when sand is added, of 2 to 35 wt. %,

5. · A process according to any preceding clain, wherein the brine has a claciun chloride content of 15 to 25% by weight of tho magnesium chloride in the brine.

6. A process according to any preceding clain, wherein the particles supplied to the fluidized bed contain at least 55 wt. % magnesium oxide, preferably at least 75¾t or contain 55 to 6 % magnesium oxide v/hen sand is added.

7. A process according to any preceding claim, v/herein the calcium chloride content of the particles supplied to the bed is not more th^n 15 "wt. %, and not mora than 8% when sand is added.

8. A process according to any preceding claim, wherein the particles supplied to the bed arc 650 - 1000 nicrons, preferably 65Ο -85Ο microns, in r.icdian diameter*

9. · A process according to claim 2, wherein the silica sand particles are 350 to 800 microns in median diameter.

10. A process according to any preceding claim, wherein the concentration of the brine is such that the -olar ratio of H 0 to lie-is 2 - 3 between 3:1 ~-nd 7:1} or the molar ratio of H O to MgCl is between 2:1 and 10:1.

11. A process according to any preceding claim, wherein the particulate product is purified by washing with water, to obtain a magnesium oxide product containing not more than 1% of calcium oxide.

12. A process according to any of claims 1 to 10, wherein the particulate product is slurried with water at a temperature of 80 o to 100 C. to form a slurry of magnesium hydroxide which is filtered off, calcined and dried.

13. A process according to any preceding claim, wherein the hydrogen chloride gas formed in the process is recovered and dissolved in water to form dilute hydrochloric acid.

14. Magnesium oxide or hydrogen chloride, when prepared by p. process according to any preceding claim. Dated this 11th day of March, 1968 For /the Applicants DR. RE Bto COH AND, PAETN#RS By: