Resources, Conservation and Recycling, 3 (1990) 231-239 Elsevier Science Publishers B.V./Pergamon Press p lc - Printed in The Netherlands

231

Production of magnesiumfrom desalination brines

I.S. AI Mutaz and K.M. Wagialia Department of Chemical Engineering, King Saud University, P.O. Box 800, Riyadh 11421 (Kingdom

of Saudi Arabia)

(Received July 1, 1989; accepted in revised form February 22, 1990)

ABSTRACT

AI Mutaz, I.S. and Wagialla, K.M., 1990. Production of magnesium from desalination brines. Resour. Conserv. Recycl., 3:231-239.

Recovery of minerals from desalination brines is considered to be a very attractive source of min- erals. It is usually recommended for reducing fresh-water production cost and minimizing waste dis- posal. This paper discusses the production of magnesium from a Saudi desalination brine with refer- ence to the Arabian Gulf conditions.

INTRODUCTION

Seawater is considered an important source of several minerals such as so- dium, magnesium, sulphur, calcium, potassium, etc. The world's seawaters contain about 166 million tons of these minerals. In comparison with the lim- ited in-land mineral resources, seawater provides an endless supply of various minerals.

Table 1 shows the composition of some important minerals in seawater. Table 2 gives the average Arabian Gulf water analysis.

The project under consideration would be connected as a downstream plant to a desalination plant in the Arabian Peninsula where the installed desalina- tion capacity increased from 0.5 10 6 m3/day in 1973 to 5.8 10 6 m 3 day by 1984, over a ten-fold increase. The capacity (in million gallon per day ) of the large plants installed during that period were as follows:

Jubail II plant, Saudi Arabia (Arabian Gulf ) Um EI-Nar plant, Abu Dhabi (Arabian Gul f ) Jeddah IV, Saudi Arabia (Red Sea) AI-Khobar II, Saudi Arabia (Arabian Gul f ) Doha Plant, Qatar (Arabian Gul f )

253 mgd 95.0 mgd 58.0 mgd 51.0 mgd 50.0 mgd

Elsevier Science Publishers B.V./Pergamon Press plc

232 l.S. AL MUTAZ AND K.M. WAGIALLA

Shuaiba, Kuwait (Arabian Gulf) 45.0 mgd

The feedstock to the magnesium extraction plant would be the highly con- centrated brines (blow down) leaving an upstream desalination facility in the Arabian Gulf. Table 3 gives a typical analysis for a desalination brine in the Arabian Gulf.

The manufacture of magnesium compounds from salines has been prac- ticed successfully in many places. At an average concentration of 1.25 mg Mg 1 - l of seawater, a cubic kilometer of seawater, for example, contains 1250 tons of magnesium [ 1 ]. The total world production of magnesite is 8.92 mil- lion tons per year in 1988. More than 33% is from saline sources, mainly seawaters. The United States, the USSR and Norway are considered the ma- jor producers of magnesium. At present, the west depends on seawater-based

TABLE 1

Composition of seawater (specific gravity: 1.024)

Species Concentration (gl -~ )

NaCI 27.319 MgC12 4.176 MgSO4 1.668 MgBr2 0.076 CaSO4 1.268 Ca(HCO3)2 0.178 K2SO2 0.869 B203 0.029 SiO2 0.008 Iron/Alumina 0.022

TABLE 2

Average Arabian Gulf water analysis (ppm)

Total dissolved salt (TDS) 45 000 Na+K 14 800 Ca 600 Mg 1600 HCO~ 130 SO~- 3450 CI 25 000 COl- 40 F 2 Fe, AI, SiO2, NO2 nil pH 8.2

PRODUCTION OF MAGNESIUM FROM DESALINATION BRINES

TABLE 3

Approximate Arabian Gulf desalination brine analysis (ppm)

233

Chloride, CI- 35 800 Sodium Na 25 650 Sulfate, SO 2- 5000 Magnesium, Mg 2 2750 Calcium, Ca 2 80 Potassium, K 720 Bicarbonate, HCO~- 220 Bromide, Br- 120 Silicon, Si~- 2 Total dissolved salt (TDS) 70 000

technologies for producing over half of its requirements of calcined magnesia for refractories production. No magnesium processing facilities are located in the Arabian Gulf countries, although they have some of the largest desalina- tion facilities in the world.

The Arabian Gulf countries have about 60% of the total world desalination capacity. About 67.6% of the total installed capacity is of the multi-stage flash (MSF) type. Moreover, the MSF plants account for over 84% of the large size plants erected so far [ 2 ]. Mineral extraction from seawater has attracted spe- cial attention in the Arabian Gulf countries recently. Seawater is available in high concentrations (42 000-45 000 vs. 35 000 ppm for normal seawater). The region has limited natural mineral resources. The hot weather and the virtually unlimited space available in the Arabian Gulf region make minerals recovery very attractive.

This study concentrates on the production of magnesium from desalination brines. Only MSF plants are considered because they usually have large ca- pacities and are normally associated with the production of surplus electric power.

Magnesium is found in seawater as MgCI2. Processing 12.5 mgd of normal seawater will produce 105 000 ton yr-~ of MgC12 which could yield 26 000 ton yr- l of magnesium. The Dow Chemical process is the classical process of Mgcl2 extraction. It is accomplished by adding lime to seawater bitterns to precipitate Mg(OH)2. Then, by adding HC1, MgC1 is formed. Electrolysis is then required to produce Mg metal. Crystallization and the soda ash process could also be used to produce MgC12.

On a world-wide scale, 50% of the magnesium produced is used in the prep- aration of high quality aluminium alloys, which find several applications in structural and industrial fields. Other uses of aluminium include the removal of sulphur from iron and steel, the production of cast iron in protection against corrosion, and minerals extraction.

234

PROCESS DESCRIPTION

1.S. AL MUTAZ AND K.M. WAGIALLA

There are two competing groups of processes for the production of metallic magnesium (see Fig. 1 ):

( 1 ) The electrolytic processes which are based on the electrolysis of mag- nesium chloride. These are seawater-based processes. Several commercial processes use this method, such as the Dow Chemical progress, the soda ash process, the I.G. Farben process, the Norsk Hydro process, the Amax process, and the Russian process.

(2) The thermal processes generally involve the ferrosilicon reduction of calcined dolomite. These processes use minerals such as dolomite, magnesite and ferrosilicons as raw materials. Under this heading come the carbon ther- mal process, the pidgeon process and the Magnatherm process.

Of these two alternate processing routes the electrolytic processes are con- sidered to be more economic. The thermal processes are generally more suited to small scale units (5000-15 000 tons yr-~ of magnesium), while the sea- water-based processes (the electrolytic processes) are more suited to large scale production (over 1500 tons yr-~ of magnesium). As far as energy con-

ELECTROLYSIS OF MAGNESIUM CHLORIDE

FERROSILICON REDUCTION PROCESS

Oxide Byproduct Brine Sea Water MgO+C+CI 2 of potash wells Mg(OH) 2

mines bitt- + HCI @ 850C ern

+CO MgCl2

gCl 2-~H20

Dehydrators

MgCl 2-H~O I Heat + I electricity

IElec ce, lW / , I ;o-73ooc /

Magnesium ne HcI+02 + H20

99. g%

Electric Blast Dolomite furnace furnace quarry

-r-zT-T Calcined

Ferro- dolomite silicon CaO-M~O 75% Si j

Briqueting Machine

I | Vacuum retorts @2140oc 12MgO+Si. = 2M~+Si02 2~aO Condenser

Remel ter Slag solid I ~ 2CaO'Si02 rigors of Mg

Fig. 1. Outline of alternate processes for the production of metallic magnesium [ l ].

PRODUCTION OF MAGNESIUM FROM DESALINATION BRINES 235

sumption is concerned, the thermal processes use about 14 000 kWh of elec- tricity for each ton of magnesium produced (including the preparation of the ferrosilicon), while the seawater-based processes require about 18 000 kWh per ton of magnesium produced.

Magnesium chloride can be recovered from seawater bitterns by different chemical and physical processes depending on the availability and cost of chemicals and energy. The chemical process is the conventional process often used for the production of magnesium from saline water. As indicated in Ta- ble 1, MgCI2 and MgSO4 are the major magnesium compounds found in sea- waters. In the Dow Chemical process, calcined limestones are used to precip- itate magnesium hydroxide, which is then treated by hydrochloric acid to form magnesium chloride according to the following reactions:

Calcination: 2CACO3 , 2CaO + 2CO2

Slaking: CaO+H20 , Ca(OH)2

Precipitation: MgC12 + Ca (OH2) --, Mg (OH) 2 "~" CaC12 MgSO4 + Ca ( OH2 ) q- 2H20 ~ Mg (OH) 2 "~ CaSO4 2H20

AH= + 9.46 kJ AH= - 13.3 kJ

Hydrochlorination: Mg (OH) 2 + 2HCI~ MgC12 + 2H20 AH=44.7 kJ

The production of each ton of magnesium according to the Dow Chemical process requires the treatment of 180 000 gallons of seawater. In the Dow Texas plant, 50 million gallons of water are processed each day for the pro- duction of 90 000-100 000 tons of magnesium per year. Figure 2 shows an outline of the Dow Chemical process.

An alternative technology achieves the separation of MgC12 hydrate by crystallization through evaporation/concentration. This process requires a salt free solution in order to avoid precipitation of other salts with the magnesium chloride hydrate.

When seawater is frozen to below -36C ( -32 .8F) , magnesium chlo- ride in the dodecahydrate form (MgC12-12H20) is formed. Also, potassium chloride begins to separate at the same time. Further freezing below to - 54 C ( - 65 F) will cause calcium-bearing crystals to form as calcium chloride hex- ahydrate (CaC12- 6H20 ). So freezing seawater to between - 34 C to - 43 C will precipitate mainly magnesium chloride. The potassium chloride can be washed and separated by gravity in a hydroclone. Potassium chloride is a heavy salt which has a density of 1.984 gcm -3.

236 I.S. AL MUTAZ AND K.M. WAGIALLA

Dolomite or lime stone

I Crushing I

I Calcination I

Precipitation of Mg(OH) 2

I Filtration ]

Sea Water (Mg(OH) 2)

I Acld I Recovery

LI Aci0 I v~ Treatment MgCI 2

Drying of Magnesium Chloride

f ' I Electrolytic Cells 1

Magnesium

Fig. 2. An outline of the Dow Chemical process.

Cl 2

The magnesium chloride hydrate obtained is melted to form a solution of about 19% MgC12. It should be pointed out here that this process has, so far, never been carried out commercially.

Figure 3 shows the soda ash process schematically. Lime and CO2 are used to produce Mg(HCO3 )2 which is then reacted with sodium chloride to yield sodium bicarbonate precipitate and magnesium chloride solution. The bicar- bonate can be processed to produce soda ash (which is how the process got

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238 I.S. AL MUTAZ AND K.M. WAGIALLA

its name). The magnesium chloride is then concentrated to form solid MgC12 hydrates.

The magnesium chloride solution produced must be concentrated and then crystallized to obtain magnesium chloride hexahydrate of at least 45% con- centration. Triple effect evaporators are often used for concentration pur- poses. Once Mgcl2 is obtained from any of the above processes, it is dehy- drated and the magnesium chloride is fed to electrolytic cells where it is decomposed into the metal and chlorine gas. The power requirements for electrolysis are 63.4 MJ kg-1 of magnesium produced. About 180 000 gal of normal seawater are required to produce one ton of magnesium. Figure 2 shows the Dow Chemical process block diagram for magnesium production from desalination brines.

A PRELIMINARY ECONOMIC EVALUATION

Table 4 presents an order-of-magnitude economic estimate for the produc-

TABLE4

Economic estimate for the production of 2000 ton yr- ~ of magnesium from desalination brines produced by a desalination plant in the Arabian Gulf

Time basis: Fixed capital investiment: Raw material requirements:

Power requirements: Fuel requirements: Water:

1988 100 million US$ 9 million tons of desalination brines 100 000 tons of dolomite/limestone 18 000 kwh ton- l 140 106 Btu ton -~ 5 ton ton-

A breakdown of production costs (US$) is as follows (per ton of magnesium produced): Raw materials:

Dolomite/lime stone ( @ US$5 ton- ~ ) 25 Desalination brines (assumed free)

Chemicals 120 Power ( @ US~I.5 kWh-l ) 270 Fuel ( @US~50 M Btu- 1 ) 70 Cooling water ( @ US~ 1.9 m- 3 ) 2 Operating labour ( @ US$20 man h- l ) 500 Operating supplies ( @ 10% of operating labour) 50 Control laboratory ( @ 20% of operating labour) 100 Maintenance ( @ 20% per year of total fixed investment) 100 Plant overhead (@ 100% of operating labour + maintenance 600

labour + control laboratory labour) Taxes and insurance ( @ 2% per year of total fixed investiment) 20 Depreciation (@ 10% of TFI) 500 To t al 2357

PRODUCTION OF MAGNESIUM FROM DESALINATION BRINES 239

tion of 2000 ton yr- ~ of magnesium from desalination brines produced by a desalination plant in the Arabian Gulf.

The price of magnesium metal in the United States in 1986 was US$3370 ton-i. Thus the above preliminary economic assessment indicates that the extraction of magnesium from the Arabian Gulf desalination brines is eco- nomically viable and the necessity for preparing a techno-economic feasibil- ity study is indicated.

REFERENCES

1 Austin, G.T., 1984. Shreve's Chemical Process Industries. McGraw Hill, New York, 5th ed. 2 A1-Mutaz, I.S. and Wagialla, K.M., 1988. Desalination, 69: 297. 3 Editorial Board, 1974. Chemical Engineering News, 52 (9): 18.