Sea Water Softening with the Lime-Magnesium Carbonate (LMC) Process

Abstract

A pilot plant to remove 70% of the calcium from sea water using the lime-magnesium carbonate (LMC) process was tested and improved. The calcium-deficient LMC product water was used as feed to a distillation desalting plant which was operated scale-free at 335'F and above. LMC pretreatment is estimated to cost 5.1&/1000 gal of treated water for a plant sized to supply a 50 million gal/day desalting plant. The process may find use with desalting processes using feed water more saline than the oceans. Sea Water Softening with the LMC ProcessThe total cost for commercially desalting sea water is the sum of operating and capital costs. For large distillation desalting plants, more than half of the operating cost is for energy used to heat the brine. As energy costs increase, desalting plants must become increasingly more efficient if product water costs are to remain competitive with other sources of fresh water. Higher thermal efficiency can be achieved by raising the maximum brine temperature of the distillation plant, but there is a practical limit at 250-265OF due to the formation of calcium sulfate scale on the heat transfer surfaces.The lime-magnesium carbonate process (LMC) was devised to remove the maximum temperature restriction by precipitating calcium from sea water as calcium carbonate (W. R. Grace & Co., 1966). The economic advantage of this process does not hinge on the sale of byproducts as has been the case with many other proposed schemes for the removal of scale-forming materials. All materials produced in the process are recycled for use in some other part of the process, and only nominal chemical makeup is required. Process DescriptionTwo feed stocks are required to sustain the LMC process: limestone and fuel oil. All other materials required in the process are derived, directly or indirectly, from these two. The process can be represented by the four basic operations shown in Figure 1.Softening. The softening step is described by two reactions in which calcium precipitates as calcium carbonate and bicarbonate is converted to carbonate.These reactions are carried out in a sludge contact reactor. The clarified water from this reactor is the final LMC product, depleted of 70% of its original calcium content, and is used as distillation plant feed water.Calcining. Calcium carbonate sludge from the softener is supplemented with purchased limestone and both are calcined. Offgassed carbon dioxide is used in the manufacture of magnesium carbonate trihydrate from magnesium hydroxide, and lime is used to neutralize bicarbonate and to produce magnesium hydroxide from sea water.Magnesium Hydroxide Production. Magnesium hydroxide is precipitated from a sea water slip stream with lime. The magnesium hydroxide floc is separated and used in the carbonation process, and the calcium-rich liquid phase is discharged as waste.Ca?' + MgC0,.3H20 --+ CaCOl + Mg*' + 3H20 Magnesium Hydroxide Carbonation. Magnesium hydmxide and carbon dioxide from the kiln are combined in a sparged reactor to form magnesium carbonate trihyd...

Bio-cementation of sandy soil using microbially induced carbonate precipitation for marine environments

“…In marine environments, the contribution of the magnesium carbonate hydrate to the strength development is of potential interest and will be investigated in a following paper. In particular, it would be interesting to study to what extent a re-crystallisation of carbonate from magnesium to calcium carbonate occurs under the seawater conditions (Mavis & Checkovich, 1975) and how it effects the duration and stability of bio-cemented sand.…”

“…The magnesium carbonate hydrate produced is known to recrystallise as calcium carbonate in seawater over time. In fact magnesium carbonate has been used to remove calcium from seawater (Mavis & Checkovich, 1975). …”