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Water Treatment Chemicals – The Role of Molybdenum Compounds

2020

Written by: Ritesh Trivedi

A shiny grey metal, which looks quite a lot like steel, molybdenum has been around in nature since time immemorial, however because of its limited occurrence, it has always been considered extremely valuable. Molybdenum has been used widely as an alloying element as well as a metallic coating. The oxides of moly find usage in several industries, including petrochemical, water and chemical treatment.

Let’s take a quick look at the types of the moly compounds and their usages:

Molybdenum disulphide, or Molybdenite, is a naturally occurring ore and molybdates are chemical products that are made from this ore. The formulation is done by swapping out the sulphur atoms with oxygen ones and this leads to the formation of molybdic acid or pure molybdic oxide. Depending on what base is being used and under what conditions the basification is being done, multiple molybdates can be produced and the most common ones include pure molybdic oxide, sodium molybdate dihydrate and ammonium dimolybdate. 

Molybdic oxide finds several uses – as a component in certain types of steel, for the manufacture of other moly compounds and most importantly, as a chemical in water treatment facilities in the form of corrosion inhibitor formulations. It can help prevent corrosion on steel and aluminium and in places where the water needs to be heated, such as in boilers and water heating systems, sodium molybdate is most commonly used. Other uses include temporary coatings against rusting, pitting inhibitors for stainless steels, hydraulic and metalwork fluids, in rinses for phosphate conversion coating and aluminium anodizing processes.

Here’s a little history about the usage of molybdates as corrosion inhibitors:

The earliest patents related to the usage of molybdates as corrosion inhibitors, date all the way back to 1939, wherein they were considered as appropriate for coolants for motor engine vehicles. Recent studies have proven that molybdate is an anodic inhibitor – this means that it inhibits the anode component in the cells that causes corrosion, by increasing the polarization. In simple terms, the molybdate when used under the correct circumstances forms a protective layer, which inhibits the corrosion caused due to aluminium, ferrous and cuprous metals. More importantly, molybdates do not lose any of their chemical properties or efficacy, when they encounter elevated temperatures or breakdown in systems that have chlorine in them.

Perhaps the biggest advantage of using molybdates is that very low concentration levels are required to be considered effective corrosion inhibitors. The most common usage of molybdates for corrosion inhibition is in open and closed water cooling systems. In a typical commercial treatment program, there could be four to five other compounds being used with the molybdate, as it will be the combination that will ward off corrosion, regulate pH levels, control the growth of algae or other microorganisms.

Moving onto why molybdenum compounds are used in water treatment chemicals:

Most large scale commercial complexes, such as office buildings, hospitals and other such commercial institutions will utilise some version of a cooling tower. The heat that is generated through the HVAC systems in the building needs to be dispelled and this takes place through the cooling towers and the process of evaporation. When evaporation happens, there is a concentration of mineral salts and then solubility levels of these salts hit saturation point, they will start to form scales. The water in the cooling towers can be recycled several times, however there is always the need for certain chemicals, because with water, there is always the chance of corrosion and fungal or microbial build-up. Some of the most commonly used chemical products for corrosion inhibition include nitrite, borate and silicate and phosphoric acid for scale inhibition.

The use of sodium molybdate for closed water systems has been a long accepted practice and it is able to not only prevent corrosion, but also assist in the elongation of cooling tower lifecycles. Sodium molybdate inhibits the corrosion of low carbon steel, copper as well as brass in cooling water systems, which function on a recirculating basis and are environmentally safe. Previously, it was chromate that was used, however, when chromate was found to be toxic, it was banned, spurring the increased usage of molybdenum.  

Not only do these molybdate compounds have several commercial applications, they are also considered the most economical when taken in reference to water treatment; what makes it the most appropriate choice is the fact that molybdenum is safe and non-toxic for humans, in most forms. Studies and trials have shown that sodium molybdate’s efficacy might be altered depending on the water composition and other water treatment chemicals being used.

Here is where Rubamin comes in:

Rubamin is a leading manufacturer and global supplier of highly pure molybdenum compounds such as sodium molybdate, ammonium molybdate and molybdenum trioxide commercially, manufactured from spent catalysts from oil refineries. The state of art technology for producing highly pure molybdenum compounds was developed in-house by a strong technical team of R&D chemists, engineers and scientists. Our inorganic molybdates are quite popular because of their favourable properties and behaviour.

Molybdates provide an advantage of corrosion inhibition over earlier practice of using sodium nitrite because it is dosed at a lower Parts Per Million or ppm level, and this allows for lower conductivity of the circulating water  systems, thereby greatly decreasing the potential for galvanic corrosion. Sodium molybdate (SMD) dosing of 50-100 ppm levels, offer the same levels of corrosion inhibition as compared to that of sodium nitrite levels of more than 800 ppm levels dosing. By utilizing lower concentrations of sodium molybdate, the conductivity of water is kept at a minimum and thus the galvanic corrosion potentials are decreased.

In order to meet the requirement for superior quality of molybdates for water treatment applications, we, at Rubamin, a company that is the top sodium molybdate manufacturer in Gujarat, ensure that our sodium molybdate is extremely pure. This is so because there is a very low level of impurity, no insolubles and heavy metals. The impurities in our sodium molybdate are stable at all levels, due to advanced purification technology developed in-house and adopted in the processing of the molybdenum source raw materials, backed by our strong in-house R&D team.  Over the past several years, we have been able to supply the highest grade sodium molybdate for coolants and boilers as well as closed water systems, successfully, due to our dedication to offer only the best, while maintaining our commitment towards remaining sustainable and environmentally friendly.