Animal Feed Supplement Having Reduced Corrosion Characteristics

This invention relates to an improved animal feed supplement having reduced metal corrosion characteristics comprising an aqueous solution containing from 87-90 percent by weight 2-hydroxy-4-(methylthio)-butanoic acid, i.e. MHA.RTM. acid, and to the method for preparing same.

The method for preparing MHA acid and its utility as an animal feed supplement are well known and set forth in commonly assigned U.S. Pat. No. 2,745,745 which is incorporated herein by reference. Such patent describes the preparation of MHA acid by hydrolysis of the corresponding cyanohydrin. Thus, the MHA-nitrile is hydrolyzed in excess aqueous mineral acid, such as hydrochloric acid or sulfuric acid, to afford the MHA acid and, e.g., ammonium chloride in a dilute aqueous hydrochloric acid slurry. The slurry is then dehydrated to remove water and aid in the precipitation of most of the ammonium chloride. During this process, most of the hydrochloric acid is also removed. The resulting thick slurry is then filtered and centrifuged to separate a further amount of ammonium chloride and then water is added back to dilute the mixture to a concentration of 87-90 percent by weight of MHA acid.

The acid solution thus prepared is suitable as an animal feed supplement but has presented problems in storage, transportation and handling in metal equipment because of its corrosion characteristics. Accordingly, the object of the present invention is to provide an aqueous MHA acid solution containing from 87 to 90 percent by weight MHA acid which shows reduced corrosion characteristics in contact with metal surfaces such as carbon steel, stainless steel and aluminum, such metals being used as materials of construction in storage, transportation and handling facilities.

In accordance with the present invention, it has been found that the MHA acid as prepared above, contains from about 0.3 to about 0.5% residual mineral acid from the hydrolysis step. This residual mineral acid presents corrosion problems relating to storage, transportation, handling and safety.

It has now been found that if the aqueous MHA acid solution is partially neutralized to the point where the residual mineral acid is neutralized but the MHA acid is not neutralized, an aqueous MHA acid solution is formed having reduced corrosion characteristics. Thus, if the MHA acid solution is neutralized to a pH of from about 0.9 to about 1.2, the residual mineral acid is neutralized and the aqueous acid solution may then be stored or transported and handled in contact with metal surfaces which could not be used with the non-neutralized material. It is critical, however, that the partial neutralization be very carefully done in order that only the residual mineral acid is neutralized and that no neutralization of the MHA acid occurs.

There are three stages in the process for preparing MHA acid wherein the excess mineral acid may be neutralized. Thus, the excess mineral acid may be neutralized (1) after hydrolysis and before dehydration, (2) after dehydration and before centrifuging and (3) after centrifuging when the concentrated MHA acid has been diluted to 87 to 90 percent by weight.

An 87 to 90 percent aqueous solution of MHA acid neutralized to a pH of from about 0.9 to about 1.2 contains essentially no free mineral acid and retains physical characteristics which are comparable to those of the non-neutralized material.

The neutralizing agents which may be used in the process of this invention include ammonia, ammonium hydroxide, sodium hydroxide, potassium hydroxide and calcium hydroxide.

To compare the corrosion rates of neutralized aqueous solutions of MHA acid containing 88 percent by weight MHA acid and non-neutralized aqueous solutions containing 88 percent by weight MHA acid, various metal surfaces were exposed to both the vapor and liquid state of the neutralized and the non-neutralized aqueous solutions at 40.degree. C. The test duration was 40 days.

The results of the tests are set forth in Tables 1 and 2 below. It is readily seen that the neutralized material had significantly reduced corrosion characteristics as compared to the non-neutralized aqueous material.