The state treasurer, with the approval of the industrial commission, shall invest the fund designated as the real estate bond sinking fund in any securities designated as legal investments by section 15-03-04. At the request of the commission, the state treasurer shall redeem and take up out of the real estate bond sinking fund any series of real estate bonds outstanding which may be called by the commission. Upon notification of the call, the state treasurer shall cause to be published a notice of call as directed by the commission but not less than forty-five days prior to the date of call. No other disposition by appropriation, or otherwise, ever may be made of the money in the fund until the bonds are paid fully or until the time limit provided by law for the payment thereof has expired, but if any of the bonds issued and delivered to the commission, as hereinbefore provided, are returned to the state treasurer not sold, then the returned bonds may not be deemed a part of the bond issue secured by the fund.