International Telephone & Telegraph Corp. announced last night that costs associated with closing a Canadian pulp mill during September resulted in sharply reduced profits for 1979.

Preliminary, unaudited results show that earnings declined to $2.65 a share from $4.66 a share in 1978 despite a 13 percent increase in worldwide sales to a record 21.8 billion. Actual net income figures weren't available.

Excluding a $320 million reserve of funds equal to $2.32 a share to cover costs for shutting a pulp mill in Port Cartier, Quebec, ITT's earnings would have increased 7 percent, the firm stated.

Chairman Rand Araskog said the increased sales for the year reflected strong gains in all lines of business for the conglomerate, which include international communications, finance, insurance, electronics, auto parts and food production.

ITT said a more detailed report on 1979 results will be published after a board meeting in March.

Rockwell International Corp. reported a 6 percent boost in net income during the first quarter of fiscal 1980, which ended Dec. 31.

The Pittsbrugh-based corporation said yesterday that net income totaled $68.7 million ($1.88 a share) compared with $64.7 million ($1.83) during the same quarter a year ago. The company said sales in the three-month period totaled $1.56 billion, up 9 percent from sales of $1.43 billion during the first quarter a year ago.

Monsanto Co., a giant chemical producer, reported a gain in earnings in 1979 but a 75.2 percent drop in final-quarter profit despite an increase in sales.

Chairman John W. Hanley said a sharp drop in shipments of chemical products for the automotive and residential construction industries caused fourth-quarter profits to drop to $10.9 million (30 cents a share) on sales of $1.548 billion from $44 million ($1.21) a year earlier on sales of $1.304 billion.

For the year, Monsanto earned $331.1 million ($9.11) on sales of $6.195 billion compared with $302.6 million ($8.29) in 1978 on sales of $5.019 billion. The sales gain was 23.4 percent.

The fourth-quarter figures were depressed further by foreign currency translation losses of 28 cents a share and a reserve of 20 cents a share for costs of the planned closing of a plastics plant in Spain.

Consolidated Edison Co. of New York reported a 5.1 percent increase in earnings per common share in 1979, and the utility's directors raised the common stock dividend by 7 cents to 67 cents a share.

The higher dividend is payable March 15 to shareholders of record Feb. 6. The company noted that inflation had eroded the dividend payment to the point where the new annual rate of $2.68 a share has less purchasing power that the $1.80 annual rate paid in 1967.

Net income amounted to $323.91 million compared with $309.57 million.After preferred stock dividends, the net was equal to $4.51 a common share, up from $4.29 a share.

Total operating revenues in 1979 were $3.33 billion against $3.01 billion.

Fourth-quarter net income for common stock was $59.05 million, (94 cents a share) on operating revenues of $856.97 million compared with $44.96 million (72 cents) on revenues of $707.59 million.